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Interference from HONO in the measurement of ambient air NO2 via photolytic conversion and quantification of NO
Abstract
The reference method to quantify mixing ratios of the criteria air pollutant nitrogen dioxide (NO2) is NO-O3 chemiluminescence (CL), in which mixing ratios of nitric oxide (NO) are measured by sampling ambient air directly, and mixing ratios of NOx (= sum of NO and NO2) are measured by converting NO2 to NO using, for example, heated molybdenum catalyst or, more selectively, photolytic conversion (P-CL). In this work, the nitrous acid (HONO) interference in the measurement of NO2 by P-CL was investigated. Results with two photolytic NO2 converters are presented. The first used radiation centered at 395 nm, a wavelength region commonly utilized in P-CL. The second used light at 415 nm, where the overlap with the HONO absorption spectrum and hence its photolysis rate are less. Mixing ratios of NO2, NOx and HONO entering and exiting the converters were quantified by Thermal Dissociation Cavity Ring-down Spectroscopy (TD-CRDS). Both converters exhibited high NO2 conversion efficiency (CFNO2; > 90%) and partial conversion of HONO. Plots of CF against flow rate were consistent with photolysis frequencies of 4.2 s⁻¹ and 2.9 s⁻¹ for NO2 and 0.25 s⁻¹ and 0.10 s⁻¹ for HONO at 395 nm and 415 nm, respectively. CFHONO was larger than predicted from the overlap of the emission and HONO absorption spectra. The results imply that measurements of NO2 by P-CL marginally but systematically overestimate true NO2 concentrations, and that this interference should be considered in environments with high HONO:NO2 ratios such as the marine boundary layer or in biomass burning plumes.
... 2.5 (Reed et al., 2016). With increasing residence time in the converter and high atmospheric HONO / NO 2 ratios, photolysis of HONO could become relevant, as recently shown by Gingerysty et al. (2021). ...
Nitrogen oxides (NOx≡NO+NO2) are centrally involved in the photochemical processes taking place in the Earth's atmosphere. Measurements of NO2, particularly in remote areas where concentrations are of the order of parts per trillion by volume (pptv), are still a challenge and subject to extensive research. In this study, we present NO2 measurements via photolysis–chemiluminescence during the research aircraft campaign CAFE Africa (Chemistry of the Atmosphere – Field Experiment in Africa) 2018 around Cabo Verde and the results of laboratory experiments to characterize the photolytic converter used. We find the NO2 reservoir species MPN (methyl peroxy nitrate) to produce the only relevant thermal interference in the converter under the operating conditions during CAFE Africa. We identify a memory effect within the conventional photolytic converter (type 1) associated with high NO concentrations and rapidly changing water vapor concentrations, accompanying changes in altitude during aircraft measurements, which is due to the porous structure of the converter material. As a result, NO2 artifacts, which are amplified by low conversion efficiencies, and a varying instrumental background adversely affect the NO2 measurements. We test and characterize an alternative photolytic converter (type 2) made from quartz glass, which improves the reliability of NO2 measurements in laboratory and field studies.
A well-characterized source of nitrous acid vapour (HONO) is essential for accurate ambient air measurements by instruments requiring external calibration. In this work, a compact HONO source is described in which gas streams containing dilute concentrations of HONO are generated by flowing hydrochloric acid (HCl) vapour emanating from a permeation tube over continuously agitated dry sodium nitrite (NaNO2) heated to 50 ∘C. Mixing ratios of HONO and potential by-products including NO, NO2, and nitrosyl chloride (ClNO) were quantified by Fourier transform infrared (FTIR) and thermal-dissociation cavity ring-down spectroscopy (TD-CRDS). A key parameter is the concentration of HCl, which needs to be kept small (<4 ppmv) to avoid ClNO formation. The source produces gas streams containing HONO in air in >95 % purity relative to other nitrogen oxides. The source output is rapidly tuneable and stabilizes within 90 min. Combined with its small size and portability, this source is highly suitable for calibration of HONO instruments in the field.
Reactive nitrogen (Nr, defined as all nitrogen-containing compounds except for N2 and N2O) is one of the most important classes of compounds emitted from wildfire, as Nr impacts both atmospheric oxidation processes and particle formation chemistry. In addition, several Nr compounds can contribute to health impacts from wildfires. Understanding the impacts of wildfire on the atmosphere requires a thorough description of Nr emissions. Total reactive nitrogen was measured by catalytic conversion to NO and detection by NO–O3 chemiluminescence together with individual Nr species during a series of laboratory fires of fuels characteristic of western US wildfires, conducted as part of the FIREX Fire Lab 2016 study. Data from 75 stack fires were analyzed to examine the systematics of nitrogen emissions. The measured Nr / total-carbon ratios averaged 0.37 % for fuels characteristic of western North America, and these gas-phase emissions were compared with fuel and residue N/C ratios and mass to estimate that a mean (±SD) of 0.68 (±0.14) of fuel nitrogen was emitted as N2 and N2O. The Nr detected as speciated individual compounds included the following: nitric oxide (NO), nitrogen dioxide (NO2), nitrous acid (HONO), isocyanic acid (HNCO), hydrogen cyanide (HCN), ammonia (NH3), and 44 nitrogen-containing volatile organic compounds (NVOCs). The sum of these measured individual Nr compounds averaged 84.8 (±9.8) % relative to the total Nr, and much of the 15.2 % “unaccounted” Nr is expected to be particle-bound species, not included in this analysis.
A number of key species, e.g., HNCO, HCN, and HONO, were confirmed not to correlate with only flaming or with only smoldering combustion when using modified combustion efficiency, MCE=CO2/(CO+CO2), as a rough indicator. However, the systematic variations in the abundance of these species relative to other nitrogen-containing species were successfully modeled using positive matrix factorization (PMF). Three distinct factors were found for the emissions from combined coniferous fuels: a combustion factor (Comb-N) (800–1200 ∘C) with emissions of the inorganic compounds NO, NO2, and HONO, and a minor contribution from organic nitro compounds (R-NO2); a high-temperature pyrolysis factor (HT-N) (500–800 ∘C) with emissions of HNCO, HCN, and nitriles; and a low-temperature pyrolysis factor (LT-N) (<500 ∘C) with mostly ammonia and NVOCs. The temperature ranges specified are based on known combustion and pyrolysis chemistry considerations. The mix of emissions in the PMF factors from chaparral fuels (manzanita and chamise) had a slightly different composition: the Comb-N factor was also mostly NO, with small amounts of HNCO, HONO, and NH3; the HT-N factor was dominated by NO2 and had HONO, HCN, and HNCO; and the LT-N factor was mostly NH3 with a slight amount of NO contributing. In both cases, the Comb-N factor correlated best with CO2 emission, while the HT-N factors from coniferous fuels correlated closely with the high-temperature VOC factors recently reported by Sekimoto et al. (2018), and the LT-N had some correspondence to the LT-VOC factors. As a consequence, CO2 is recommended as a marker for combustion Nr emissions, HCN is recommended as a marker for HT-N emissions, and the family NH3 / particle ammonium is recommended as a marker for LT-N emissions.
This work describes an incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for quantification of HONO and NO2 mixing ratios in ambient air. The instrument is operated in the near-ultraviolet spectral region between 361 and 388 nm. The mirror reflectivity and optical cavity transmission function were determined from the optical extinction observed when sampling air and helium. To verify the accuracy of this approach, Rayleigh scattering cross sections of nitrogen and argon were measured and found to be in quantitative agreement with literature values. The mirror reflectivity exceeded 99.98 %, at its maximum near 373 nm, resulting in an absorption path length of 6 km from a 1 m long optical cavity. The instrument precision was assessed through Allan variance analyses and showed minimum deviations of ±58 and ±210 pptv (1σ) for HONO and NO2, respectively, at an optimum acquisition time of 5 min. Measurements of HONO and NO2 mixing ratios in laboratory-generated mixtures by IBBCEAS were compared to thermal dissociation cavity ring-down spectroscopy (TD-CRDS) data and agreed within combined experimental uncertainties. Sample ambient air data collected in Calgary are presented.
This work describes a state-of-the-art, incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for quantification of HONO and NO2 mixing ratios in ambient air. The instrument is operated in the near-ultraviolet spectral region between 361 and 388 nm. The mirror reflectivity and optical cavity transmission function were determined from the optical extinction observed when sampling air and helium. To verify the accuracy of this approach, Rayleigh scattering cross-sections of nitrogen and argon were measured and found in quantitative agreement with literature values. The mirror reflectivity exceeded 99.98 %, at its maximum near 373 nm, resulting in an absorption pathlength of 6 km from a 1 m long optical cavity. The instrument precision was assessed through Allan variance analyses and showed minimum deviations of ±58 pptv and ±210 pptv (1σ) for HONO and NO2, respectively, at an optimum acquisition time of 5 min. Measurements of HONO and NO2 mixing ratios in laboratory-generated mixtures by IBBCEAS were compared to thermal dissociation cavity ring-down spectroscopy (TD-CRDS) data and agreed within combined experimental uncertainties. Sample ambient air data collected in Calgary are presented.
Heating of quartz crystals in order to study melt and high-temperature fluid inclusions is a common practice to constrain major physical and chemical parameters of magmatic and hydrothermal processes. Diffusion and modification of trace element content in quartz and its hosted melt inclusions have been investigated through step-heating experiments of both matrix-free quartz crystals and quartz crystals associated with sulfides and other minerals using a Linkam TS1500 stage. Magmatic and hydrothermal quartz were successively analyzed after each heating step for Cu, Al, and Ti using electron probe micro-analyzer. After the last heating step, quartz crystals and their hosted melt inclusions were analyzed by laser ablation inductively coupled plasma mass spectrometry and compared to unheated samples. Heated samples reveal modification of Cu, Li, Na, and B contents in quartz and modification of Cu, Li, Ag, and K concentrations in melt inclusions. Our results show that different mechanisms of Cu, Li, and Na incorporation occur in magmatic and hydrothermal quartz. Heated magmatic quartz records only small, up to a few ppm, enrichment in Cu and Na, mostly substituting for Li. By contrast, heated hydrothermal quartz shows enrichment up to several hundreds of ppm in Cu, Li, and Na, which substitute for originally present H. This study reveals that the composition of both quartz and its hosted melt inclusions may be significantly modified upon heating experiments, leading to erroneous quantification of elemental concentrations. In addition, each quartz crystal also becomes significantly enriched in Cu in the sub-surface layer during heating. We propose that sub-surface Cu enrichment is a direct indication of Cu diffusion in quartz externally sourced from both the surrounding sulfides as well as the copper pins belonging to the heating device. Our study shows that the chemical compositions of both heated quartz and its hosted inclusions must be interpreted with great caution to avoid misleading geological interpretations.
Characterization of daytime sources of nitrous acid (HONO) is crucial to
understand atmospheric oxidation and radical cycling in the planetary
boundary layer. HONO and numerous other atmospheric trace constituents were
measured on the Mediterranean island of Cyprus during the CYPHEX
(CYprus PHotochemical EXperiment) campaign in summer 2014. Average volume
mixing ratios of HONO were 35 pptv (±25 pptv) with a
HONO ∕ NOx ratio of 0.33, which was considerably higher than
reported for most other rural and urban regions. Diel profiles of HONO showed
peak values in the late morning (60 ± 28 pptv around 09:00 local
time) and persistently high mixing ratios during daytime
(45 ± 18 pptv), indicating that the photolytic loss of HONO is
compensated by a strong daytime source. Budget analyses revealed unidentified
sources producing up to
3.4 × 106 molecules cm−3 s−1 of HONO and up to
2.0 × 107 molecules cm−3 s−1 NO. Under humid
conditions (relative humidity > 70 %), the source strengths of HONO and NO exhibited
a close linear correlation (R2 = 0.72), suggesting a common source that may
be attributable to emissions from microbial communities on soil surfaces.
Measurements of HONO were carried out at an urban background site near
central London as part of the Clean air for London (ClearfLo) project in summer 2012. Data
were
collected from 22 July to 18 August 2014, with peak values of
up to 1.8 ppbV at night and non-zero values of between 0.2 and 0.6 ppbV seen
during the day. A wide range of other gas phase, aerosol, radiation, and
meteorological measurements were made concurrently at the same site,
allowing a detailed analysis of the chemistry to be carried out. The peak
HONO/NOx ratio of 0.04 is seen at ∼ 02:00 UTC, with the
presence of a second, daytime, peak in HONO/NOx of similar magnitude
to the night-time peak, suggesting a significant secondary daytime HONO
source. A photostationary state calculation of HONO involving formation from
the reaction of OH and NO and loss from photolysis, reaction with OH, and dry
deposition shows a significant underestimation during the day, with
calculated values being close to 0, compared to the measurement average
of 0.4 ppbV at midday. The addition of further HONO sources from the
literature, including dark conversion of NO2 on surfaces, direct
emission, photolysis of ortho-substituted nitrophenols, the postulated
formation from the reaction of HO2 × H2O with NO2,
photolysis of adsorbed HNO3 on ground and aerosols, and HONO produced
by photosensitized conversion of NO2 on the surface increases the
daytime modelled HONO to 0.1 ppbV, still leaving a significant missing
daytime source. The missing HONO is plotted against a series of parameters
including NO2 and OH reactivity (used as a proxy for organic material),
with little correlation seen. Much better correlation is observed with the
product of these species with j(NO2), in particular NO2 and the
product of NO2 with OH reactivity. This suggests the missing HONO
source is in some way related to NO2 and also requires sunlight.
Increasing the photosensitized surface conversion rate of NO2 by a
factor of 10 to a mean daytime first-order loss of ∼ 6 × 10−5 s−1 (but which varies as a function of j(NO2)) closes
the daytime HONO budget at all times (apart from the late afternoon),
suggesting that urban surfaces may enhance this photosensitized source. The
effect of the missing HONO to OH radical production is also investigated and
it is shown that the model needs to be constrained to measured HONO in order
to accurately reproduce the OH radical measurements.
Nitrous acid (HONO) often plays an important role in tropospheric photochemistry as a major precursor of the hydroxyl radical (OH) in early morning hours and potentially during the day. However, the processes leading to formation of HONO and its vertical distribution at night, which can have a considerable impact on daytime ozone formation, are currently poorly characterized by observations and models. Long-path differential optical absorption spectroscopy (LP-DOAS) measurements of HONO during the 2006 TexAQS II Radical and Aerosol Measurement Project (TRAMP), near downtown Houston, TX, show nocturnal vertical profiles of HONO, with mixing ratios of up to 2.2 ppb near the surface and below 100 ppt aloft. Three nighttime periods of HONO, NO2 and O3 observations during TRAMP were used to perform model simulations of vertical mixing ratio profiles. By adjusting vertical mixing and NOx emissions the modeled NO2 and O3 mixing ratios showed very good agreement with the observations.
Using a simple conversion of NO2 to HONO on the ground, direct HONO emissions, as well as HONO loss at the ground and on aerosol, the observed HONO profiles were reproduced by the model for 1–2 and 7–8 September in the nocturnal boundary layer (NBL). The unobserved increase of HONO to NO2 ratio (HONO/NO2) with altitude that was simulated by the initial model runs was found to be due to HONO uptake being too small on aerosol and too large on the ground. Refined model runs, with adjusted HONO uptake coefficients, showed much better agreement of HONO and HONO/NO2 for two typical nights, except during morning rush hour, when other HONO formation pathways are most likely active. One of the nights analyzed showed an increase of HONO mixing ratios together with decreasing NO2 mixing ratios that the model was unable to reproduce, most likely due to the impact of weak precipitation during this night.
HONO formation and removal rates averaged over the lowest 300 m of the atmosphere showed that NO2 to HONO conversion on the ground was the dominant source of HONO, followed by traffic emission. Aerosol did not play an important role in HONO formation. Although ground deposition was also a major removal pathway of HONO, net HONO production at the ground was the main source of HONO in our model studies. Sensitivity studies showed that in the stable NBL, net HONO production at the ground tends to increase with faster vertical mixing and stronger NOx emission. Vertical transport was found to be the dominant source of HONO aloft.
A complete understanding of the formation mechanism of nitrous acid (HONO) in the ambient atmosphere is complicated by a lack of understanding of processes occurring when aqueous water is present. We report nocturnal measurements of HONO, SO2 and NO2 by differential optical absorption spectroscopy over the ocean surface in a polluted marine environment. In this aqueous environment, we observed reproducible pseudo steady states (PSS) of HONO every night, that are fully formed shortly after sunset, much faster than seen in urban environments. During the PSS period, HONO is constant with time, independent of air mass source and independent of the concentration of NO2. The independence of HONO on the concentration of NO2 implies a 0° order formation process, likely on a saturated surface, with reversible partitioning of HONO to the gas phase, through vaporization and deposition to the surface. We observed median HONO/NO2 ratios starting at 0.13 at the beginning of the PSS period (with an apparent lower bound of 0.03), rising to median levels of ~0.30 at the end of the PSS period (with an upper bound >1.0). The implication of these numbers is that they suggest a common surface mechanism of HONO formation on terrestrial and aqueous surfaces, with an increase in the HONO/NO2 ratio with the amount of water available at the surface. The levels of HONO during the nocturnal PSS period are positively correlated with temperature, consistent with a partitioning of HONO from the surface to the gas phase with an apparent enthalpy of vaporization of Δ H SNL (HONO)=55.5±5.4 kJ mol−1. The formation mechanism on aqueous surfaces is independent of relative humidity (RH), despite observation of a negative HONO-RH correlation. A conceptual model for HONO formation on ambient aqueous surfaces is presented, with the main elements being the presence of a surface nanolayer (SNL), highly acidic and saturated with N(IV) precursors, production of HNO3, that diffuses to underlying water layers, and HONO, which partitions reversibly between the SNL and the gas phase. Implications of the conceptual model are discussed.
Vegetation commonly managed by prescribed burning was collected from five southeastern and southwestern US military bases and burned under controlled conditions at the US Forest Service Fire Sciences Laboratory in Missoula, Montana. The smoke emissions were measured with a large suite of state-of-the-art instrumentation including an open-path Fourier transform infrared (OP-FTIR) spectrometer for measurement of gas-phase species. The OP-FTIR detected and quantified 19 gas-phase species in these fires: CO2, CO, CH4, C2H2, C2H4, C3H6, HCHO, HCOOH, CH3OH, CH3COOH, furan, H2O, NO, NO2, HONO, NH3, HCN, HCl, and SO2. Emission factors for these species are presented for each vegetation type burned. Gas-phase nitrous acid (HONO), an important OH precursor, was detected in the smoke from all fires. The HONO emission factors ranged from 0.15 to 0.60 g kg−1 and were higher for the southeastern fuels. The fire-integrated molar emission ratios of HONO (relative to NOx) ranged from approximately 0.03 to 0.20, with higher values also observed for the southeastern fuels. The majority of non-methane organic compound (NMOC) emissions detected by OP-FTIR were oxygenated volatile organic compounds (OVOCs) with the total identified OVOC emissions constituting 61±12% of the total measured NMOC on a molar basis. These OVOC may undergo photolysis or further oxidation contributing to ozone formation. Elevated amounts of gas-phase HCl and SO2 were also detected during flaming combustion, with the amounts varying greatly depending on location and vegetation type. The fuels with the highest HCl emission factors were all located in the coastal regions, although HCl was also observed from fuels farther inland. Emission factors for HCl were generally higher for the southwestern fuels, particularly those found in the chaparral biome in the coastal regions of California.
Characterization of daytime sources of nitrous acid (HONO) is crucial to understand atmospheric oxidation and radical cycling in the planetary boundary layer. HONO and numerous other atmospheric trace constituents were measured on the Mediterranean island of Cyprus during the CYPHEX campaign (CYPHEX = CYprus PHotochemical EXperiment) in summer 2014. Average volume mixing ratios of HONO were 35 pptv (±25 pptv) with a HONO/NOx ratio of 0.33, which was considerably higher than reported for most other rural and urban regions. Diel profiles of HONO showed peak values in the late morning (60±28 pptv around 09:00 local time), and persistently high mixing ratios during daytime (45±18 pptv) indicating that the photolytic loss of HONO is compensated by a strong daytime source. Budget analyses revealed unidentified sources producing up to 3.4 × 106 molecules cm−3 s−1 of HONO and up to 2.0 × 107 molecules cm−3 s−1 NO. Under humid conditions (RH >70 %), the source strengths of HONO and NO exhibited a close linear correlation (R2 = 0.78), suggesting a common source that may be attributable to emissions from microbial communities on soil surfaces.
Measurement of NO2 at low concentrations (tens of ppts) is
non-trivial. A variety of techniques exist, with the conversion of
NO2 into NO followed by chemiluminescent detection of NO being
prevalent. Historically this conversion has used a catalytic approach
(molybdenum); however, this has been plagued with interferences. More
recently, photolytic conversion based on UV-LED irradiation of a reaction
cell has been used. Although this appears to be robust there have been
a range of observations in low-NOx environments which have measured higher NO2 concentrations than might be expected from steady-state analysis of simultaneously measured NO, O3, jNO2, etc. A range of explanations exist in the literature, most of which focus on an unknown and unmeasured “compound X” that is able to convert NO to NO2 selectively. Here we explore in the laboratory the interference on the photolytic NO2 measurements from the thermal decomposition of peroxyacetyl nitrate (PAN) within the photolysis cell. We find that approximately 5 % of the PAN decomposes within the instrument, providing a potentially significant interference. We parameterize the decomposition in terms of the temperature of the light source, the ambient temperature, and a mixing timescale ( ∼ 0.4 s for our instrument) and expand the
parametric analysis to other atmospheric compounds that decompose readily to
NO2 (HO2NO2, N2O5, CH3O2NO2, IONO2, BrONO2, higher PANs). We apply these parameters to the output of a global atmospheric model (GEOS-Chem) to investigate the global impact of this interference on (1) the NO2 measurements and (2) the NO2 : NO ratio, i.e. the Leighton relationship. We find that there are significant interferences in cold regions with low NOx concentrations such as the Antarctic, the remote Southern Hemisphere, and the
upper troposphere. Although this interference is likely instrument-specific,
the thermal decomposition to NO2 within the instrument's photolysis
cell could give an at least partial explanation for the anomalously high
NO2 that has been reported in remote regions. The interference can be
minimized by better instrument characterization, coupled to instrumental
designs which reduce the heating within the cell, thus simplifying
interpretation of data from remote locations.
During the last decades, nitrous acid (HONO) has attracted significant attention as a major source of the OH radical, the detergent of the atmosphere. However, the different daytime sources identified in the laboratory are still the subject of controversial discussion. In the present study, one of these postulated HONO sources, the heterogeneous photolysis of nitric acid (HNO3), was studied on quartz glass surfaces in a photo flow-reactor under atmospherically relevant conditions. In contrast to other investigations, a very low HNO3 photolysis frequency for HONO formation of J(HNO3→HONO) = 2.4•10-7 s-1 (0° SZA, 50 % r.h.) was determined. If these results can be transferred to atmospheric surfaces, HNO3 photolysis cannot explain significant HONO levels in the daytime atmosphere. In addition, it is demonstrated that even the small measured yields of HONO did not result from the direct photolysis of HNO3 but rather from the consecutive heterogeneous conversion of the primary photolysis product NO2 on the humid surfaces. The secondary NO2 conversion was not photo-enhanced on pure quartz glass surfaces in good agreement with former studies. A photolysis frequency for the primary reaction product NO2 of J(HNO3→NO2) = 1.1•10-6 s-1 has been calculated (0° SZA, 50 % r.h.), which indicates that renoxification by photolysis of adsorbed HNO3 on non-reactive surfaces is also a minor process in the atmosphere.
Measurement of NO2 at low concentrations is non-trivial. A variety of techniques exist, with the conversion of NO2 into NO followed by chemiluminescent detection of NO be- ing prevalent. Historically this conversion has used a catalytic approach (Molybdenum); however this has been plagued with interferences. More recently, photolytic conversion based on UV-LED irradiation of a reaction cell has been used. Although this appears to be robust there have been a range of observations in low NOx environments which have measured higher NO2 concentrations than might be expected from steady state analysis of simultaneously measured NO, O3, JNO2 etc. A range of explanations exist
10 in the literature most of which focus on an unknown and unmeasured “compound X ” that is able to convert NO to NO2 selectively. Here we explore in the laboratory the interference on the photolytic NO2 measurements from the thermal decomposition of peroxyacetyl nitrate (PAN) within the photolysis cell. We find that approximately 5 % of the PAN decomposes within the instrument providing a potentially significant interference. We parameterize the decomposition in terms of the temperature of the light source, the ambient temperature and a mixing timescale (∼ 0.4 s for our instrument) and expand the parametric analysis to other atmospheric compounds that decompose readily to NO2 (HO2NO2, N2O5, CH3O2NO2, IONO2, BrONO2, Higher PANs). We ap- ply these parameters to the output of a global atmospheric model (GEOS-Chem) to investigate the global impact of this interference on (1) the NO2 measurements and (2) the NO2 : NO ratio i.e. the Leighton relationship. We find that there are significant in- terferences in cold regions with low NOx concentrations such as Antarctic, the remote Southern Hemisphere and the upper troposphere. Although this interference is likely instrument specific, it appears that the thermal decomposition of NO2 within the instrument’s photolysis cell may give an explanation for the anomalously high NO2 that has been reported in remote regions, and would reconcile measured and modelled NO2 to NO ratios without having to invoke novel chemistry. Better instrument characterization, coupled to instrumental designs which reduce the heating within the cell seem likely to minimize the interference in the future, thus simplifying interpretation of data from remote locations.
Micro-contamination exerts ever-increasing adverse impact on semiconductor manufacturing as device integration scale keeps increasing and device geometry continues decreasing. In particular, contaminations from particles, trace metals, and/or organic compounds can reduce device yield, quality, and reliability [. Metallic impurities from materials used for process equipment are one of the major contamination sources.
The photolysis of nitrate embedded in ice and snow can be a significant source of volatile nitrogen oxides affecting the composition of the planetary boundary layer. In this work, we examined the nitrogen oxides evolved from irradiated frozen solutions containing nitrate. Products were monitored by cavity ring-down spectroscopy (CRDS), NO-O3 chemiluminescence (CL), and chemical ionization mass spectrometry (CIMS). Under acidic conditions, the nitrogen oxides volatilized were mainly in the form of NOz, i.e., nitrous (HONO), nitric (HONO2), peroxynitrous (HOONO), and peroxynitric acid (HO2NO2). Identification of acidic nitrogen oxides by CIMS and possible HOONO, HONO2 and HO2NO2 formation pathways are discussed.
Nitric acid (HNO3) is the dominant end product of NOx (= NO + NO2) oxidation in the troposphere, and its dry deposition is considered to be a major removal pathway for the atmospheric reactive nitrogen. Here we present both field and laboratory results to demonstrate that HNO3 deposited on ground and vegetation surfaces may undergo effective photolysis to form HONO and NOx, 1–2 orders of magnitude faster than in the gas phase and aqueous phase. With this enhanced rate, HNO3 photolysis on surfaces may significantly impact the chemistry of the overlying atmospheric boundary layer in remote low-NOx regions via the emission of HONO as a radical precursor and the recycling of HNO3 deposited on ground surfaces back to NOx.
[ 1] Recent decreases in nitrogen oxide (NO(x) = NO + NO(2)) emissions from eastern U. S. power plants and their effects on regional ozone are studied. Using the EPA 1999 National Emission Inventory as a reference emission data set, NO(x) and sulfur dioxide (SO(2)) emission rates at selected power plants are updated to their summer 2003 levels using Continuous Emission Monitoring System ( CEMS) measurements. The validity of the CEMS data is established by comparison to observations made on the NOAA WP-3 aircraft as part of the 2004 New England Air Quality Study. The impacts of power plant NO(x) emission decreases on O(3) are investigated using the WRF-Chem regional chemical forecast model. Summertime NO(x) emission rates decreased by approximately 50% between 1999 and 2003 at the subset of power plants studied. The impact of NO(x) emission reductions on ozone was moderate during summer 2004 because of relatively cool temperatures and frequent synoptic disturbances. Effects in individual plant plumes vary depending on the plant's NO(x) emission strength, the proximity of other NO(x) sources, and the availability of volatile organic compounds (VOCs) and sunlight. This study provides insight into the ozone changes that can be anticipated as power plant NO(x) emission reductions continue to be implemented throughout the United States.
At a site near Boulder, Colorado, simultaneous atmospheric measurements were made of NO, NO(x), and NO(y) in a field intercomparison of instruments in involving two currently employed techniques of NO(x) and NO(y) measurement. Both NO(y) instruments depended upon the reduction of the NO(y) species to NO with detection by chemiluminescence, but different catalysts were employed for the reduction: (1) a gold catalyst (with addition of 0.3% CO) at 300°C, or (2) a molybdenum catalyst at 400°C. The two systems of NO(x) detection involveld (1) photolysis of NO 2, and (2) reduction of NO 2 by solid ferrous sulfate. Several times during the intercomparison the response to calibrated samples of reference compounds NO, NO 2, PAN, HNO 3, n-propyl nitrate (NPN), and NH 3 was measured. From the results the following conclusions were made: (1) The two NO instruments agreed on NO mixing ratios that were measured during the daytime hours over a range from the limits of detection to 35 parts per billion by volume (ppbv), (2) the two NO(y) instruments gave similar estimates of NO(y) in ambient air over a wide range of mixing ratios (0.4-10 ppbv), and (3) the ferrous sulfate converter used for NO(x) detection showed a significant interference from NPN and PAN.
Commercially available ultraviolet light-emitting diodes (UV-LEDs) are evaluated in the laboratory as a light source for photolysis
of atmospheric nitrogen dioxide (NO2) followed by chemiluminescence detection (P-CL) of the resulting nitric oxide (NO). Sensitivity, selectivity, and engineering
simplicity of three UV-LED sources are compared. The most powerful source uses two 9-W Nichia LED modules and provides an
NO2 photolysis frequency (j) of 8.4 s−1 corresponding to a nine-fold improvement over a 100-W Hg arc lamp; this source provides the most sensitivity, but requires
water cooling. A pair of Hamamatsu 0.25-W LED modules provides intermediate sensitivity with photolysis efficiency comparable
to a 100-W Hg arc lamp. The Hamamatsu LEDs require no additional cooling and are a simple and inexpensive package, providing
sensitivity and stability appropriate for long-term, unattended measurements at remote surface sites. A Droplet Measurement
Technologies Blue-Light Converter (BLC) represents the most complete off-the-shelf system available for atmospheric measurements
of NO2. The BLC provides the least sensitivity of the LED-based converters, but is less subject to interference from HONO. Implementing
the Nichia LEDs into the NOAA WP-3D aircraft P-CL instrument, in conjunction with improved sample gas handling, improves instrument
time response by a factor of eight and permits 1-Hz retrieval of ambient NO and NO2 using a single detector simply by modulating the LEDs on and off. The performance and stability of the Nichia LEDs are further
assessed during the CalNex (California Research at the Nexus of Air Quality and Climate Change) field study in May and June
of 2010.
KeywordsNitrogen dioxide–Photolysis–UV-LEDs–Chemiluminescence–Troposphere
Reliable measurements of atmospheric trace gases are necessary for both, a better understanding of the chemical processes occurring in the atmosphere, and for the validation of model predictions. Nitrogen dioxide (NO2) is a toxic gas and is thus a regulated air pollutant. Besides, it is of major importance for the oxidation capacity of the atmosphere and plays a pivotal role in the formation of ozone and acid precipitation. Detection of NO2 is a difficult task since many of the different commercial techniques used are affected by interferences. The chemiluminescence instruments that are used for indirect NO2 detection in monitoring networks and smog chambers use either molybdenum or photolytic converters and are affected by either positive (NOy) or negative interferences (radical formation in the photolytic converter). Erroneous conclusions on NO2 can be drawn if these interferences are not taken into consideration. In the present study, NO2 measurements in the urban atmosphere, in a road traffic tunnel and in a smog-chamber using different commercial techniques, i.e. chemiluminescence instruments with molybdenum or photolytic converters, a Luminol based instrument and a new NO2-LOPAP, were compared with spectroscopic techniques, i.e. DOAS and FTIR. Interferences of the different instruments observed during atmospheric measurements were partly characterised in more detail in the smog chamber experiments. Whereas all the commercial instruments showed strong interferences, excellent agreement was obtained between a new NO2-LOPAP instrument and the FTIR technique for the measurements performed in the smog chamber.
Vegetation commonly managed by prescribed burning was collected from five southeastern and southwestern US military bases and burned under controlled conditions at the US Forest Service Fire Sciences Laboratory in Missoula, Montana. The smoke emissions were measured with a large suite of state-of-the-art instrumentation including an open-path Fourier transform infrared (OP-FTIR) spectrometer for measurement of gas-phase species. The OP-FTIR detected and quantified 19 gas-phase species in these fires: CO2, CO, CH4, C2H2, C2H4, C3H6, HCHO, HCOOH, CH3OH, CH3COOH, furan, H2O, NO, NO2, HONO, NH3, HCN, HCl, and SO2. Emission factors for these species are presented for each vegetation type burned. Gas-phase nitrous acid (HONO), an important OH precursor, was detected in the smoke from all fires. The HONO emission factors ranged from 0.15 to 0.60 g kg−1 and were higher for the southeastern fuels. The fire-integrated molar emission ratios of HONO (relative to NOx) ranged from approximately 0.03 to 0.20, with higher values also observed for the southeastern fuels. The majority of non-methane organic compound (NMOC) emissions detected by OP-FTIR were oxygenated volatile organic compounds (OVOCs) with the total identified OVOC emissions constituting 61 ± 12% of the total measured NMOC on a molar basis. These OVOC may undergo photolysis or further oxidation contributing to ozone formation. Elevated amounts of gas-phase HCl and SO2 were also detected during flaming combustion, with the amounts varying greatly depending on location and vegetation type. The fuels with the highest HCl emission factors were all located in the coastal regions, although HCl was also observed from fuels farther inland. Emission factors for HCl were generally higher for the southwestern fuels, particularly those found in the chaparral biome in the coastal regions of California.
In March 2006 two instrumented aircraft made the first detailed field measurements of biomass burning (BB) emissions in the Northern Hemisphere tropics as part of the MILAGRO project. The aircraft were the National Center for Atmospheric Research C-130 and a University of Montana/US Forest Service Twin Otter. The initial emissions of up to 49 trace gas or particle species were measured from 20 deforestation and crop residue fires on the Yucatan peninsula. This included two trace gases useful as indicators of BB (HCN and acetonitrile) and several rarely, or never before, measured species: OH, peroxyacetic acid, propanoic acid, hydrogen peroxide, methane sulfonic acid, and sulfuric acid. Crop residue fires emitted more organic acids and ammonia than deforestation fires, but the emissions from the main fire types were otherwise fairly similar. The Yucatan fires emitted unusually high amounts of SO2 and particle chloride, likely due to a strong marine influence on this peninsula. As smoke from one fire aged, the ratio ΔO3/ΔCO increased to ~15% in 1×10^7 molecules/cm^3) that were likely caused in part by high initial HONO (~10% of NO_y). Thus, more research is needed to understand critical post emission processes for the second-largest trace gas source on Earth. It is estimated that ~44 Tg of biomass burned in the Yucatan in the spring of 2006. Mexican BB (including Yucatan BB) and urban emissions from the Mexico City area can both influence the March-May air quality in much of Mexico and the US.
Data from a recent field campaign in Mexico City are used to evaluate the performance of the EPA Federal Reference Method for monitoring the ambient concentrations of NO2. Measurements of NO2 from standard chemiluminescence monitors equipped with molybdenum oxide converters are compared with those from Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS) and Differential Optical Absorption Spectroscopy (DOAS) instruments. A significant interference in the chemiluminescence measurement is shown to account for up to 50% of ambient NO2 concentration during afternoon hours. As expected, this interference correlates well with non-NOx reactive nitrogen species (NOz) as well as with ambient O3 concentrations, indicating a photochemical source for the interfering species. A combination of ambient gas phase nitric acid and alkyl and multifunctional alkyl nitrates is deduced to be the primary cause of the interference. Observations at four locations at varying proximities to emission sources indicate that the percentage contribution of HNO3 to the interference decreases with time as the air parcel ages. Alkyl and multifunctional alkyl nitrate concentrations are calculated to reach concentrations as high as several ppb inside the city, on par with the highest values previously observed in other urban locations. Averaged over the MCMA-2003 field campaign, the chemiluminescence monitor interference resulted in an average measured NO2 concentration up to 22% greater than that from co-located spectroscopic measurements. Thus, this interference has the potential to initiate regulatory action in areas that are close to non-attainment and may mislead atmospheric photochemical models used to assess control strategies for photochemical oxidants.
Mixing ratios of the criteria air contaminant nitrogen dioxide (NO2) are commonly quantified by reduction to nitric oxide (NO) using a photolytic converter followed by NO-O3 chemiluminescence (CL). In this work, the performance of a photolytic NO2 converter prototype originally designed for continuous emission monitoring and emitting light at 395 nm was evaluated. Mixing ratios of NO2 and NOx (= NO + NO2) entering and exiting the converter were monitored by blue diode laser cavity ring-down spectroscopy (CRDS). The NO2 photolysis frequency was determined by measuring the rate of conversion to NO as a function of converter residence time and found to be 4.2 sec⁻¹. A maximum 96% conversion of NO2 to NO over a large dynamic range was achieved at a residence time of (1.5±0.3) sec, independent of relative humidity.
Interferences from odd nitrogen (NOy) species such as peroxyacyl nitrates (PAN; RC(O)O2NO2), alkyl nitrates (AN; RONO2), nitrous acid (HONO), and nitric acid (HNO3) were evaluated by operating the prototype converter outside its optimum operating range (i.e., at higher pressure and longer residence time) for easier quantification of interferences. Four mechanisms that generate artifacts and interferences were identified: direct photolysis, foremost of HONO at a rate constant of 6% that of NO2, thermal decomposition, primarily of PAN, surface promoted photochemistry, and secondary chemistry in the connecting tubing. These interferences are likely present to a certain degree in all photolytic converters currently in use but are rarely evaluated or reported. Recommendations for improved performance of photolytic converters include operating at lower cell pressure and higher flow rates, thermal management that ideally results in a match of photolysis cell temperature with ambient conditions, and minimization of connection tubing length. When properly implemented, these interferences can be made negligibly small when measuring NO2 in ambient air.
The effect of a single water molecule on the atmospheric reaction between nitrous acid (HONO) and the hydroxyl radical (OH) has been investigated in this work. The CCSD(T)/aug-cc-pVTZ//ωB97X-D/aug-cc-pVTZ level of theory was used to obtain all stationary points on the energy surface and to determine the rate constants. Due to the two HONO configurations (cis- and trans-) existing in the atmosphere, the water-free HONO + OH reaction has two major elementary channels, both based on the HONO hydrogen abstraction by the hydroxyl radical. The products, NO2 + H2O, are formed with substantial energy gain, but separated by relatively low energy barriers. In the presence of water, the reaction becomes more complex, proceeding through twelve different channels, each starting with the formation of a binary complex between water and one reactant followed by its interaction with the third species. The products are similar to those of the water-free reaction. At 298 K, the rate constants of water-free cis-HONO + OH and trans-HONO + OH reactions are 1.34×10-12 and 1.00×10-15 cm3 molecule-1 s-1, respectively. The calculated rate constants for H2O-complexed HONO or OH increase by one to two orders of magnitude, but weighted for their relative abundances, the H2O-complexed fractions of the reactants in the atmosphere are so small that the effect of H2O on the overall reaction rate is minor.
Nitrous acid (HONO) serves as a key source of hydroxyl radicals and plays important roles in atmospheric photochemistry. In this study, gaseous HONO and related species and parameters were measured in autumn of 2016 at a marine background site on Tuoji Island in eastern Bohai Sea, China. The HONO concentration in marine boundary layer (MBL) was on average 0.20 ± 0.20 ppbv (average ± standard deviation) with a maximum hourly value of 1.38 ppbv. It exhibited distinct diurnal variations featuring with elevated concentrations in the late night and frequent concentration peaks in the early afternoon. During nighttime, the HONO was produced at a fast rate with the NO 2 -HONO conversion rate ranging from 0.006 to 0.036 h ⁻¹ . The fast HONO production and the strong dependence of temperature implied the enhancement of nocturnal HONO formation caused by air-sea interactions at high temperature. At daytime, HONO concentration peaks were frequently observed between 13:00–15:00. The observed daytime HONO concentrations were substantially higher than those predicted in the photostationary state in conditions of intensive solar radiation and high temperature. Strong or good correlations between the missing HONO production rate and temperature or photolysis frequency suggest a potential source of HONO from the photochemical conversions of nitrogen-containing compounds in sea microlayer. The source intensity strengthened quickly when the temperature was high. The abnormally high concentrations of daytime HONO contributed a considerable fraction to the primary OH radicals in the MBL.
Nitric acid, a well-known sink of NOx gases in the atmosphere, has been found to be photoactive while adsorbed on tropospheric particles. When adsorbed onto semiconductive metal oxides, nitrate’s photochemical degradation can be interpreted as a photocatalytic process. Yet, the photolysis of nitrate ions on the surface of aerosols can also be initiated by changes in the symmetry of the ion upon adsorption. In this study, we use quantum chemistry to model the vibrational spectra of adsorbed nitrate on TiO2, a semiconductor component of atmospheric aerosols, and determine the kinetics of the heterogeneous photochemical degradation of nitrate under simulated solar light. Frequencies and geometry calculations suggest that the symmetry of chemisorbed nitrate ion depends strongly on coadsorbed water, with water changing the reactive surface of TiO2. Upon irradiation, surface nitrate undergoes photolysis to yield nitrogen-containing gaseous products including NO2, NO, HONO and N2O, in proportions that depend on relative humidity (RH). In addition, the heterogeneous photochemistry rate constant decreases an order of magnitude, from (5.7±0.1)×10⁻⁴ s⁻¹ on a dry surface to (7.1±0.8)×10⁻⁵ s⁻¹ when nitrate is coadsorbed with water above monolayer coverage. Little is known about the roles of coadsorbed water on the heterogeneous photochemistry of nitrates on TiO2, along with its impact on the chemical balance of the atmosphere. This work discusses the roles of water in the photolysis of surface nitrates on TiO2 and the concomitant renoxification of the atmosphere.
The sources and distribution of tropospheric nitrous acid (HONO) were examined using airborne measurements over the Southeast U.S. during the SENEX experiment in June and July 2013. HONO was measured once per second using a chemical ionization mass spectrometer on the NOAA WP-3D aircraft to assess sources that affect HONO abundance throughout the planetary boundary layer (PBL). The aircraft flew at altitudes between 0.12 and 6.4 km above ground level on 18 research flights that were conducted both day and night, sampling emissions from urban and power plant sources that were transported in the PBL. At night, HONO mixing ratios were greatest in plumes from agricultural burning, where they exceeded 4 ppbv and accounted for 2 − 14% of the reactive nitrogen emitted by the fires. The HONO to carbon monoxide ratio in these plumes from flaming stage fires ranged from 0.13 − 0.52%. Direct HONO emissions from coal-fired power plants were quantified using measurements at night, when HONO loss by photolysis was absent. These direct emissions were often correlated with total reactive nitrogen with enhancement ratios that ranged from 0 − 0.4%. HONO enhancements in power plant plumes measured during the day were compared with a Lagrangian plume dispersion model, showing that HONO produced solely from the gas phase reaction of OH with NO explained the observations. Outside of recently emitted plumes from known combustion sources, HONO mixing ratios measured several hundred m above ground level were indistinguishable from zero within the 15 pptv measurement uncertainty. The results reported here do not support the existence of a ubiquitous unknown HONO source that produces significant HONO concentrations in the lower troposphere.
Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed. A 'renoxification' process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth's surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.
Chemiluminescence instruments for measuring NO in situ are known to have the high sensitivity that is needed to detect this important tropospheric molecule in remote regions of the earth's atmosphere. In this paper a combination NO and NOâ detector is described that has a high sensitivity for both NO and NOâ, along with a high specificity for NOâ detection.
We present a sensitive, compact detector that measures total reactive nitrogen (NOy), as well as NO2, NO, and O3. In all channels, NO2 is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 405 nm. Ambient O3 is converted to NO2 in excess NO for the O3 measurement channel. Likewise, ambient NO is converted to NO2 in excess O3. Ambient NOy is thermally dissociated at ∼700° to form NO2 or NO in a heated quartz inlet. Any NO present in ambient air or formed from thermal dissociation of other reactive nitrogen compounds is converted to NO2 in excess O3 after the thermal converter. We measured thermal dissociation profiles for six of the major NOy components, and compared ambient measurements with other instruments during field campaigns in Utah and Alabama. Alabama measurements were made in a rural location with high biogenic emissions, and Utah measurements were made in the wintertime in unusual conditions that form high ozone from emissions related to oil and gas production. The NOy comparison in Alabama, to an accepted standard measurement method (a molybdenum catalytic converter/chemiluminescence instrument), agreed to within 12%, which we define as an upper limit to the accuracy of the NOy channel. The 1σ precision is <30 pptv at 1 second and <4 pptv at 1 minute time resolution for all measurement channels. The accuracy is 3% for the NO2 and O3 channels, and 5% for the NO channel. The precision and accuracy of this instrument make it a versatile alternative to standard chemiluminescence-based NOy instruments.
A compact blue diode laser catalytic thermal dissociation cavity
ring-down spectrometer (cTD-CRDS) to detect vapors of
nitroaromatic explosives is described. The instrument uses heated
platinum(IV) oxide catalyst to convert nitroaromatic compounds to
NO2, which is detected at 405 nm. Using the relatively
volatile nitrobenzene as a test compound, we show by Fourier Transform
Infrared Spectroscopy (FTIR) in off-line experiments that nitroaromatics
can be quantitatively converted to NO2. The cTD-CRDS
detection limit was 0.3 parts-per-billion by volume (ppbv) and
sufficiently low to allow the detection of a room temperature sample of
2,4,6-trinitrotoluene (TNT) without sample preconcentration.
Numerous studies infer the existence of an “unknown” daytime HONO source based on the assumption that HONO is at photostationary state (PSS). Secondary HONO production rate as high as 1.1 ppb hr−1 can be estimated from this approach, based on measurements made during the Study of Houston Atmospheric Radical Precursors campaign in May of 2009. We argue, however, that the PSS assumption might not have been valid because the transport time from nearby NOx emission sources to the measurement site was likely less than the time required for HONO in vehicle exhaust to reach PSS. Using a chemical box model, we demonstrate that there is initially net HONO formation as high levels of NO in exhaust react with ambient OH. Net production is followed by a period of net HONO loss dominated by photolysis that is sustained for several minutes to hours depending on time of day. The presence of relatively fresh exhaust in sampled air can partially, if not fully, account for the observed measurement PSS discrepancy. We also show that a large fraction of the observed nighttime increase in HONO/NOx ratio is explained by NO2 oxidation. These results do not rule out the existence of an unexplained secondary HONO source but suggest that great care must be exercised when applying the PSS method to quantify its strength.
[1] The nocturnal conversion of dinitrogen pentoxide (N2O5) to nitryl chloride (ClNO2) on chloride-containing aerosol can be a regionally important NOx (= NO + NO2) recycling and halogen activation pathway that affects oxidant photochemistry the following day. Here we present a comprehensive measurement data set acquired at Pasadena, California, during the CalNex-LA campaign 2010 that included measurements of odd nitrogen and its major components (NOy = NOx + NO3 + 2N2O5 + ClNO2 + HNO3 + HONO + peroxyacyl, alkyl, and aerosol nitrates) and aerosol size distribution and composition. Nitryl chloride was present during every night of the study (median mixing ratio at sunrise 800 pptv) and was usually a more significant nocturnal NOx and odd oxygen (Ox = O3 + NO2 + 3N2O5 + ClNO2) reservoir species than N2O5 (whose concentrations were calculated from its equilibrium with NO2 and NO3). At sunrise, ClNO2 accounted for 21% of NOz (=NOy − NOx), 4% of NOy, and 2.5% of Ox, respectively (median values). Kinetic parameters for the N2O5 to ClNO2 conversion were estimated by relating ClNO2 concentrations to their time-integrated heterogeneous production from N2O5 and were highly variable between nights. Production of ClNO2 required conversion of N2O5 on submicron aerosol with average yield (φ) and N2O5 reactive uptake probability (γ) of γφ = 0.008 (maximum 0.04), scaled with submicron aerosol chloride content, and was suppressed by aerosol organic matter and liquid water content. Not all of the observed variability of ClNO2 production efficiency could be rationalized using current literature parameterizations.
We describe a new laboratory-based method for in situ detection of nitrous acid (HONO) using a combination of thermal dissociation (TD) and chemiluminescent (CL) detection of nitric oxide. A prototype was built using a commercial NO sensor. Laboratory tests for possible chemical interferences show that measurements are affected in predictable ways by NO2NO2, peroxy nitrates, alkyl nitrates, HNO3HNO3, O3O3 and H2OH2O.
A sensitive, small detector was developed for atmospheric NO2 and NOx concentration measurements. NO2 is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 404 nm. The sum of NO and NO2 (=NOx) is simultaneously measured in a second cavity by quantitative conversion of ambient NO to NO2 in excess ozone. Interferences due to absorption by other trace gases at 404 nm, such as ozone and water vapor, are either negligible or small and are easily quantified. The limit of detection is 22 pptv (2sigma precision) for NO2 at 1 s time resolution. The conversion efficiency of NO to NO2 is 99% in excess O3. The accuracy of the NO2 measurement is mainly limited by the NO2 absorption cross section to +/-3%. Because of the formation of undetectable higher nitrogen oxides in subsequent reactions of NO2 with ozone in the NOx channel, the (1sigma) accuracy of the NOx measurement is increased to approximately +/-5% depending on the level of NOx. The new instrument was designed to be easily deployed in the field with respect to size, weight and consumables. Measurements were validated against a photolysis/chemiluminescence detector during six days of sampling ambient air with colocated inlets. The data sets for NO2, NO and NOx exhibit high correlation and good agreement within the combined accuracies of both methods. Linear fits for all three species give similar slopes of 0.99 in ambient air.
A new photolytic converter for NO2 measurement is described
and its performance assessed using laboratory, ground-, and
aircraft-based field data. Focusing the output of a 200-W short-arc Hg
lamp into a photolytic cell attains conversion fractions of
NO2 to NO greater than 0.70 in cell residence times of less
than a second. Limiting lamp output to wavelengths greater than 350 nm
by means of optical filters increases specificity for NO2,
affording a peroxyacetyl nitrate conversion fraction of less than 0.006
and negligible conversion of HNO3. Unwanted (artifact) signal
in clean synthetic air is also greatly minimized through the use of
optical filters. Fast instrument response is achieved by minimizing
NO2 inlet line and photolysis cell residence times. NO and
NO2 sample residence times are matched in a multichannel
instrument so that signal from ambient NO may be easily subtracted from
the total signal and ambient NO2 calculated by difference at
high time resolution. Induced change in the ambient ratio of NO to
NO2, due to reaction of ozone and other oxidants with NO
during sampling, is minimized in the new design. This configuration
permits simple and accurate retrieval of NO2 concentrations
in aircraft transects of power plant plumes, where ambient NO
concentrations can change over several orders of magnitude in seconds.
At lower concentrations found in the planetary boundary layer, agreement
between calculated and observed NO2 is within +/-(40pptv+7%)
for a 10-s average. The new converter consumes less power, is more
efficient, and is less expensive to operate than previous photolysis
designs.
An intercomparison was made near Niwot Ridge, Colorado, of three different instruments for measuring NO2 at low concentrations in ambient air: (1) the photolysis/chemiluminescence (PC) instrument, (2) the tunable diode laser absorption spectrometer (TDLAS), and (3) the Luminox instrument. Calbrated mixtures of NO2 in air and NO2 with possible interferants (HNO3, peroxy-acetyl nitrate (PAN), H2O2, n-propyl nitrate, and O3) were provided in simultaneous tests. In addition, ambient air measurements were made using the three instruments. Blind procedures were followed in preparing all results. Several conclusions were reached concerning the performance of these instruments during this intercomparison: (1) For NO2 levels above 2 parts per billion by volume (ppbv), similar results were obtained for all instruments; (2) Below 2 ppbv, the expected interferences from ozone and PAN influenced the NO2 measurements made using the Luminox instruments. Those interferences were sufficiently consistent that they could be corrected for by using the measured values of O3 and PAN down to about 0.3 ppbv NO2; (3) The ozone interference on the Luminox instruments was removed by an ozone scrubber placed in the sampled air stream of the Luminox instrument. However, this did not remove PAN. In addition, the scrubber appeared to remove about 50% of the NO2 as well; (4) Although no interferences were identified for the TDLAS technique, care must be taken in the data analysis near (or below) the detection limit for the instrument. At these levels the data reduction program provided with the TDLAS will tend to find background noise that is correlated with the reference NO2 spectrum and calculate levels of NO2 that are too high; (5) No interferences or artifacts were found for the final results reported by the PC technique. However, these results for ambient measurements were corrected by subtracting an artifact that averaged 5 parts per trillion by volume (pptv) and by calculating a correction for the effect of ambient ozone. This latter correction averaged 1.0% in magnitude.
Pegmatite quartz from different occurrences in Norway and Namibia was investigated by a combination of ICP-MS, Electron Spin Resonance (ESR), Capillary Ion Analysis (CIA) and Gas Chromatography (GC) to quantify trace elements in very low concentrations and to determine their position in the quartz structure.The studied quartz samples show similar geochemical characteristics with low contents of most trace elements. Remarkable are the elevated concentrations of Al (36–636 ppm), Ti (1.6–25.2 ppm), Ge (1.0–7.1 ppm), Na (5.2 to >50 ppm), K (1.6 to >100 ppm) and Li (2.1–165.6 ppm). These elements are preferentially incorporated into the quartz lattice on substitutional (Al, Ti, Ge) and interstitial (Li, Na, K) positions. Li+ was found to be the main charge compensating ion for Al, Ge and Ti, whereas some ppm of Na and K may also be hosted by fluid inclusions. Ti may be incorporated as substitutional ion for Si or bound on mineral microinclusions (rutile). The results of the ESR measurements show that there may be a redistribution of alkali ions during irradiation. The diamagnetic [AlO4/M+]0 center transforms into the paramagnetic [AlO4]0 center, whilst the compensating ions diffuse away and may be captured by the diamagnetic precursor centers of [GeO4]0 and [TiO4]0 to form paramagnetic centers ([TiO4/Li+]0, [GeO4/Li+]0).In general, fluid inclusions in pegmatite quartz can be classified as H2O-CO2-NaCl type inclusions with water as the predominant volatile. Among the main elements hosted by fluid inclusions in quartz are Na, K, NH4, Ca, Mg and the anionic complexes Cl−, NO3−, HCO3− and SO42−. Gas analysis of trapped fluids shows volatile components in the following order of abundance: H2O > CO2 > N2(+) ≥ CH4 > COS > C2 and C3 hydrocarbons. Additionally, traces of Co, Ni, Zn, Pb, and Cu were detected by CIA in fluid inclusions of some samples. There are indications that the REE and Rb are also bound in fluid inclusions, however, the concentrations of these elements are too low to be measured by CIA. Assuming that the REE preferentially occur in fluid inclusions, the typical chondrite normalized REE distribution patterns with tetrad effects and a distinct negative Eu anomaly would reflect the composition of the mineralizing fluid.For a number of elements, especially those with extremely low concentrations, the “type” of incorporation in quartz could not directly be determined. We conclude that these ions either are too large to substitute for the small Si4+ ion or they are not soluble in the mineralizing fluids to be hosted by fluid inclusions. Some of these elements, which are concentrated in the specific mineralization of certain pegmatites, are not present in elevated concentrations in the paragenetic pegmatite quartz itself. This was observed, for instance, for Be, Cs and Rb in the Li (Be-Cs-Rb) pegmatites of Rubicon or for Nb and Ta for Nb-Ta bearing pegmatites from Norway. It may be concluded that the concentrations of these trace elements in quartz do not reflect the mineralization and that these elements thus, cannot be used as petrogenetic indicator.
A method to photochemically generate stable outputs of peroxyacetic, peroxypropionic, or peroxyisobutanoic nitric anhydride (PAN, PPN, or PiBN) in dilute gas streams is described. The PANs are generated by photolysis of excess acetone, diethyl ketone, or diisopropyl ketone in the presence of oxygen and either nitric oxide or nitrogen dioxide. The source output was characterized using a commercial NOy monitor, an in-house constructed thermal dissociation cavity ring-down spectrometer (TD-CRDS) equipped with a heated inlet for quantification of NO2, total peroxyacyl nitrates ([summation operator]PAN), and total alkyl nitrates ([summation operator]AN), and a thermal dissociation chemical ionization mass spectrometer (TD-CIMS) operated with iodide reagent ion. The TD-CIMS was calibrated (against TD-CRDS) using diffusion sources containing synthetic PAN standards. Response factors of 21, 19, and 5 counts per pptv, normalized to 1 million counts of iodide reagent ion, were found for PAN (monitored at m/z 59), PPN (m/z 73), and PiBN (m/z 87), respectively. The photo source was found to generate the three PANs in high yield. CIMS response factors derived using the photo source and TD-CRDS were identical to those derived from synthetic standards for PAN and PPN; hence, the photochemical PAN and PPN sources may be used to calibrate TD-CIMS (against TD-CRDS). For PiBN, the response factor derived using the photo source was 60% larger than that derived using the synthetic standard, limiting its use to deliver a calibrated stream of PiBN.
A new technique is described for the real-time, quantitative conversion of several XNOâ compounds to NO, where X denotes a radical group. The technique utilizes the reduction of these compounds by CO at heated gold catalyst to yield NO. The NO product was detected by a sensitive chemiluminescence detector. The technique was demonstrated to convert NOâ, HNOâ, and n-propyl nitrate quantitatively to NO for mixing ratios in the range of 1-120 ppbv in synthetic air at atmospheric pressure. Conversion results were obtained for CO mixing ratios in the range of 0.2-1000 ppmv and for temperatures in the range of 20-600 °C. The Gormley-Kennedy equation was used to analyze the results for consistency with known diffusion coefficients. The CO reduction process is discussed in terms of the thermochemistry of the net conversion reactions and catalytic properties of the gold surface. In addition, the potential of this technique for atmospheric measurements is noted. 8 figures, 3 tables.
The nitrate radical, NO(3), is photochemically unstable but is one of the most chemically important species in the nocturnal atmosphere. It is accompanied by the presence of dinitrogen pentoxide, N(2)O(5), with which it is in rapid thermal equilibrium at lower tropospheric temperatures. These two nitrogen oxides participate in numerous atmospheric chemical systems. NO(3) reactions with VOCs and organic sulphur species are important, or in some cases even dominant, oxidation pathways, impacting the budgets of these species and their degradation products. These oxidative reactions, together with the ozonolysis of alkenes, are also responsible for the nighttime production and cycling of OH and peroxy (HO(2) + RO(2)) radicals. In addition, reactions of NO(3) with biogenic hydrocarbons are particularly efficient and are responsible for the production of organic nitrates and secondary organic aerosol. Heterogeneous chemistry of N(2)O(5) is one of the major processes responsible for the atmospheric removal of nitrogen oxides as well as the cycling of halogen species though the production of nitryl chloride, ClNO(2). The chemistry of NO(3) and N(2)O(5) is also important to the regulation of both tropospheric and stratospheric ozone. Here we review the essential features of this atmospheric chemistry, along with field observations of NO(3), N(2)O(5), nighttime peroxy and OH radicals, and related compounds. This review builds on existing reviews of this chemistry, and encompasses field, laboratory and modelling work spanning more than three decades.
Doping of TiO2 with altervalent cations, Fe3+, Cr3+, Co2+ and Mg2+ leads to a red shift in the onset of absorption. The role of the dopant ions is primarily to improve the charge separation by means of a permanent electric field. Doping of TiO2 also influences the adsorption of reagent species and the resulting photoactivity is a consequence of these two factors. An optimum level of dopant ions is beneficial for the photocatalytic activity of TiO2.
The absorption cross-sections of NO2 at atmospheric temperatures (223–293 K) and pressures (100 and 1000 mbar) were measured in the 250–800 nm (12500–40000 cm−1) region using Fourier-transform spectroscopy, at spectral resolutions of 0.5 cm−1 above 435 nm and 1.0 cm−1 below 435 nm (corresponding to about 8 and 16 pm at this wavelength). The wavenumber accuracy of the new cross-sections is better than 0.1 cm−1 (about 0.5 pm at 250 nm and about 6.4 pm at 800 nm), validated by recording of I2 absorption spectra in the visible using the same experimental set-up (light source, beam splitter, interferometer optics). The NO2 absorption spectra were recorded at five different sample temperatures between 223 and 293 K, and at each temperature at two total pressures (100 and 1000 mbar) using pure N2 as buffer gas. Despite the weakness of this effect compared to the density of the NO2 absorption structures, pressure-broadening was clearly observed at all temperatures. The pressure-broadening was partially modeled using a convolution of the low-pressure NO2 absorption spectra with a Lorentzian lineshape. The pressure-broadening coefficient increases significantly with decreasing temperature, as already observed in the mid- and near-infrared vibration–rotation spectra of NO2. This effect is of importance for high-resolution spectroscopy of the earth’s atmosphere in the UV–visible region.
A photolytic converter of nitrogen dioxide (NO(2)) to nitric oxide (NO) using light-emitting diodes (LEDs) has been designed to measure NO(2) in the troposphere. The typical electrical power consumption of the photolytic converter (PLC) is only 44 W. The maximum conversion efficiency of NO(2) to NO of the photolytic converter is around 90%, which is higher than that of metal halides or high-pressure Xe arc lamps (up to ∼70%). The conversion efficiency of the PLC was almost constant for at least 2.5 months. The conversion efficiency of peroxyacetyl nitrate by the LED-PLC was measured to be 2.6 ± 0.1% (1σ). The interference of HONO using the PLC was experimentally estimated to be less than 3%, which is within the uncertainty of the instrument. An intercomparison of NO(2) measurements between the PLC-CLD and the laser-induced fluorescence (LIF) technique was conducted, and the NO(2) concentrations measured by the PLC-CLD method were in agreement with those obtained by the LIF technique, within the uncertainties of the instruments. Continuous observations were made on Fukue Island, a remote area. These results demonstrate the performance of the PLC for continuous ambient measurements.
Peroxycarboxylic nitric anhydrides (PANs) have long been recognized as important trace gas constituents of the troposphere. Here, we describe a blue diode laser thermal dissociation cavity ring-down spectrometer for rapid and absolute measurements of total peroxyacyl nitrate (SigmaPAN) abundances at ambient concentration levels. The PANs are thermally dissociated and detected as NO2, whose mixing ratios are quantified by optical absorption at 405 nm relative to a reference channel kept at ambient temperature. The effective NO2 absorption cross-section at the diode laser emission wavelength was measured to be 6.1 x 10(-19) cm2 molecule(-1), in excellent agreement with a prediction based on a projection of a high-resolution literature absorption spectrum onto the laser line width. The performance, i.e., accuracy and precision of measurement and matrix effects, of the new 405 nm thermal dissociation cavity ring-down spectrometer was evaluated and compared to that of a 532 nm thermal dissociation cavity ring-down spectrometer using laboratory-generated air samples. The new 405 nm spectrometer was considerably more sensitive and compact than the previously constructed version. The key advantage of laser thermal dissociation cavity ring-down spectroscopy is that the measurement can be considered absolute and does not need to rely on external calibration.