Publications (12)0 Total impact
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Article: Detection of HO<sub>2</sub> by laser-induced fluorescence: calibration and interferences from RO<sub>2</sub> radicals
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ABSTRACT: HO2 concentration measurements are widely accomplished by chemical conversion of HO2 to OH including reaction with NO and subsequent detection of OH by laser-induced fluorescence. RO2 radicals can be converted to OH via a similar radical reaction sequence including reaction with NO, so that they are potential interferences for HO2 measurements. Here, the conversion efficiency of various RO2 species to HO2 is investigated. Experiments were conducted with a radical source that produces OH and HO2 by water photolysis at 185 nm, which is frequently used for calibration of LIF instruments. The ratio of HO2 and the sum of OH and HO2 concentrations provided by the radical source was investigated and was found to be 0.50 ± 0.02. RO2 radicals are produced by the reaction of various organic compounds with OH in the radical source. Interferences via chemical conversion from RO2 radicals produced by the reaction of OH with alkanes (H-atom abstraction) are negligible consistent with measurements in the past. However, RO2 radicals from OH plus alkene- and aromatic-precursors including isoprene (mainly OH-addition) are detected with a relative sensitivity larger than 80% with respect to that for HO2 for the configuration of the instrument with which it was operated during field campaigns. Also RO2 from OH plus methyl vinyl ketone and methacrolein exhibit a relative detection sensitivity of 60%. Thus, previous measurements of HO2 radical concentrations with this instrument were biased in the presence of high RO2 radical concentrations from isoprene, alkenes or aromatics, but were not affected by interferences in clean air, when the OH reactivity was dominated by small alkanes. By reducing the NO concentration and/or the transport time between NO addition and OH detection, interference from these RO2 species are suppressed to values below 20% relative to the HO2 detection sensitivity. The HO2 conversion efficiency is also smaller by a factor of four, but this is still sufficient for atmospheric HO2 concentration measurements for a wide range of conditions.Atmospheric Measurement Techniques Discussions. 01/2011; -
Article: Measurements of gaseous H<sub>2</sub>SO<sub>4</sub> by AP-ID-CIMS during CAREBeijing 2008 Campaign
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ABSTRACT: As part of the 2008 Campaign of Air Quality Research in Beijing and Surrounding Regions (CAREBeijing 2008), measurements of gaseous sulfuric acid (H2SO4) have been conducted at an urban site in Beijing, China from 7 July to 25 September 2008 using atmospheric pressure ion drift – chemical ionization mass spectrometry (AP-ID-CIMS). This represents the first gaseous H2SO4 measurements in China. Diurnal profile of sulfuric acid is strongly dependent on the actinic flux, reaching a daily maximum around noontime and with an hourly average concentration of 5 × 106 molecule cm−3. Simulation of sulfuric acid on the basis of the measured sulfur dioxide concentration, photolysis rates of ozone and nitrogen dioxide, and aerosol surface areas captures the trend of the measured H2SO4 diurnal variation within the uncertainties, indicating that photochemical production and condensation onto preexisting particle surface dominate the observed diurnal H2SO4 profile. The frequency of the peak H2SO4 concentration exceeding 5 × 106 molecule cm−3 increases by 16% during the period of the summer Olympic Games (8–23 August 2008), because of the implementation of air quality control regulations. Using a multivariate statistical method, the critical nucleus during nucleation events is inferred, containing two H2SO4 molecules (R2 = 0.85). When neither nucleation nor precipitation occurs, the condensation rate of H2SO4 correlates with the daytime sulfate mass concentration of the Aitken mode, but not with that of the accumulation mode aerosols.Atmospheric Chemistry and Physics Discussions. 01/2011; -
Article: Intercomparison of measurements of NO<sub>2</sub> concentrations in the atmosphere simulation chamber SAPHIR during the NO3Comp campaign
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ABSTRACT: NO<sub>2</sub> concentrations were measured by various instruments during the NO3Comp campaign at the atmosphere simulation chamber SAPHIR at Forschungszentrum Jülich, Germany, in June 2007. Analytical methods included photolytic conversion with chemiluminescence (PC-CLD), broadband cavity ring-down spectroscopy (BBCRDS), pulsed cavity ring-down spectroscopy (CRDS), incoherent broadband cavity-enhanced absorption spectroscopy (IBB-CEAS), and laser-induced fluorescence (LIF). All broadband absorption spectrometers were optimized for the detection of the main target species of the campaign, NO<sub>3</sub>, but were also capable of detecting NO<sub>2</sub> simultaneously with reduced sensitivity. NO<sub>2</sub> mixing ratios in the chamber were within a range characteristic of polluted, urban conditions, with a maximum mixing ratio of approximately 75 ppbv. The overall agreement between measurements of all instruments was excellent. Linear fits of the combined data sets resulted in slopes that differ from unity only within the stated uncertainty of each instrument. Possible interferences from species such as water vapor and ozone were negligible under the experimental conditions.Atmospheric Measurement Techniques. 01/2010; -
Article: Relationship between the NO<sub>2</sub> photolysis frequency and the solar global irradiance
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ABSTRACT: Representative values of the atmospheric NO<sub>2</sub> photolysis frequency j (NO<sub>2</sub>) are required for the adequate calculation and interpretation of NO and NO<sub>2</sub> concentrations and exchange fluxes near the surface. Direct measurements of j (NO<sub>2</sub>) at ground level are often not available in field studies. In most cases, modeling approaches involving complex radiative transfer calculations are used to estimate j (NO<sub>2</sub>) and other photolysis frequencies for air chemistry studies. However, important input parameters for accurate modeling are often missing, most importantly with regard to the radiative effects of clouds. On the other hand, solar global irradiance ("global radiation", G ) is nowadays measured as a standard parameter in most field experiments and in many meteorological observation networks around the world. Previous studies mainly reported linear relationships between j (NO<sub>2</sub>) and G . We have measured j (NO<sub>2</sub>) using spectro- or filter radiometers and G using pyranometers side-by-side at several field sites. Our results cover a solar zenith angle range of 0–90°, and are based on nine field campaigns in temperate, subtropical and tropical environments during the period 1994–2008. We show that a second-order polynomial function (intercept = 0): j (NO<sub>2</sub>)=(1+α)× ( B <sub>1</sub>× G + B <sub>2</sub>× G <sup>2</sup>), with α defined as the site-dependent UV-A surface albedo and the polynomial coefficients: B <sub>1</sub>=(1.47± 0.03)×10<sup>-5</sup> W<sup>−1</sup> m<sup>2</sup> s<sup>−1</sup> and B <sub>2</sub>=(-4.84±0.31)×10<sup>-9</sup> W<sup>−2</sup> m<sup>4</sup> s<sup>−1</sup> can be used to estimate ground-level j (NO<sub>2</sub>) directly from G , independent of solar zenith angle under all atmospheric conditions. The absolute j (NO<sub>2</sub>) residual of the empirical function is ±6×10<sup>-4</sup> s<sup>−1</sup>(2σ). The relationship is valid for sites below 800 m a.s.l. and with low surface albedo (α<0.2). It is not valid in high mountains, above snow or ice and sandy or dry soil surfaces.Atmospheric Measurement Techniques. 01/2009; -
Article: Relationship between the NO<sub>2</sub> photolysis frequency and the solar global irradiance
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ABSTRACT: Representative values of the atmospheric NO<sub>2</sub> photolysis frequency, ( j (NO<sub>2</sub>)), are required for the adequate calculation and interpretation of NO and NO<sub>2</sub> concentrations and exchange fluxes near the surface. Direct measurements of j (NO<sub>2</sub>) at ground level are often not available in field studies. In most cases, modeling approaches involving complex radiative transfer calculations are used to estimate j (NO<sub>2</sub>) and other photolysis frequencies for air chemistry studies. However, important input parameters for accurate modeling are often missing, most importantly with regard to the radiative effects of clouds. On the other hand, solar global irradiance ("global radiation", G ) is nowadays measured as a standard parameter in most field experiments and in many meteorological observation networks around the world. A linear relationship between j (NO<sub>2</sub>) and G was reported in previous studies and has been used to estimate j (NO<sub>2</sub>) from G in the past 30 years. We have measured j (NO<sub>2</sub>) using spectro- or filter radiometers and G using pyranometers side-by-side at several field sites. Our results cover a solar zenith angle range of 0–90°, and are based on nine field campaigns in temperate, subtropical and tropical environments during the period 1994–2008. We show that a second-order polynomial function (intercept=0): j (NO<sub>2</sub>)=(1+α)×( B <sub>1</sub>× G + B <sub>2</sub>×G<sup>2</sup>), with α defined as the site-dependent UV-A surface albedo and the polynomial coefficients (including uncertainty ranges): B <sub>1</sub>=(1.47±0.03)×10<sup>−5</sup> W<sup>−1</sup> m<sup>2</sup> s<sup>−1</sup> and B <sub>2</sub>=(−4.84±0.31)×10<sup>−9</sup> W<sup>−2</sup> m<sup>4</sup> s<sup>−1</sup> can be used to estimate ground-level j (NO<sub>2</sub>) directly from G , independent of solar zenith angle under all atmospheric conditions. The absolute j (NO<sub>2</sub>)↓ residual of the empirical function is ±6×10<sup>−4</sup> s<sup>−1</sup> (95.45% confidence level). The relationship is valid for sites below 800 m a.s.l. and under low background albedo conditions. It is not valid in alpine regions, above snow or ice and sandy or dry soil surfaces. Our function can be applied to estimate chemical life times of the NO<sub>2</sub> molecule with respect to photolysis, and is useful for surface-atmosphere exchange and photochemistry studies close to the ground, e.g., above fields with short vegetation and above forest canopies.Atmospheric Measurement Techniques Discussions. 01/2009; -
Article: Isotope effect in the formation of H<sub>2</sub> from H<sub>2</sub>CO studied at the atmospheric simulation chamber SAPHIR
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ABSTRACT: Formaldehyde of known, near-natural isotopic composition was photolyzed in a large photochemical reactor under ambient conditions. The isotopic composition of the product H<sub>2</sub> was used to determine the isotope effects in formaldehyde photolysis. The experiments are sensitive to the molecular photolysis channel, and the radical channel has only a second order effect and can thus not be derived with high precision. The molecular channel kinetic isotope effect (KIE<sub>mol</sub>), the ratio of photolysis frequencies j (HCHO→CO+H<sub>2</sub>)/ j (HCDO→CO+HD) under tropospheric conditions is determined to be KIE<sub>mol</sub>=1.63±0.03. Combining this result with the total KIE from a recent relative rate experiment, it is likely that KIE<sub>mol</sub> and KIE<sub>rad</sub> are not as different as described previously in the literature.Atmospheric Chemistry and Physics Discussions. 01/2009; -
Article: Atmospheric OH reactivities in the Pearl River Delta – China in summer 2006: measurement and model results
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ABSTRACT: Total atmospheric OH reactivities (kOH) have been measured as reciprocal OH lifetimes by a newly developed instrument at a rural site in the densely populated Pearl River Delta (PRD) in Southern China in summer 2006. The deployed technique, LP-LIF, uses laser flash photolysis (LP) for artifical OH generation and laser-induced fluorescence (LIF) to measure the time-dependent OH decay in samples of ambient air. The reactivities observed at PRD covered a range from 10 s−1 to 120 s−1, indicating a large load of chemical reactants. On average, kOH exhibited a pronounced diurnal profile with a mean maximum value of 50 s−1 at daybreak and a mean minimum value of 20 s−1 at noon. The reactivity was dominated by anthropogenic pollutants (e.g., CO, NOx, light alkenes and aromatic hydrocarbons) at night, while it was strongly influenced by local, biogenic emissions of isoprene at day. The comparison of reactivities calculated from measured trace gases with measured kOH reveals a missing reactivity of about a factor of 2 at day and night. Box model calculations initialized by measured parameters reproduce the observed OH reactivity well and suggest that the missing reactivity is contributed by unmeasured, secondary chemistry products (mainly aldehydes and ketones) that were photochemically formed by hydrocarbon oxidation. Overall, kOH was dominated by organic compounds, which had a maxium contribution of 85% in the afternoon. The paper demonstrates the usefulness of direct reactivity measurements and emphasizes the need for direct measurements of oxygenated organic compounds in atmospheric chemistry studies.Atmospheric Chemistry and Physics Discussions. 01/2009; -
Article: Photolysis frequency measurement techniques: results of a comparison within the ACCENT project
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ABSTRACT: An intercomparison of different radiometric techniques measuring atmospheric photolysis frequencies j(NO2), j(HCHO) and j(O1D) was carried out in a two-week field campaign in June 2005 at Jülich, Germany. Three double-monochromator based spectroradiometers (DM-SR), three single-monochromator based spectroradiometers with diode-array detectors (SM-SR) and seventeen filter radiometers (FR) (ten j(NO2)-FR, seven j(O1D)-FR) took part in this comparison. For j(NO2), all spectroradiometer results agreed within ±3%. For j(HCHO), agreement was slightly poorer between −8% and +4% of the DM-SR reference result. For the SM-SR deviations were explained by poorer spectral resolutions and lower accuracies caused by decreased sensitivities of the photodiode arrays in a wavelength range below 350 nm. For j(O1D), the results were more complex within +8% and −4% with increasing deviations towards larger solar zenith angles for the SM-SR. The direction and the magnitude of the deviations were dependent on the technique of background determination. All j(NO2)-FR showed good linearity with single calibration factors being sufficient to convert from output voltages to j(NO2). Measurements were feasible until sunset and comparison with previous calibrations showed good long-term stability. For the j(O1D)-FR, conversion from output voltages to j(O1D) needed calibration factors and correction functions considering the influences of total ozone column and altitude of the sun. All instruments showed good linearity at photolysis frequencies exceeding about 10% of maximum values. At larger solar zenith angles, the agreement was non-uniform with deviations explainable by insufficient correction functions. Comparison with previous calibrations for some j(O1D)-FR indicated drifts of calibration factors.Atmospheric Chemistry and Physics Discussions. 01/2008; -
Article: Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol
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ABSTRACT: The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO2) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO2 is converted on the humic acid aerosol into nitrous acid (HONO), which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: In the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradiation with visible light. Under simulated atmospheric conditions with respect to the actinic flux, relative humidity and NO2-concentration, reactive uptake coefficients γrxn for the NO2→HONO conversion on the aerosol between γrxn −7 (in the dark) and γrxn=6×10−6 were observed. The observed uptake coefficients decreased with increasing NO2-concentration in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity (60% RH). The measured uptake coefficients for the NO2→HONO conversion are too low to explain the HONO-formation rates observed near the ground in rural and urban environments by the conversion of NO2→HONO on organic aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that the processes leading to HONO formation on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles.Atmospheric Chemistry and Physics. 01/2007; -
Article: Characterisation of the photolytic HONO-source in the atmosphere simulation chamber SAPHIR
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ABSTRACT: HONO formation has been proposed as an important OH radical source in simulation chambers for more than two decades. Besides the heterogeneous HONO formation by the dark reaction of NO<sub>2</sub> and adsorbed water, a photolytic source has been proposed to explain the elevated reactivity in simulation chamber experiments. However, the mechanism of the photolytic process is not well understood so far. As expected, production of HONO and NO<sub>x</sub> was also observed inside the new atmospheric simulation chamber SAPHIR under solar irradiation. This photolytic HONO and NO<sub>x</sub> formation was studied with a sensitive HONO instrument under reproducible controlled conditions at atmospheric concentrations of other trace gases. It is shown that the photolytic HONO source in the SAPHIR chamber is not caused by NO<sub>2</sub> reactions and that it is the only direct NO<sub>y</sub> source under illuminated conditions. In addition, the photolysis of nitrate which was recently postulated for the observed photolytic HONO formation on snow, ground, and glass surfaces, can be excluded in the chamber. A photolytic HONO source at the surface of the chamber is proposed which is strongly dependent on humidity, on light intensity, and on temperature. An empirical function describes these dependencies and reproduces the observed HONO formation rates to within 10%. It is shown that the photolysis of HONO represents the dominant radical source in the SAPHIR chamber for typical tropospheric O<sub>3</sub>/H<sub>2</sub>O concentrations. For these conditions, the HONO concentrations inside SAPHIR are similar to recent observations in ambient air.Atmospheric Chemistry and Physics. 01/2005; -
Article: Model-aided radiometric determination of photolysis frequencies in a sunlit atmosphere simulation chamber
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ABSTRACT: In this work diurnal and seasonal variations of mean photolysis frequencies for the atmosphere simulation chamber SAPHIR at Forschungszentrum Jülich are calculated. SAPHIR has a complex construction with UV permeable teflon walls allowing natural sunlight to enter the reactor volume. The calculations are based on external measurements of solar spectral actinic flux and a model considering the time-dependent impact of shadows from construction elements as well as the influence of the teflon walls. Overcast and clear-sky conditions are treated in a consistent way and different assumptions concerning diffuse sky radiance distributions are tested. Radiometric measurements inside the chamber are used for an inspection of model predictions. Under overcast conditions we obtain 74% and 67% of external values for photolysis frequencies j(NO2) (NO2+hν→NO+O(3P)) and j(O1D) (O3+hν→O2+O(1D)), respectively. On a clear sky summer day these values are time-dependent within ranges 0.65–0.86 and 0.60–0.73, for j(NO2) and j(O1D), respectively. A succeeding paper (Bohn et al., 2004) is dealing with an on-road test of the model approach by comparison with photolysis frequencies from chemical actinometry experiments within SAPHIR.Atmospheric Chemistry and Physics Discussions. 01/2004; -
Article: Actinometric measurements of NO<sub>2</sub> photolysis frequencies in the atmosphere simulation chamber SAPHIR
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ABSTRACT: The simulation chamber SAPHIR at Forschungszentrum Jülich has UV permeable teflon walls facilitating atmospheric photochemistry studies under the influence of natural sunlight. Because the internal radiation field is strongly affected by construction elements, we use external, radiometric measurements of spectral actinic flux and a model to calculate mean photolysis frequencies for the chamber volume (Bohn and Zilken, 2004). In this work we determine NO<sub>2</sub> photolysis frequencies j (NO<sub>2</sub>) within SAPHIR using chemical actinometry by injecting NO<sub>2</sub> and observing the chemical composition during illumination under various external conditions. In addition to a photo-stationary approach, a time-dependent method was developed to analyse the data. These measurements had two purposes. Firstly, to check the model predictions with respect to diurnal and seasonal variations in the presence of direct sunlight and secondly to obtain an absolute calibration factor for the combined radiometry-model approach. We obtain a linear correlation between calculated and actinometric j (NO<sub>2</sub>). A calibration factor of 1.34±0.10 is determined, independent of conditions in good approximation. This factor is in line with expectations and can be rationalised by internal reflections within the chamber. Taking into account the uncertainty of the actinometric j (NO<sub>2</sub>), an accuracy of 13% is estimated for the determination of j (NO<sub>2</sub>) in SAPHIR. In separate dark experiments a rate constant of (1.93±0.12)×10<sup>−14</sup> cm<sup>3</sup> s<sup>−1</sup> was determined for the NO+O<sub>3</sub> reaction at 298 K using analytical and numerical methods of data analysis.Atmospheric Chemistry and Physics Discussions. 01/2004;
