No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Assessment of field monitoring of plant fluxes of oxidized nitrogen with two types of detectors
Abstract
Chemiluminescence-based measurements of oxidized nitrogen can be specific to NOx or can also detect other NOy compounds, depending on the equipment. We monitored chamber fluxes of oxidized nitrogen in Scots pine shoots under field conditions, and changed from a NOy measurement to the more NOx-specific measurement. The aims of this study were to evaluate how the NOx measurement system performs in comparison with the NOy measurement system in dynamic field measurements, and whether the new measurements provide information on the composition of the NOy emissions reported in earlier studies. We found that absolute NOx concentrations were slightly more inaccurate than the earlier NOy concentrations but that the new analyzer led to an improvement in the measurement of NOx fluxes. Simultaneous NOy and NOx flux measurements from chambers indicated that the measured NOy fluxes often include compounds other than NOx. We found no clear plant-related NOx emissions.
ResearchGate has not been able to resolve any citations for this publication.
The nitrogen dioxide (NO2) exchange between the atmosphere
and needles of Picea abies L. (Norway Spruce) was studied under
uncontrolled field conditions using a dynamic chamber system. This
system allows measurements of the flux density of the reactive
NO-NO2-O3 triad and additionally of the
non-reactive trace gases CO2 and H2O. For the
NO2 detection a highly NO2 specific blue light
converter was used, which was coupled to chemiluminescence detection of
the photolysis product NO. This NO2 converter excludes known
interferences with other nitrogen compounds, which occur by using more
unspecific NO2 converters. Photo-chemical reactions of NO,
NO2, and O3 inside the dynamic chamber were
considered for the determination of NO2 flux densities,
NO2 deposition velocities, as well as NO2
compensation point concentrations. The calculations based on a
bi-variate weighted linear regression analysis (y- and x-errors
considered). The NO2 deposition velocities for spruce, based
on projected needle area, ranged between 0.07 and 0.42 mm
s-1. The calculated NO2 compensation point
concentrations ranged from 7.4 ± 6.40 to 29.0 ± 16.30 nmol
m-3 (0.17-0.65 ppb) but the compensation point concentrations
were all not significant in terms of compensation point concentration is
unequal zero. These data challenge the existence of a NO2
compensation point concentration for spruce. Our study resulted in lower
values of NO2 gas exchange flux densities, NO2
deposition velocities and NO2 compensation point
concentrations in comparison to most previous studies. It is essential
to use a more specific NO2 analyzer and to consider
photo-chemical reactions between NO, NO2, and O3
inside the chamber.
Systems have been developed and deployed at a North Michigan forested
site to measure ambient HONO and vertical HONO flux. The modified HONO
measurement technique is based on aqueous scrubbing of HONO using a coil
sampler, followed by azo dye derivatization and detection using a
long-path absorption photometer (LPAP). A
Na2CO3-coated denuder is used to generate "zero
HONO" air for background correction. The lower detection limit of the
method, defined by 3 times of the standard deviation of the signal, is 1
pptv for 1-min averages, with an overall uncertainty of ±(1 +
0.05 [HONO]) pptv. The HONO flux measurement technique has been
developed based on the relaxed eddy accumulation approach, deploying a
3-D sonic anemometer and two HONO measurement systems. The overall
uncertainty is estimated to be within ±(8 × 10-8
+ 0.15 FHONO) mol m-2 h-1, with a
20-min averaged data point per 30 min. Ambient HONO and vertical HONO
flux were measured simultaneously at the PROPHET site from 17 July to 7
August 2008. The forest canopy was found to be a net HONO source, with a
mean upward flux of 0.37 × 10-6 moles m-2
h-1. The HONO flux reached a maximal mean of ~0.7 ×
10-6 moles m-2 h-1 around solar noon,
contributing a major fraction to the HONO source strength required to
sustain the observed ambient concentration of ~70 pptv. There were no
significant correlations between [NOx] and daytime HONO flux
and between JNO2 × [NO2] and HONO
flux, suggesting that NOx was not an important precursor
responsible for HONO daytime production on the forest canopy surface in
this low-NOx rural environment. Evidence supports the
hypothesis that photolysis of HNO3 deposited on the forest
canopy surface is a major daytime HONO source.
Nitrogen dioxide (NO2) plays an important role in atmospheric pollution, in particular for tropospheric ozone production. However, the removal processes involved in NO2 deposition to terrestrial ecosystems are still the subject of on-going discussion. This study reports NO2 flux measurements made over a meadow using the eddy covariance method. The measured NO2 deposition fluxes during daytime were about a factor of two lower than a priori calculated fluxes using the Surfatm model without taking into account an internal (also called mesophyllic or sub-stomatal) resistance. Neither an underestimation of the measured NO2 deposition flux due to chemical divergence or an in-canopy NO2 source nor an un-derestimation of the resistances used to model the NO2 deposition explained the large difference between measured and modelled NO2 fluxes. Thus, only the existence of the internal resistance could account for this large discrepancy between model and measurements. The median internal resistance was estimated to be 300 s m−1 during daytime, but exhibited a large variability (100–800 s m−1). In comparison, the stom-atal resistance was only around 100 s m−1 during daytime. Hence, the internal resistance accounted for 50–90 % of the total leaf resistance to NO2 . This study presents the first clear evidence and quantification of the internal resistance using the eddy covariance method; i.e. plant functioning was not affected by changes of microclimatological (turbulent) conditions that typically occur when using enclosure methods.
The dynamic behavior of nitrogen oxides (NOx = NO + NO2) and ozone (O3) above and within the canopy at the University of Michigan Biological Station AmeriFlux (UMBS Flux) site was investigated by continuous multi-height vertical gradient measurements during the summer and the fall of 2008. A daily maximum in nitric oxide (NO) mixing ratios was consistently observed during the morning hours between 06:00 and 09:00 EST above the canopy. Daily NO maxima ranged between 0.1 and 2 ppbv (with a median of 0.3 ppbv), which were 2 to 20 times above the atmospheric background. The sources and causes of the morning NO maximum were evaluated using NOx and O3 measurements and synoptic and micrometeorological data. Numerical simulations with a multi-layer canopy-exchange model were done to further support this analysis. The observations indicated that the morning NO maximum was caused by the photolysis of NO2 from non-local air masses, which were transported into the canopy from aloft during the morning breakup of the nocturnal boundary layer. The analysis of simulated process tendencies indicated that the downward turbulent transport of NOx into the canopy compensates for the removal of NOx through chemistry and dry deposition. The sensitivity of NOx and O3 concentrations to soil and foliage NOx emissions was also assessed with the model. Uncertainties associated with the emissions of NOx from the soil or from leaf-surface nitrate photolysis did not explain the observed diurnal behavior in NOx (and O3) and, in particular, the morning peak in NOx mixing ratios. However, a ~30% increase in early morning NOx and NO peak mixing ratios was simulated when a foliage exchange NO2 compensation point was considered. This increase suggests the potential importance of leaf-level, bidirectional exchange of NO2 in understanding the observed temporal variability in NOx at UMBS.
Nitrogen dioxide (NO2) exchange between the atmosphere and five European tree species was investigated in the laboratory using a dynamic branch enclosure system (consisting of two cuvettes) and a highly specific NO2 analyzer. NO2 measurements were performed with a sensitive gas phase chemiluminescence NO detector combined with a NO2 specific (photolytic) converter, both from Eco-Physics (Switzerland). This highly specific detection system excluded bias from other nitrogen compounds. Investigations were performed at two light intensities (Photosynthetic Active Radiation, PAR, 450 and 900 μmol m−2 s−1) and NO2 concentrations between 0 and 5 ppb. Ambient parameters (air temperature and relative humidity) were held constant. The data showed dominant NO2 uptake by the respective tree species under all conditions. The results did not confirm the existence of a compensation point within a 95% confidence level, though we cannot completely exclude emission of NO2 under very low atmospheric concentrations. Induced stomatal stricture, or total closure, by changing light conditions, as well as by application of the plant hormone ABA (Abscisic Acid) caused a corresponding decrease of NO2 uptake. No loss of NO2 to plant surfaces was observed under stomatal closure and species dependent differences in uptake rates could be clearly related to stomatal behavior.
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.
Nitrogen oxides (NOx=NO+NO2) are a common group of nitrogen-containing trace gases that interact significantly with plant canopies and play a critical role in atmospheric chemistry including the formation of ozone, smog and nitric acid. The controls over leaf–atmosphere exchange of NOx (NOx=NO+NO2) have been characterized for a few plant species and important drivers have been identified including stomatal conductance, the partial pressure gradient from the interior to the exterior of the leaf, and, most recently the level of ultraviolet radiation. However, our ability to predict leaf fluxes has been hampered by a dearth of information describing the reactions occurring between NOx and internal constituents of the leaf (often referred to as the mesophyllic resistance). In this study, we investigated NOx uptake in four important agricultural plant species and specifically tested the hypothesis that ascorbic acid within the leaf was a significant control over mesophyllic resistance. Measurements were made using a dynamic leaf chamber at ambient mixing ratios typical of urban atmospheres. Measured NO and NO2 leaf fluxes were lower than the potential flux calculated assuming stomatal conductance as the sole limitation (i.e., no mesophyllic resistance). The ratios of leaf intercellular to ambient concentrations (Ci/Ca) for NO across partial pressures and plant species was usually >0.99 and the leaf uptake rates of NO were negligible within the ranges examined. In contrast, leaf uptake for NO2 was much greater and Ci/Ca were often as low as 0.75. For both gases, Ci/Ca ratios>1 were observed at low ambient mixing ratios indicating emission from leaves. Across plant species, plants with higher leaf nitrogen contents showed higher NO2 emission rates and plants with higher leaf ascorbate concentrations had higher leaf NO2 uptake rates. These results suggest that in addition to the known effects of stomatal conductance and incident UV radiation, leaf nitrogen content and the capacity for the scavenging of NOx by ascorbic acid may explain the variation in the ability of plants to absorb atmospheric nitrogen oxides.
We monitored NOx fluxes in an empty chamber and in chambers containing Scots pine shoots. Our aims were to observe how the chamber blank behaves in non-controlled field conditions and at low ambient NOx concentrations, to find ways of reducing deleterious wall effects, and to evolve a method for a blank correction. We found that solar ultraviolet radiation induced NOx emission from the chamber walls. Comparisons between blanks of several chambers showed that the chambers were not identical. We therefore recommend that a separate blank chamber will be monitored together with the branch chambers in long-term field measurements. At regular time intervals, all of the chambers should be measured simultaneously while empty and the regression between the UV irradiance and NOx flux determined for each chamber. In case the chambers differ from each other, the blank could be corrected with the UV-regression coefficients obtained.
When spruce ( Picea abies L., Karst.) branches were exposed to 5.2–18.7 nl NO 2 l ⁻¹ the flux to the shoots increased linearly with increasing NO 2 concentrations. At NO 2 concentrations below 2.6 nl l ⁻¹ uptake of NO 2 by the shoots was not observed. The measured flux of NO 2 to the shoots was found to be lower than the NO 2 flux predicted from the shoot conductance to diffusion of H 2 O. These results are consistent with the existence of internal resistances for NO 2 influx, e.g. production and emission of NO 2 by the leaves. However, emission of NO 2 was not observed. When the NO 2 flux to the shoots was plotted versus transpiration rate, a linear increase was found with an y‐axis intercept. The intercept may be interpreted as the NO 2 flux to the cuticle and the bark; its value increased with increasing atmospheric NO 2 concentrations.
The flux of NO 2 to the shoots showed diurnal variation with high levels in the light and low levels during darkness. NO 2 flux to the branches was dependent on light intensity. This dependency can largely be explained by light dependent changes in shoot conductance. Daytime light intensity determined also the night‐time shoot conductance and, hence, the NO 2 flux to the branches during the night. The ratio of NO 2 flux to transpiration rate was higher in the dark than in the light. Whether this observation can be explained by a light dependency of internal resistances remains to be elucidated.
The absorption of NO 2 by the shoots enhanced the in vitro NR activity of the needles, while NiR and GS activities were not increased significantly. One day after exposure to NO 2 concentrations of 60 nl l ⁻¹ , nitrate reductase (NR) activity was three times higher than that of untreated controls. However, after three days of NO 2 exposure the NR activity declined to the level of untreated controls. Apparently, the increased in vitro NR activity upon NO 2 fumigation is a transient phenomenon in spruce needles. The regulatory events that may modulate NR activity of the needles, when exposed to atmospheric NO 2 , are discussed.
Tropical forest soils are known to emit large amounts of reactive nitrogen oxide compounds, often referred to collectively as NOy (NOy = NO + NO2 + HNO3 + organic nitrates). Plants are known to assimilate and emit NOy and it is therefore likely that plant canopies affect the atmospheric concentration of reactive nitrogen compounds by assimilating or emitting some fraction of the soil-emitted NOy. It is crucial to understand the magnitude of the canopy effects and the primary environmental and physiological controls over NOy exchange in order to accurately quantify regional NOy inventories and parameterize models of tropospheric photochemistry. In this study we focused on nitrogen dioxide (NO2), which is the component of NOy that most directly catalyzes the chemistry of O3 dynamics, one of the most abundant oxidative species in the troposphere, and which has been reported as the NOy species that is most readily exchanged between plants and the atmosphere. Leaf chamber measurements of NO2 flux were measured in 25 tree species growing in a wet tropical forest in the Republic of Panama. NO2 was emitted to the atmosphere at ambient NO2 concentrations below 0.53-1.60 ppbv (the NO2 compensation point) depending on species, with the highest rate of emission being 50 pmol m(-2) s(-1) at <0.1 ppbv. NO2 was assimilated by leaves at ambient NO2 concentrations above the compensation point, with the maximum observed uptake rate being 1,550 pmol m(-2) s(-1) at 5 ppbv. No seasonal variation in leaf NO2 flux was observed in this study and leaf emission and uptake appeared to be primarily controlled by leaf nitrogen and stomatal conductance, respectively. When scaled to the entire canopy, soil NO emission rates to the atmosphere were estimated to be maximally altered ±19% by the overlying canopy.
Nitrogen oxides (NOx = NO + NO2) in the atmosphere
are often measured using instruments equipped with molybdenum
converters. NO2 is catalytically converted to NO on a heated
molybdenum surface and subsequently measured by chemiluminescence after
reaction with ozone. The drawback of this technique is that other
oxidized nitrogen compounds such as peroxyacetyl nitrate and nitric acid
are also partly converted to NO. Thus such NO2 measurements
are really surrogate NO2 measurements because the resultant
values systematically overestimate the true value because of
interferences of these compounds, especially when sampling
photochemically aged air masses. However, molybdenum converters are
widely used, and a dense network of surrogate NO2
measurements exists. As an alternative with far less interference,
photolytic converters using ultraviolet light are nowadays applicable
also for long-term measurements. This work presents long-term collocated
NO2 measurements using molybdenum and photolytic converters
at two rural sites in Switzerland. On a relative scale, the molybdenum
converter instruments overestimate the NO2 concentrations
most during spring/summer because of prevalent photochemistry. On a
monthly basis, only 70-83% of the "surrogate" NO2 can be
attributed to "real" NO2 at the non-elevated site and even
less (43-76%) at the elevated one. The observed interferences have to be
taken into account for monitoring and regulatory issues and to be
considered when using these data for ground-truthing of satellite data
or for validation of chemical transport models. Alternatively, an
increased availability of artifact-free data would also be beneficial
for these issues.
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.
Herbicide-treated soybean [Glycine max(L.) Merr. var. “Amsoy”] leaves and intact plants formed NO and NO2 within the leaves and released them into the surrounding atmosphere. These gaseous products are thought to be due to reactions between herbicide-caused free nitrite accumulations and plant metabolites. Untreated soybean leaves and plants did not accumulate free nitrite nor evolve these nitrogen oxide gases. At relatively low concentrations of free nitrite in green leaves only NO was detected. Higher nitrite concentrations were required for NO2 evolution. Emissions of NO and NO2 were proportional to the amount of herbicide used. The technique for detection and measurement of NOx emissions from soybean plants is described.
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.
Emission of nitric oxide (NO) from a variety of plant species was observed in a continuously stirred tank reactor. During daytime and at NO concentrations below 1 ppb in the chamber air, NO emissions were observed for all studied nitrate-nourished plant species. A relation was found between the NO emission rates during daytime and the uptake rates of CO2. The ratio of the NO emission rate to the CO2 uptake was similar for all plants. Changes of the net rate of photosynthesis induced by variations of light intensity or changes of CO2 concentrations changed the NO emission rates correspondingly. The link between NO emissions and CO2 uptake during daytime allowed estimation of the potential of the vegetation to evolve NO on a global scale as 0.23TgNyr-1. Strong NO emissions during nights were observed when the nitrate concentration in the nutrient solution was enhanced. Then NO emissions were observed with flux densities comparable to the highest emission rates found from soils.
The exchange of NO2, NO, and O3 between the atmosphere and coniferous forests has been studied by using a dynamic flow-through chamber technique. Generally, the flux of NO2 was towards the vegetation surface, and only in a few cases a small emission of NO2 was detected. The average deposition velocities per projected needle area were 1.5 ± 1.3 mm s-1 for Norway spruce and 1.4 ± 1.1 mm s-1 for Scots pine. The results from Jadraas are slightly different; the deposition velocity of NO2, at concentrations lower than 1 ppbv, was always lower than the stomatal conductance and net emissions of NO2 were observed at concentrations below 0.5-0.7 ppbv. The average deposition velocity per projected needle area observed in Jadraas was 0.8 ± 0.7 mm s-1. The ratio between the deposition velocity for O3 and stomatal conductance was in general larger than one, indicating deposition of O3 to the external surfaces of the vegetation. The residual deposition of O3 showed a marked diurnal variation with maximum values around noon. -from Authors
Nitrogen oxides are trace gases that critically affect atmospheric chemistry and aerosol formation1. Vegetation is usually regarded as a sink for these gases, although nitric oxide and nitrogen dioxide have been detected as natural emissions from plants2, 3. Here we use in situ measurements to show that solar ultraviolet radiation induces the emission of nitrogen oxide radicals (NOx) from Scots pine (Pinus sylvestris) shoots when ambient concentrations drop below one part per billion. Although this contribution is insignificant on a local scale, our findings suggest that global NOx emissions from boreal coniferous forests may be comparable to those produced by worldwide industrial and traffic sources.
Measurements of the deposition of NO2 to seedlings of Scots pine and Norway spruce at ppbv and sub ppbv levels were made in the laboratory using a flow through chamber technique. The results show that NO2 deposition at air concentrations above 1 ppbv was governed by stomatal uptake. The possible influence of other factors controlling the NO2 deposition was also explored. Mean NO2 compensation points for both Scots pine and Norway spruce were lower than 0.2 ppbv, which can be considered negligible. Throughout the experiments, there was no evidence of internal resistance to NO2 uptake by the seedlings. At low NO2 concentrations (0.2–1 ppbv), the NO2 flux was generally below the detection limit of the technique.
Users of automatic air pollution monitors are largely unaware of how certain parameters, like pressure, can affect readings. The present work examines the influence of inlet pressure changes on chemiluminescence NOx measurements. These changes have been grouped into two categories: (i) those due to changes in atmospheric pressure and (ii) those produced by any other reason (e.g., clogs in the inlet sampling line). Atmospheric pressure changes were simulated varying both the inlet and the outlet sample line pressures in the same way, whereas sample pressure changes were produced modifying only the inlet sample line pressure. Analyzer calibration was performed with zero air and 200 nmol mol−1 of NO at 101.3 kPa. The test pressures ranged from −10 to +10 kPa with respect to the calibration pressure. ANOVA and Dunnett's tests were performed to look for significant differences between measurements obtained under calibration conditions and those obtained under different pressure conditions. The differences in measurements were practically negligible for fixed analyzers subjected to usual changes in atmospheric pressure (<±5 kPa); however, differences could be as high as 7% if ±10 kPa pressure changes took place in the inlet sample line for causes included in the second group.Highlights► Pressure changes on NOx chemiluminescence measurements have been studied. ► Deviations in measurements are negligible for fixed analyzers subjected to usual changes in atmospheric pressure (<±5 kPa). ► If pressure changes are due to different causes, deviations can be as high as 7% for ±10 kPa pressure changes. ► Operational procedures are given in order to minimise these deviations.
In the present study NH3 and NO2 exchange between the atmosphere and needles of adult spruce (Picea abies) trees at a field site ('Hoglwald') exposed to high loads of atmospheric nitrogen was assessed. Twigs were fumigated with different NH3 (C-NH3) or NO2 (C-NO2) concentrations using the dynamic chamber technique. Beside fluxes of NH3 (J(NH 3)) and NO2 (J(NO 2)) transpiration (J(H 2 O)), leaf conductance for water vapour (g(H 2 O)), photosynthetic activity (J(CO 2)), photosynthetic photon fluence rate (PPFR), air temperature (T) and relative air humidity (RH) were determined. Both fluxes, J(NH 3) and J(NO 2), depended linearly on C-NH 3 and C-NO 2 in concentration ranges representative for the field site and g(H 2 O) as a measure of stomatal aperture. For both trace gases compensation points could be determined amounting to 2.5 nmol mol(-1) for NH3 and to 1.7 nmol mol(-1) for NO2. The fluxes of NH3 and NO2 could not be explained exclusively by exchange through the stomata. In both NH3 and NO2 fumigation experiments additional deposition onto the needle surface was observed and increased with increasing C-NH 3 and C-NO 2. N-15[NH3] fumigation experiments with adult spruce trees confirmed the results of gas exchange measurements and revealed that NH3-N deposited to spruce needles is subjected to long distance transport within the plant, supplying the plant with additional nitrogen from the atmosphere.
Over 60 published papers reporting field measurements of emissions of nitric oxide (NO) from soil are reviewed, and over 100 annual estimates of NO emissions were made for various types of ecosystems, including agricultural fields. These data were stratified by biome and the mean of each stratum was multiplied by an estimate of the biome area. A few strata were identified as clearly having low NO emissions: montane forests, swamps and marshes, tundra, and temperate forests that are not heavily affected by N deposition. The largest emissions were observed in tropical savanna/woodland, chaparral, and cultivated agriculture, but variation in NO emissions within these strata was also large. Although the stratification scheme fails to partition this within-stratum variation, it does clearly identify these biomes as globally important sources of NO and as areas where more research is needed to investigate within-biome variation in NO emissions. It is too early to tell whether differences in NO emissions between temperate and tropical agriculture are significant, but it is clear that agriculture is an important source of NO and that management practices affect NO emissions. The best current estimate of the global soil source of NO is 21 Tg N yr-1. Adsorption of NOx onto plant canopy surfaces may reduce emissions to the atmosphere to as low as 13 Tg N yr-1, although the absorption effect is probably smaller than this. An error term for the global estimate is difficult to determine, but it is at least 4 and perhaps as large at 10 Tg N yr-1. Hence, only modest progress has been made in narrowing uncertainties in the estimate of the global soil source of NO, although some published lower estimates appear unlikely. This inventory reconfirms that the soil source of NO is similar in magnitude to fossil fuel emissions of NOx. Further narrowing of the uncertainty of the estimate of global soil NO emissions will require more sophisticated and carefully chosen stratification schemes to address variation within biomes based on soil fertility, soil texture, climate, and management and will require linking this type of inventory and stratification with mechanistic models.
The detection of NO2 by its chemiluminescent reaction with luminol is a rapid and sensitive means of measuring atmospheric NO2. However, testing and field use of a commercial NO2 monitor employing this detection scheme have shown that several corrections are necessary in order to obtain accurate NO2 data at low concentrations. In use aboard aircraft, the NO2 data must be corrected for zero offset, altitude (i.e. pressure), nonlinearity of response, and interferences from ozone and PAN. Detector response is dependent on the age of luminol reagent solution. This paper describes the tests performed to determine correction factors, the algorithms and order of precedence for applying the corrections, and other observations regarding detector performance.
The compensation point of bidirectional NOx fluxes of plants is often determined in chamber measurements by varying the NOx concentration and searching for the point where the net flux is zero. We hypothesized that the compensation point can vary with changing conditions so much that one constant value is not sufficiently accurate. The compensation point for NOx fluxes of Scots pine was analysed with a model describing the NOx consumption and production processes. Consumption was assumed to occur via the stomata and on needle surfaces, with the rate depending on ambient concentration and degree of stomatal opening. Production was assumed to occur on the needle surfaces depending on solar UV-A irradiance. We used NOy flux data measured with two gas-exchange chambers in southern Finland to parameterize the model. The estimated compensation points increased with increasing UV-A irradiance and decreased with increasing stomatal conductance. With open stomata, it varied several ppb even under field conditions.
Values of ozone (O3) flux measured on the shoot scale by enclosure techniques can provide detailed information about the partition and seasonality of the removal pathways on the foliage. However, O3 reactivity makes such measurements potentially inaccurate, particularly in field conditions where ambient concentration is low. Special consideration should be given to the losses on the chamber wall material, which can rival in magnitude and dynamics the measure of shoot removal. We propose a way of taking into account this dynamic behaviour in the context of field monitoring with simple chambers based on dynamic principles. We accommodated an existing field monitoring gas exchange system for CO2 and water vapour to measure simultaneous,but independent, O3 fluxes. We report here the system's performance based on the analysis of 19-month data. The background noise of the measurements is mainly imposed by the analyser and is around 2 μg m−3. The resolution of the system is in the order of 10−2 μg m−2 s−1. The disturbance due to wall-losses was corrected thoroughly but it was mostly needed when the shoot activity was low and the mass flow signals produced by the shoots and the walls came close to each other. In spite of the disfavouring combination of chamber material, O3 reactivity and low concentrations, we have successfully followed diurnal and seasonal trends of O3 removal in Scots pine foliage.
Production of NOy on Scots pine branch surfaces by ultraviolet radiation is observed in Hyytiälä, southern Finland. Similar irradiance-dependent NOy emissions from snow and different chamber surfaces have been observed to originate from HNO3 or nitrate photolysis. In this study, we investigate whether also the NOy emitted from pine shoots could originate from photolysis of HNO3 attached to the needle surface. We used field data of several years from Hyytiälä to test this hypothesis. The HNO3 deposition, estimated for the Hyytiälä site, had been high enough to account for the NOy emission rates observed from the chambers. We could not find the particular characteristics of the daily pattern of CO2 exchange or stomatal control reflected in the NOy flux. When a pine branch was rinsed, which reduced the amount of water-soluble nitrogen compounds (e.g., HNO3, nitrates and HONO) from the needle surface, NOy emissions from that branch decreased compared to another non-rinsed branch. Therefore, we conclude that the results support the hypothesis and that HNO3 photolysis on plant surfaces needs to be taken into account both from air chemistry and plant sciences point of view.
In the United States, fertilized corn fields, which make up approximately 5% of the total land area, account for approximately 45% of total soil NOx emissions. Leaf chamber measurements were conducted of NO and NO2 fluxes between individual corn leaves and the atmosphere in (1) field-grown plants near Champaign, IL (USA) in order to assess the potential role of corn canopies in mitigating soil–NOx emissions to the atmosphere, and (2) greenhouse-grown plants in order to study the influence of various environmental variables and physiological factors on the dynamics of NO2 flux. In field-grown plants, fluxes of NO were small and inconsistent from plant to plant. At ambient NO concentrations between 0.1 and 0.3 ppbv, average fluxes were zero. At ambient NO concentrations above 1 ppbv, NO uptake occurred, but fluxes were so small (14.3±0.0 pmol m−2 s−1) as to be insignificant in the NOx inventory for this site. In field-grown plants, NO2 was emitted to the atmosphere at ambient NO2 concentrations below 0.9 ppbv (the NO2 compensation point), with the highest rate of emission being 50 pmol m−2 s−1 at 0.2 ppbv. NO2 was assimilated by corn leaves at ambient NO2 concentrations above 0.9 ppbv, with the maximum observed uptake rate being 643 pmol m−2 s−1 at 6 ppbv. When fluxes above 0.9 ppbv are standardized for ambient NO2 concentration, the resultant deposition velocity was 1.2±0.1 mm s−1. When scaled to the entire corn canopy, NO2 uptake rates can be estimated to be as much as 27% of the soil-emitted NOx. In greenhouse-grown and field-grown leaves, NO2 deposition velocity was dependent on incident photosynthetic photon flux density (PPFD; 400–700 nm), whether measured above or below the NO2 compensation point. The shape of the PPFD dependence, and its response to ambient humidity in an experiment with greenhouse-grown plants, led to the conclusion that stomatal conductance is a primary determinant of the PPFD response. However, in field-grown leaves, measured NO2 deposition velocities were always lower than those predicted by a model solely dependent on stomatal conductance. It is concluded that NO2 uptake rate is highest when N availability is highest, not when the leaf deficit for N is highest. It is also concluded that the primary limitations to leaf-level NO2 uptake concern both stomatal and mesophyll components.
During the 1999 summer field season at Summit, Greenland, we conducted several series of experiments to follow up on our 1998 discovery that NOx is released from the sunlit snowpack. The 1999 experiments included measurements of HONO in addition to NO and NO2, and were designed to confirm, for Greenland snow, that the processes producing reactive nitrogen oxides in the snow are largely photochemical. Long duration experiments (up to 48 h) in a flow-through chamber and in the natural snowpack revealed sun-synchronous diurnal variations of all three reactive nitrogen oxides. In a second set of experiments we alternately shaded or exposed snow (again in the natural snowpack and in the chamber) to ambient sunlight for short periods to reduce any temperature changes during variations in light intensity. All three N oxides increased (decreased) very rapidly when sunlit (shaded). In all experiments NO2 was approximately 3-fold more abundant than NO and HONO (which were at similar levels). Higher concentrations of NO3− in the snow resulted in higher mixing ratios of HONO, NO and NO2 in the snow pore air, consistent with our hypothesis that photolysis of NO3− is the source of the reactive N oxides.
Users of automatic air pollution monitors are largely unaware of how certain parameters, like temperature, can affect readings. The present work examines the influence of temperature changes on chemiluminescence NO(x) measurements made with a Thermo Scientific 42i analyzer, a model widely used in air monitoring networks and air pollution studies. These changes are grouped into two categories according to European Standard EN 14211: (1) changes in the air surrounding the analyzers and (2) changes in the sampled air. First, the sensitivity tests described in Standard EN 14211 were performed to determine whether the analyzer performance was adapted to the requirements of the standard. The analyzer met the performance criteria of both tests; however, some differences were detected in readings with temperature changes even though the temperature compensator was on. Sample temperature changes were studied more deeply as they were the most critical (they cannot be controlled and differences of several tens of degrees can be present in a single day). Significant differences in readings were obtained when changing sample temperature; however, maximum deviations were around 3% for temperature ranges of 15°C. If other possible uncertainty contributions are controlled and temperature variations with respect to the calibration temperature are not higher than 15°C, the effect of temperature changes could be acceptable and no data correction should have to be applied.
We describe a time-gated laser-induced fluorescence instrument designed for accurate (+/- 5%, 1 sigma), continuous, autonomous, in situ observations of NO2 with the sensitivity (15 ppt/10 s at S/N = 2) and portability necessary to study NO2 anywhere in the troposphere. The technique is advantageous because it is spectroscopically specific and direct in that it does not require conversion of NO2 into another species (e.g., NO) prior to detection, eliminating a class of potential interferences. Performance of the instrument is illustrated with 15 weeks (July-Oct 1998) of observations at the University of California, Blodgett Forest field station located in the foothills of the Sierra Nevada and 4 weeks (June 15-July 15, 1999) in Nashville, TN during the Southern Oxidants Study. Ambient concentrations of NO2 at Blodgett Forest varied from below 50 ppt to 4000 ppt and NO2 ranged from 5 to 50% of the total reactive nitrogen; while in Nashville, TN, concentrations ranged from 1 to 75 ppb.
Hereafter, an assessment of the ability of the chemiluminescence method to measure ambient NO2 with an accuracy within 15%, as requested by the data quality objective of European directive 1999/30/CE, is presented. In general, uncertainty is evaluated using the response to reference materials or by means of inter-comparisons used to determine some statistics like repeatability, reproducibility and calibration bias. These are incomplete approaches and the method of the Guide to the Expression of Uncertainty in Measurement, advised by the Directive, should be preferred. In fact, even if it requires a large data set, it allows the relative influence of all possible sources of uncertainty to be studied. The extent of NO2 uncertainty is mainly dependent on the level of NO. It is decreased by NOx and the correlation between NOx and NO. Furthermore, the uncertainty budget reveals that the contribution of accuracy of calibration standard, linearity, converter efficiency and drift of the analyser between calibration checks to the overall uncertainty is less important than the contribution of interference, mainly humidity and PAN in rural areas. The relative expanded uncertainty of the NO2 hourly average exceeds 30% for NO2 concentrations lower than 40 microg m(-3). Nevertheless, the data quality objective of 15% is reached for 200 microg m(-3), the hourly limit value of the European directive. On the contrary, at the limit value on the annual average, 40 microg m(-3), the data quality objective is not met if NO is higher than 100 microg m(-3). However, the data quality objective could be reached by correcting the measurements with the bias due to interference.
Nitric oxide (NO) is a diffusible, very reactive gas that is involved in the regulation of many processes in plants. Several enzymatic sources of NO production have been identified in recent years. Nitrate reductase (NR) is one of them and it has been shown that this well-known plant protein, apart from its role in nitrate reduction and assimilation, can also catalyse the reduction of nitrite to NO. This reaction can produce large amounts of NO, or at least more than is needed for signalling, as some escape of NO to the outside medium can be detected after NR activation. A role for NO and NR in stomata functioning in response to abscisic acid has also been proposed. The question that remains is whether this NR-derived NO is a signalling molecule or the mere product of an enzymatic side reaction like the products generated by the oxygenase activity of RuBisCO.