NO2 Mixing Ratios Research Articles

Implications of iodine chemistry for daytime HO2 levels at Rishiri Island

The observed midday maximum in the mixing ratio of HO2 at Rishiri Island in June 2000 was ∼10 pptv, but photochemical box model simulations overpredicted HO2 at this location by an average of 70%. This overestimation was significant only when the mixing ratio of NO was lower than 300 pptv, and was coincident with overprediction of the NO/NO2 ratio. We detected several organoiodines, presumably emitted from seaweeds, and propose the presence of the IO radical. IO could reduce HO2 mixing ratios via the formation of HOI that may subsequently be scavenged by aerosols or lost by photolysis and may also convert NO to NO2 directly. Model calculations with known iodine chemistry could reproduce the observed HO2 with 12–25 pptv of IO. Although iodine chemistry is unlikely to explain the entire discrepancy in HO2, several pptv of IO could significantly reduce HO2 mixing ratios and NO/NO2 ratios.

Effects of biomass burning, lightning, and convection on O3, CO, and NOy over the tropical Pacific and Australia in August–October 1998 and 1999

In situ aircraft measurements of O3, CO, total reactive nitrogen (NOy), NO, and non‐methane hydrocarbons (NMHCs) were made over the western Pacific Ocean and Australia during the Biomass Burning and Lightning Experiment (BIBLE) A and B conducted in August–October 1998 and 1999. Generally, similar features were seen in the BIBLE A and B data in the latitudinal variations of these species in the troposphere from 35°N to 28°S at longitudes of 120°–150°E. The focus of this paper is to describe the characteristics of air masses sampled at 15°N–10°S (tropical Pacific) and 10°S–28°S (over Australia). With the exception of occasional enhancements in reactive nitrogen seen over New Guinea associated with lightning activities, the tropical Pacific region is distinguished from the rest of the region by smaller concentrations of these trace species. This can be explained in terms of the absence of surface sources over the ocean, lack of stratospheric intrusion, and the preclusion of midlatitude air and air from the west due to active convection throughout the troposphere. The median O3, CO, NOy, and NO mixing ratios in tropical air above 4 km were about 15–20 parts per billion by volume (ppbv), 60–75 ppbv, 20–100 parts per trillion by volume (pptv), and 5–40 pptv, respectively. Data obtained from PEM‐West A and B conducted in 1991 and 1994 showed similar latitudinal features, although the PEM‐West A values were somewhat elevated due to dominating westerly winds in the lower troposphere associated with El Niño. Over Australia, the levels of O3, CO, NOy, NO, and NMHCs were elevated throughout the troposphere over those observed in the tropical Pacific both in 1998 and 1999. The effect from biomass burning that occurred in northern Australia was limited to within the boundary layer because of strong subsidence in the period. Analyses based on 14‐day back trajectories identified free tropospheric air over Australia that originated from Indonesia, the Indian Ocean, Africa, and southern midlatitudes. The levels of O3, CO, NOy, and NMHCs in these air masses were much higher than those from the tropical Pacific due to their stronger sources from biomass burning and lightning. These values are compared with those obtained in the South Pacific during PEM‐Tropics A. Effects of biomass burning and lightning are discussed as possible sources of O3 and its precursors in these air masses.

Photochemical production of ozone in the upper troposphere in association with cumulus convection over Indonesia

The Biomass Burning and Lightning Experiment phase A (BIBLE‐A) aircraft observation campaign was conducted from 24 September to 10 October 1998, during a La Niña period. During this campaign, distributions of ozone and its precursors (NO, CO, and nonmethane hydrocarbons (NMHCs)) were observed over the tropical Pacific Ocean, Indonesia, and northern Australia. Mixing ratios of ozone and its precursors were very low at altitudes between 0 and 13.5 km over the tropical Pacific Ocean. The mixing ratios of ozone precursors above 8 km over Indonesia were often significantly higher than those over the tropical Pacific Ocean, even though the prevailing easterlies carried the air from the tropical Pacific Ocean to over Indonesia within several days. For example, median NO and CO mixing ratios in the upper troposphere were 12 parts per trillion (pptv) and 72 parts per billion (ppbv) over the tropical Pacific Ocean and were 83 pptv and 85 ppbv over western Indonesia, respectively. Meteorological analyses and high ethene (C2H4) mixing ratios indicate that the increase of the ozone precursors was caused by active convection over Indonesia through upward transport of polluted air, mixing, and lightning all within the few days prior to observation. Sources of ozone precursors are discussed by comparing correlations of some NMHCs and CH3Cl concentrations with CO between the lower and upper troposphere. Biomass burning in Indonesia was nearly inactive during BIBLE‐A and was not a dominant source of the ozone precursors, but urban pollution and lightning contributed importantly to their increases. The increase in ozone precursors raised net ozone production rates over western Indonesia in the upper troposphere, as shown by a photochemical model calculation. However, the ozone mixing ratio (∼20 ppbv) did not increase significantly over Indonesia because photochemical production of ozone did not have sufficient time since the augmentation of ozone precursors. Backward trajectories show that many air masses sampled over the ocean south of Indonesia and over northern Australia passed over western Indonesia 4–9 days prior to being measured. In these air masses the mixing ratios of ozone precursors, except for short‐lived species, were similar to those over western Indonesia. In contrast, the ozone mixing ratio was higher by about 10 ppbv than that over Indonesia, indicating that photochemical production of ozone occurred during transport from Indonesia. The average rate of ozone increase (1.8 ppbv/d) during this transport is similar to the net ozone formation rate calculated by the photochemical model. This study shows that active convection over Indonesia carried polluted air upward from the surface and had a discernable influence on the distribution of ozone in the upper troposphere over the Indian Ocean, northern Australia, and the south subtropical Pacific Ocean, combined with NO production by lightning.

Trace gas transport and scavenging in PEM‐Tropics B South Pacific Convergence Zone convection

Analysis of chemical transport on Flight 10 of the 1999 Pacific Exploratory Mission (PEM) Tropics B mission clarifies the role of the South Pacific Convergence Zone (SPCZ) in establishing ozone and other trace gas distributions in the southwestern tropical Pacific. The SPCZ is found to be a barrier to mixing in the lower troposphere but a mechanism for convective mixing of tropical boundary layer air from northeast of the SPCZ with upper tropospheric air arriving from the west. A two‐dimensional cloud‐resolving model is used to quantify three critical processes in global and regional transport: convective mixing, lightning NOx production, and wet scavenging of soluble species. Very low NO and O3 tropical boundary layer air from the northeastern side of the SPCZ entered the convective updrafts and was transported to the upper troposphere where it mixed with subtropical upper tropospheric air containing much larger NO and O3 mixing ratios that had arrived from Australia. Aircraft observations show that very little NO appears to have been produced by electrical discharges within the SPCZ convection. We estimate that at least 90% of the HNO3 and H2O2 that would have been in upper tropospheric cloud outflow had been removed during transport through the cloud. Lesser percentages are estimated for less soluble species (e.g., <50% for CH3OOH). Net ozone production rates were decreased in the upper troposphere by ∼60% due to the upward transport and outflow of low‐NO boundary layer air. However, this outflow mixed with much higher NO air parcels on the southwest edge of the cloud, and the mixture ultimately possessed a net ozone production potential intermediate between those of the air masses on either side of the SPCZ.

Global tropospheric NO2 column distributions: Comparing three‐dimensional model calculations with GOME measurements

Tropospheric NO2 columns derived from the data products of the Global Ozone Monitoring Experiment (GOME), deployed on the ESA ERS‐2 satellite, have been compared with model calculations from two global three‐dimensional chemistry transport models, IMAGES and MOZART. The main objectives of the study are an analysis of the tropospheric NO2 data derived from satellite measurements, an interpretation of it and evaluation of its quality using global models, and an estimation the role of NO2 in radiative forcing. The measured and modeled NO2 columns show similar spatial and seasonal patterns, with large tropospheric column amounts over industrialized areas and small column amounts over remote areas. The comparison of the absolute values of the measured and modeled tropospheric column amounts are particularly dependent upon uncertainties in the derivation of the tropospheric NO2 columns from GOME and the difficulty of modeling the boundary layer in global models, both of which are discussed below. The measured tropospheric column amounts derived from GOME data are of the same order as those calculated by the MOZART model over the industrialized areas of the United States and Europe, but a factor of 2–3 larger for Asia. The modeled tropospheric NO2 columns from MOZART as well as the column amounts measured by GOME are in good agreement with NO2 columns derived from observed NO2 mixing ratios in the boundary layer in eastern North America. The comparison of the models to the GOME data illustrates the degree to which present models reproduce the hot spots seen in the GOME data. The radiative forcing of NO2 has been estimated from the calculated tropospheric NO2 columns. The local maxima in the radiative forcing of tropospheric NO2 for cloud‐free conditions over the eastern United States and western Europe represent 0.1–0.15 W m−2, while values of 0.04–0.1 W m−2 are estimated on a continental scale in these regions, of the same order of magnitude as the forcing of N2O and somewhat smaller than the regional forcing of tropospheric ozone. The globally averaged radiative forcing of tropospheric NO2 is negligible, ∼0.005 W m−2.

Isotopic enrichment of nitrous oxide (15N14NO, 14N15NO, 14N14N18O) in the stratosphere and in the laboratory

Nitrous oxide (N2O) extracted from stratospheric whole air samples has been analyzed for its 15N and 18O isotopic composition, and strong enrichments in the heavy isotopes are observed concomitant with decreasing N2O mixing ratio. Notably, the 15N enrichment is strongly different at the two nonequivalent positions in the molecule. Laboratory broadband photolysis experiments at wavelengths representative for the stratosphere confirm that photolysis is the prime cause for the observed fractionation in the stratosphere. However, the in situ stratospheric fractionation constants are significantly reduced compared to the laboratory data, reflecting the importance of dynamic processes. In addition, small but significant variations in the ratio of the two 15N fractionation constants indicate the influence of additional chemical processes like the oxidation of N2O by O(1D).

Observations of ozone and related species in the northeast Pacific during the PHOBEA campaigns: 2. Airborne observations

During late March and April of 1999 the University of Wyoming's King Air research aircraft measured atmospheric concentrations of NO, O3, peroxyacetyl nitrate (PAN), CO, CH4, VOCs, aerosols, and J(NO2) off the west coast of the United States. During 14 flights, measurements were made between 39°–48°N latitude, 125°–129°W longitude, and at altitudes from 0–8 km. These flights were part of the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) experiment, which included both ground‐based and airborne measurements. Flights were scheduled when meteorological conditions minimized the impact of local pollution sources. The resulting measurements were segregated by air mass source region as indicated by back isentropic trajectory analysis. The chemical composition of marine air masses whose 5‐day back isentropic trajectories originated north of 40°N latitude or west of 180°W longitude (WNW) differed significantly from marine air masses whose 5‐day back isentropic trajectories originated south of 40°N latitude and east of 180°W longitude (SW). Trajectory and chemical analyses indicated that the majority of all encountered air masses, both WNW and SW, likely originated from the north‐western Pacific and have characteristics of emissions from the East Asian continental region. However, air masses with WNW back trajectories contained higher mixing ratios of NO, NOx, O3, PAN, CO, CH4, various VOC pollution tracers, and aerosol number concentration, compared to those air masses with SW back trajectories. Calculations of air mass age using two separate methods, photochemical and back trajectory, are consistent with transport from the northwestern Pacific in 8–10 days for air masses with WNW back trajectories and 16–20 days for air masses with SW back trajectories. Correlations, trajectory analysis, and comparisons with measurements made in the northwestern Pacific during NASA's Pacific Exploritory Mission‐West Phase B (PEM‐West B) experiment in 1994 are used to investigate the data. These analyses provide evidence that anthropogenically influenced air masses from the northwestern Pacific affect the overall chemical composition of the northeastern Pacific troposphere.

Variations in atmospheric nitrous oxide observed at Hateruma monitoring station

In situ measurement of atmospheric nitrous oxide (N2O) has been carried out at Hateruma monitoring station (lat 24°03′N, long 123°48′E) since March 1996 by the National Institute for Environmental Studies (NIES). A fully automated gas chromatograph equipped with an electron capture detector (ECD) measures the N2O concentrations at a frequency of 3 air samples per hour. Details of the experimental methods and procedures are presented in this paper. The N2O concentrations observed from March 1996 to February 1999 increased at an average rate of 0.64 ppb/yr. The observed data also suggest that there is a weak annual cycle of N2O concentration, increasing in autumn and winter and decreasing in spring and summer, with a peak-to-peak amplitude of at most 0.3 ppb. The N2O mixing ratios, smoothed with the 24-h running average, clearly showed short-term variability with synoptic timescales and had peak-to-peak amplitudes of about 1 ppb or less. These short-term variations correlated positively with the short-term variations of CO2 during the period from winter to early spring when the air masses arriving at Hateruma are dominantly transported from the Asian continent. The ΔN2O/ΔCO2 ratios could be used to constrain the relative strengths of these fluxes on a regional scale.

Correlation relationships of stratospheric molecular constituents from high spectral resolution, ground‐based infrared solar absorption spectra

Comparisons of chemically active species with chemically inert tracers are useful to quantify transport and mixing and assess the accuracy of model predictions. We report measurements of chemically active species and chemically inert tracers in the stratosphere derived from the analysis of infrared solar absorption spectra recorded with a ground‐based Fourier transform spectrometer operated typically at 0.005‐ to 0.01‐cm−1 spectral resolution. The measurements were recorded from Kitt Peak in southern Arizona (latitude 31.9°N, 111.6°W, 2.09 km altitude). Time series of N2O, CH4, O3, and HNO3 vertical profiles have been retrieved from measurements in microwindows. From these results, correlations between N2O and CH4 stratospheric mixing ratios and between O3 and HNO3 lower stratospheric mixing ratios have been derived. The measured correlations between N2O versus CH4 mixing ratios are compact and show little variability with respect to season in quantitative agreement with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) spring and autumn measurements recorded near the same latitude. Lower stratospheric O3 versus HNO3 mixing ratios measured during low to moderate aerosol loading time periods also show a compact relations though the HNO3/O3 slope is a factor of 2 lower than obtained from November 1994 ATMOS measurements near the same latitude. We also compare Kitt Peak and ATMOS N2O versus CH4 and O3 versus HNO3 relations obtained by averaging the measurements over two broad stratospheric layers. This comparison avoids bias from the a priori profiles and the limited vertical resolution of the ground‐based observations.

Enhanced NOx by lightning in the upper troposphere and lower stratosphere inferred from the UARS Global NO2 measurements

This paper reveals a possible connection between lightning activity and the UARS NO2 data on a global scale. NO2 measured by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) is analyzed for the entire year of 1993 and compared to the seasonal and global lightning distributions inferred from the Optical Transient Detector (OTD). The results suggest a link between lightning activity and observed high levels of NO2 in the upper troposphere and lower stratosphere. Enhanced NO2 mixing ratios significantly above the background level are consistently found in areas of elevated lightning activity. This relation is strongest in the tropical regions during the summer and spring months of the Northern and Southern Hemispheres. Moreover, the results show that background levels of NO2 mixing ratios generally correspond to areas of little or no lightning activity.

Dependence of the OH concentration on solar UV

OH and the major parameters determining its concentration were measured during a field campaign in August 1994 at Mankmoos, a rural, relatively unpolluted site in northeastern Germany. The measured OH concentrations were previously shown to depend mainly on the intensity of solar UV and on the mixing ratio of NO2. In this paper we develop a simple parameterization of the dependence on solar UV and on NO2. The photolysis of O3 to O1D, of NO2 to NO, and of HCHO to HCO and H, all contribute significantly to the total dependence of OH on solar UV. We demonstrate that the photolysis frequency of O3, JO1D, is a suitable measure for that dependence which is slightly less than linear. The highly nonlinear dependence of OH on NOx is approximated by a Padé function. The parameterization provides a tool for a future quantitative intercomparison of the measured and modeled dependences of OH on UV and NO2. It also allows the removal of the variation in the measured OH induced by the dependences on the variables, UV and NO2, and thus enables a search for dependences on other, less influential parameters.

Temporal evolution of chlorine monoxide in the middle stratosphere

The temporal evolution of stratospheric chlorine monoxide (ClO) at northern midlatitudes is studied over different scales: diurnal and seasonal variations. Data sets include temperature profiles, and ClO, HCl, ClONO2, NO2, O3, H2O, and CH4 mixing ratios as measured from 10 to 0.46 hPa (i.e., from ∼30 to ∼55 km) by the Microwave Limb Sounder (MLS), by the Halogen Occultation Experiment, and by the Cryogenic Limb Array Etalon Spectrometer instruments aboard the UARS satellite from at most September 1991 to June 1997 within the band 40°–50°N. The time evolution of these species is interpreted by comparison with results from a “zero‐dimensional” (0‐D) purely photochemical model and from the SLIMCAT three‐dimensional (3‐D) chemical transport model over the Plateau de Bure station (45°N, 10°E) from September 1991 to March 1998. A maximum confidence value for the yield in the reaction ClO + OH → HCl + O2 is deduced from the analysis to be in the range 0.05–0.10±0.03, which is consistent with recent laboratory data, suggesting a value of ∼0.05–0.06±0.02. Comparisons between the ClO diurnal variations measured by MLS and calculated by the 0‐D model agree to within 10% around the maximum of ClO (∼40 km) and within 5% elsewhere. The ClO diurnal variation is induced by variations in the partitioning with HOCl and ClONO2 below 50 km and by variations in the partitioning within the ClOx (=Cl+ClO) family above. The agreement between measured and modeled seasonal variations of ClO implies an evolution above 30 km essentially dictated by the variation in the partitioning with HCl, together with the partitioning within the ClOx family above 40 km. The differences between measured and modeled ClO seasonal variations above 50 km are attributed to uncertainties in the relative rates of the sources of HCl (reactions of Cl with CH4 and HO2).

Distributions of NO, NOx, and NOy in the upper troposphere and lower stratosphere between 28° and 61°N during POLINAT 2

During the Pollution From Aircraft Emissions in the North Atlantic Flight Corridor 2 (POLINAT 2) field campaign the distribution of NO, NOx, and NOy in the upper troposphere and lowermost stratosphere over the eastern North Atlantic was measured using the Deutsches Zentrum für Luft‐und Raumfahrt research aircraft Falcon. Based from Shannon Airport in Ireland, 14 flights were carried out between September 19 and October 25, 1997. The measurements were performed in and out of the North Atlantic flight corridor covering latitudes between 28°N and 61°N. A marked latitudinal gradient in NO, NOx, and NOy, the sum of all reactive nitrogen compounds, was observed. Mean NO volume mixing ratios in the upper troposphere increased from about 50 parts per trillion by volume (pptv) at 28°N to about 180 pptv at 59°N. A similar latitude dependence was also found for NOx and NOy. In the northern part of the POLINAT 2 measuring area, NO and NOx volume mixing ratios increased significantly with increasing altitude with maximum values around the tropopause, while in the southern part of the measuring area no strong altitude gradient was observed. NO and NOx did not show a substantial gradient across the tropopause. NO/NOy and NOy/O3 ratios showed maximum values of about 0.30 ppbv/ppbv and 0.012 ppbv/ppbv, respectively, around the tropopause. The POLINAT 2 observations suggest that aircraft emissions are an important source of NOx and NOy in the region studied. Also, the present measurements contribute to the data set obtained in the North Atlantic flight corridor during the last few years and help to establish a NOx climatology around the tropopause for this region.

On the budget of hydroxyl radicals at Schauinsland during the Schauinsland Ozone Precursor Experiment (SLOPE96)

The concentration of hydroxyl radicals is estimated from the degradation of NOx and selected volatile organic compounds (VOCs) during the transport of polluted air masses from the city of Freiburg to the Schauinsland mountain through a steep valley called “Großes Tal.” The approach is based upon chemical and meteorological measurements made during the Schauinsland Ozone Precursor Experiment (SLOPE96) at two ground‐based sites and aboard a small aircraft. Separation of chemistry and transport is achieved through the degradation of chemical compounds with significantly different reactivity towards OH. The transport time of the air between the two measurements (90±5 min) and the influence of mixing with background air on the calculated OH concentration is quantified with the help of a dispersion experiment with SF6 and from airborne chemical measurements. The OH concentration (7– 0 × 106 cm−3) is almost a factor of 2 larger than what is calculated with a chemical box model constrained by the measured trace gas concentrations and photolysis rates. The radical budget can be closed within the experimental uncertainties, when an upper limit is adopted for the photolysis of nitrous acid, in addition to the radical production from photolysis of ozone, H2O2, and carbonyl compounds as well as ozonolysis of unsaturated VOCs. Biogenic VOCs (i.e., isoprene, terpenes, and a number of oxidated compounds) comprise about half of the total VOC reactivity in the transported plume. The results from SLOPE96 confirm the assumptions made in the analysis of an earlier experiment conducted in 1992, when noontime OH concentrations of 6 ‐ 8×106 cm−3 were derived in the presence of NOx mixing ratios between 70 ppb at the entrance of the valley and 15 ppb at Schauinsland. Comparison with direct measurements from different studies qualitatively reveals the expected dependence of [OH] on the NO2 mixing ratio with a maximum around 1–2 ppb of NO2.

Comparison of tropospheric NO3 radical measurements by differential optical absorption spectroscopy and matrix isolation electron spin resonance

Despite the importance of NO3 in the nighttime atmosphere only two techniques, Differential Optical Absorption Spectroscopy (DOAS) and Matrix Isolation Electron Spin Resonance (MIESR) have been applied to its detection in ambient air to date. Here we report the results of the first intercomparisons of these techniques in the atmosphere carried out at rural sites in Germany, at Deuselbach in 1983 and near Berlin in 1998. The simultaneously measured NO3 mixing ratios, which were in the range from 9 to 20 ppt and at one measurement near 100 ppt, were in good agreement within the error limits. A regression analysis yields a linear relationship between the DOAS and MIESR data with a correlation coefficient of R = 0.99 and a slope of 0.83±0.03 (1σ error) at a negligible intercept of 0.33±0.73 ppt (1σ error). The deviation from unity is within the total systematic error of both measurement techniques. This result shows the reliability of the two techniques over the past 15 years.

Diurnal and nocturnal distribution of stratospheric NO2 from solar and stellar occultation measurements in the Arctic vortex: Comparison with models and ILAS satellite measurements

NO2 mixing ratio profiles were measured at sunset between 14 and 30 km using the Limb Profile Monitor of the Atmosphere (LPMA) experiment and during the night between 13 and 31 km using the Absorption par Minoritaires Ozone et NOx (AMON) experiment inside the Arctic vortex, both on February 26, 1997. Coinciding profiles measured by the Improved Limb Atmospheric Spectrometer (ILAS) instrument on board ADEOS have been used to check the consistency between the satellite and balloon profiles for NO2, O3, and HNO3. A box model has been used for the photochemical correction of the LPMA NO2 profiles at sunset. The resulting NO2 balloon‐borne profiles of LPMA and AMON are compared to each other after accounting for the day/night photochemical variation using the box model initialized with measurements. The comparisons thus performed show an average difference less than 9% between the two measurements (considered to sample similar air masses) when the box model is initialized with little chlorine activation (i.e., when the major burden of chlorine is stored in ClONO2) for a 1 day integration. The comparison with the Reprobus 3‐D chemistry transport model (CTM) seasonal simulations clearly confirms an underestimation of NO2 by the model below 25 km, in the altitude range where aerosols lead to a complete removal of NOx in the model. Recent updates of rate coefficients for conversion of HNO3 into NO2 only slightly improve the NO2 model results in vortex conditions. These results suggest that a source of NO2 is still lacking in the CTM.

Nitric oxide emissions from a southern African savanna

NO fluxes from soils of a periodically flooded tropical savanna in southern Africa were investigated and modeled. In the laboratory, NO production rates, NO consumption rate constants, NO mixing ratios, relationships between NO emissions and soil temperature and moisture were determined for nutrient‐poor, nutrient‐rich savanna soils and a floodplain soil. The NO production rate and consumption rate constants of the floodplain soil (1.96 ng N s−1 per kilogram of soil and 2.04×10−5 m3 s−1 per kilogram of soil, respectively) were significantly higher than those of the savanna soils (average of 1.28 ng N s−1 per kilogram of soil and 1.47×10−5 m3 s−1 per kilogram of soil, respectively), but there were no significant difference between the nutrient‐rich and nutrient‐poor soils. NO flux rates increased exponentially with soil temperature. NO flux rates increased with soil moisture reaching a maximum near the field capacity (7.5–10% and 31.2% gravimetric water content for savanna and floodplain soils, respectively), after which the NO flux rate declined. These laboratory data were used in a model to estimate field NO flux rates, which were compared with actual field NO emission measurements. The NO model was modified to incorporate NO “pulsing” after the first rains of the season. Correlation between the modeled and field NO fluxes from the nutrient‐poor savanna, nutrient‐rich savanna, and the floodplain soils showed r2 values of 0.91, 0.82, and 0.74, respectively. The NO model was linked with a soil moisture and temperature model to predict annual NO emission estimates from savannas. Annual NO flux from the nutrient‐poor and nutrient‐rich savannas was estimated to be 0.16×10−3 and 0.14×10−3 kg N m−2 yr−1, respectively, which agree well with estimates from other savanna studies.

In situ measurements of the NOx distribution and variability over the eastern North Atlantic

Between 1994 and 1996 the research aircraft Falcon of the Deutsches Zentrum für Luft‐ und Raumfahrt was used to probe the upper troposphere and lowermost stratosphere over the eastern North Atlantic. In situ measurements of NO, NO2, and O3 were performed during 32 flights. The measurements were carried out during five aircraft campaigns in summer and late autumn, respectively, based from Shannon, Ireland, and Prestwick, Scotland. Most of the flights were conducted in the region of the North Atlantic flight corridor. Main objectives of these measurements included the study of the large‐scale distribution of NO and NOx and the development of reliable monthly mean values for different seasons in a region of the atmosphere that is strongly affected by aircraft emissions. Substantial variability of NO and NOx volume mixing ratios was observed in the upper troposphere and lowermost stratosphere. In spite of this variability a significant seasonal dependence was found. NO mean values (averages over all measurements made during one campaign) at altitudes between 10,500 and 11,500 m, where most of the data have been obtained, ranged between about 0.1 and 0.14 ppbv in summer and 0.03 and 0.10 ppbv in late autumn. NO and NOx did not show a significant gradient across the tropopause. The correlation between NOx and O3 in the upper troposphere and lowermost stratosphere was only very weak. The present measurements represent a suitable data set for comparison with predictions of the NOx distribution in the upper troposphere over the eastern North Atlantic by three‐dimensional models. A comparison with the NOx fields simulated with the European Center for Medium‐Range Weather Forecasts, Hamburg Version 3 (ECHAM 3) climate model extended by a simplified NOx chemistry reveals good agreement for summer and autumn conditions.

The UIUC three‐dimensional stratospheric chemical transport model: Description and evaluation of the simulated source gases and ozone

A new University of Illinois at Urbana‐Champaign (UIUC) three‐dimensional stratospheric chemical transport model is presented. The model consists of (1) a hybrid transport routine; (2) a chemical routine that includes the principal gas‐phase and heterogeneous reactions; and (3) the circulation, temperature, and tropospheric humidity fields acquired from the UIUC 24‐layer general circulation model. The model is applied to study chemistry‐transport processes in the stratosphere. The results of an 8‐year steady state simulation with 1995 boundary conditions are analyzed, and the distributions of three source gases, methane, water vapor, and nitrous oxide, and ozone are evaluated in comparison with appropriate UARS measurements. The comparison shows that the model is able to reproduce the main features of the distributions of long‐lived species obtained from satellite measurements, namely, the location of the tropical extremes in the summer hemisphere, the high horizontal gradient in the subtropics, the winter midlatitude mixing zone, and the high‐latitude minimum (or maximum) regions. The model also well captures the observed features of the ozone distribution in the stratosphere, including the intensity and location of the tropical maximum, the depletion in the lower stratosphere of the Southern Hemisphere, and the seasonal variations in the middle stratosphere. The magnitudes of the mixing ratios of the long‐lived species are found to be in reasonable agreement with the observed values, although the model overestimates the N2O mixing ratio over high latitudes and slightly underestimates the H2O mixing ratio in the stratosphere. Also, the simulated ozone mixing ratio is overestimated in the middle stratosphere and underestimated in the upper stratosphere by 5–15%. However, analysis and comparison of the simulated and observed species distributions and tracer‐to‐tracer correlations show a very good overall performance of the model.

The influence of the OH + NO2 + M reaction on the NOy, partitioning in the late Arctic winter 1992/1993 as studied with KASIMA

The northern hemispheric winter 1992/1993 is simulated with the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA). The model is a combination of an off‐line model using analyses from the European Centre for Medium Range Weather Forecasts up to a pressure altitude of 10 hPa and a mechanistic prognostic model on top with an entire altitude range from 10 up to 120 km. The chemistry scheme included in the model represents the gas phase chemistry as well as heterogeneous reactions on polar stratospheric clouds and on liquid sulfate aerosols. We compare model results with global measurements performed by the instruments of the Upper Atmosphere Research Satellite. The model is able to simulate the global distribution of source gases and long‐lived species as well as reactive species in good agreement with the observations. The most significant discrepancies occur inside the vortex where the calculated ozone mixing ratio is overestimated. Below the pressure altitude of 30 hPa inside the vortex the calculated NO2 mixing ratio is underestimated compared to the measurement. A sensitivity study with a new recommendation of the OH + NO2 + M rate constant using the full form of the falloff function for the pressure dependence has been performed. The mixing ratios of the NOy species differ by only about 5% due to the new recommended rate constant inside the vortex with no significant effect on the denoxification.