Vertical Distributions Of NO2 Research Articles

O3 sensitivity and vertical distribution of summertime HCHO, NO2, and SO2 in Shihezi, China

Shihezi City has experienced severe ozone pollution. The vertical observation of ozone precursors (NO2 and HCHO) and SO2 in this area was conducted by multi-axis differential optical absorption spectroscopy in summer, 2023. During this experiment, ozone was measured at an average concentration of 94.89 ± 37.46 μg/m3, and it mainly increased when the northerly wind dominated, accompanied by the increasing of its precursors. In addition, SO2 had a relatively high concentration and elevated with the rising ozone concentration, remarkably different from other cities in China. The average vertical distribution of HCHO in the ozone pollution period was a Gaussian type, with the peak value at about 0.2 km. In contrast, the vertical distribution of NO2 and SO2 decreased exponentially with the rise of vertical height, and their concentrations dropped rapidly within 0–1 km. The RFN range of the transitional regime in Shihezi City was [1.26, 2.80]. Given that ozone pollution mostly occurred when RFN was 1–2, the chemical production of near-surface ozone in Shihezi City was controlled by a transition regime. The RFN vertical pattern was Gaussian type and reached a maximal value at 400–600 m. RFN increased at the middle altitude (0.4–1.2 km), and the ozone formation was controlled by the transitional regime. RFN decreased above 1.2 km, and the photochemical ozone formation was mainly controlled by a VOCs-limited regime. This study provides an improved understanding of O3 precursors vertical distribution and O3 formation sensitivity in Shihezi City.

Vertical Characteristics of HCHO and NO2 in Suburban Shanghai: Understanding of Ozone formation sensitivity via Secondary HCHO

Abstract Since ozone (O3) pollution in China is on the rise, vertical distribution of nitrogen dioxide (NO2) and formaldehyde (HCHO) and their role towards O3 formation are critical towards designing O3 control strategies. Seasonal characteristics of aerosols, HCHO and NO2 were investigated over suburban Shanghai using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) from June 2020 to May 2021. For HCHO, the highest average vertical mixing ratios (VMRs) were observed for summer (0.775 ppb), followed by autumn (0.719 ppb), spring (0.718 ppb) and winter (0.672 ppb) at ground level. Seasonal trends in NO2 VMRs were observed to be opposite to that of HCHO with peak values occurring during winter (2.212 ppb) at the ground level followed by autumn (1.881 ppb), spring (1.415 ppb) and summer (1.124 ppb). In vertical, aerosols depicted a Gaussian shape with a higher values between 0.2 - 0.5 km. An exponential decay in NO2 VMRs is observed from 0.0 km to 3.0 km while the integral component of HCHO was found to be concentrated up to 2.0 km particularly during summer months. The vertical distribution of NO2 in different seasons mainly appeared below 0.5 km, while that of HCHO appeared within the range of 0.2-1.0 km. Both aerosols and NO2 tend to be higher during haze days compared to non-haze days while HCHO was found to be higher during clear days owing to high photochemical activity on a clear day. Based on the ratio of HCHO to NO2 (RFN), this study attempted to quantify the vertical characteristics of O3 sensitivity for different seasons. The results indicated that the O3 sensitivity was VOC-limited both at lower (generally, 0.0 - 0.6 km) and higher heights (1.8 - 3.0 km) while some transition limited scenarios were found at middle height (0.7 – 1.7 km) particularly during daytime. The results depicted that secondary HCHO is a crucial factor controlling on O3 sensitivity over Shanghai where enhanced ratio of VOC limited regime was observed with secondary HCHO for all the seasons.

Effects of meteorological conditions on the mixing height of Nitrogen dioxide in China using new-generation geostationary satellite measurements and machine learning

Nitrogen dioxide (NO2) plays a critical role in terms of air quality, human health, ecosystems, and its impact on climate change. While the crucial roles of the vertical structure of NO2 have been acknowledged for some time, there is currently limited knowledge about this aspect in China. The Geostationary Environment Monitoring Spectrometer (GEMS) is the world's first geostationary satellite instrument capable of measuring the hourly columnar amount of NO2. The study presented here introduces the use of mixing height for NO2 in the atmosphere. A thorough examination of spatiotemporal variations in the mixing height of NO2 was conducted using data from both the GEMS and ground-based air quality monitoring networks. A random forest model based on machine learning techniques was utilized to examine how meteorological parameters affect the mixing height of NO2. The results of our study reveal a notable seasonal fluctuation in the mixing height of NO2, with the highest values observed during the summer and the lowest values during the winter. Additionally, there was an increasing diurnal trend from early morning to mid-afternoon. Moreover, the study discovered elevated NO2 mixing heights in the dry regions of northern China. The results also indicated a positive correlation between the mixing height of NO2 and temperature and wind speed, while negative associations were found with relative humidity and air pressure. The machine learning model's predicted NO2 mixing heights were in good agreement with the measurement-based outcomes, as evidenced by a coefficient of determination (R2) value of 0.96 (0.84 for the 10-fold cross-validation). These findings emphasize the noteworthy influence of meteorological variables on the vertical distribution of NO2 in the atmosphere and enhance our comprehension of the three-dimensional variations in NO2.

Nitrogen oxides in the free troposphere: implications for tropospheric oxidants and the interpretation of satellite NO2 measurements

Abstract. Satellite-based retrievals of tropospheric NO2 columns are widely used to infer NOx (≡ NO + NO2) emissions. These retrievals rely on model information for the vertical distribution of NO2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NOx also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NOx in the free troposphere and attribute it to sources. NO2 measurements during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective Clouds and Chemistry (DC3) campaigns over the southeastern U.S. in summer show increasing concentrations in the upper troposphere above 10 km, which are not replicated by the GEOS-Chem, although the model is consistent with the NO measurements. Using concurrent NO, NO2, and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO2 reservoirs such as peroxynitric acid (HNO4) and methyl peroxy nitrate (MPN). We find that NO2 concentrations calculated from the NO measurements and NO–NO2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO2 profiles throughout the troposphere for SEAC4RS and DC3 but overestimates NO2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NOx chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NOx source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The median PSS-inferred tropospheric NO2 column density for the ATom campaign is 1.7 ± 0.44 × 1014 molec. cm−2, and the NO2 column density simulated by the four models is in the range of 1.4–2.4 × 1014 molec. cm−2, implying that the uncertainty from using modeled NO2 tropospheric columns over clean areas in the retrievals for stratosphere–troposphere separation is about 1 × 1014 molec. cm−2. We find from GEOS-Chem that lightning is the main primary NOx source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv (parts per billion by volume) in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO2 columns observed by satellites over the contiguous U.S. increases from 25 ± 11 % in winter to 65 ± 9 % in summer, according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NOx emissions from satellite NO2 column measurements.

Changes in the Column Content and Vertical Distribution of NO2 According to the Results of 30-Year Measurements at the Zvenigorod Scientific Station of the A. M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences

Changes in the Column Content and Vertical Distribution of NO2 According to the Results of 30-Year Measurements at the Zvenigorod Scientific Station of the A. M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences

Vertically increased NO3 radical in the nocturnal boundary layer

In the nocturnal boundary layer, nitrate radical (NO3) has an important contribution to atmospheric chemistry through oxidation of nitrogen oxides and hydrocarbons. Vertical distributions of NO2, O3 and NO3 were measured by four differential optical absorption spectroscopy instruments at meteorological tower in Beijing from June 1 to July 22, 2019. The results show the mean diurnal variations of NO2, O3, and NO3 display a single peak (up to 65.0 ppbv, 196.8 ppbv and 317.5 pptv, respectively) in time. O3 and NO3 mixing ratios generally increased against heights, which is opposite to NO2, suggesting the contribution of O3 to NO3 production at higher altitude. According to the correlation coefficients between NO3 production rates (PNO3) and NO2 or O3 levels, PNO3 was sensitive to NO2 mixing ratio at higher altitude but to O3 near the ground. Averaged NO3 lifetimes (τNO3) of lowest, middle, upper and highest layer intervals were 104, 118, 164 and 213 s, respectively, which indicates τNO3 increase against height and explains why NO3 mixing ratios are larger at higher altitude to some extent. Main control factors of NO3 removal changed from gas-phase reactions to N2O5 hydrolysis with height increase. When relative humidity (RH) exceeded 70% or PM2.5 level exceeded 50 μg·m−3, τNO3 was almost less than 300 s with mixing ratio lower than 70 pptv. The clear negative dependence of τNO3 on RH and PM2.5 reveals the influencing factors on indirect loss. Under polluted conditions, vertical profiles of NO2, O3 and NO3 varied drastically. Stable atmosphere (low nocturnal boundary layer height and thermal inversion), RH level and RH gradient are the main reason for the evident difference in NO3 gradient. Vertically increased NO3 radicals may imply the formation of nitrate aerosols and further increase the nitrate content in high- altitude particulate matter.

GOME-2A retrievals of tropospheric NO<sub>2</sub> in different spectral ranges – influence of penetration depth

Abstract. In this study, we present a novel nitrogen dioxide (NO2) differential optical absorption spectroscopy (DOAS) retrieval in the ultraviolet (UV) spectral range for observations from the Global Ozone Monitoring Instrument 2 on board EUMETSAT's MetOp-A (GOME-2A) satellite. We compare the results to those from an established NO2 retrieval in the visible (vis) spectral range from the same instrument and investigate how differences between the two are linked to the NO2 vertical profile shape in the troposphere.As expected, radiative transfer calculations for satellite geometries show that the sensitivity close to the ground is higher in the vis than in the UV spectral range. Consequently, NO2 slant column densities (SCDs) in the vis are usually higher than in the UV if the NO2 is close to the surface. Therefore, these differences in NO2 SCDs between the two spectral ranges contain information on the vertical distribution of NO2 in the troposphere. We combine these results with radiative transfer calculations and simulated NO2 fields from the TM5-MP chemistry transport model to evaluate the simulated NO2 vertical distribution.We investigate regions representative of both anthropogenic and biomass burning NO2 pollution. Anthropogenic air pollution is mostly located in the boundary layer close to the surface, which is reflected by large differences between UV and vis SCDs of ∼ 60 %. Biomass burning NO2 in contrast is often uplifted into elevated layers above the boundary layer. This is best seen in tropical Africa south of the Equator, where the biomass burning NO2 is well observed in the UV, and the SCD difference between the two spectral ranges is only ∼ 36 %. In tropical Africa north of the Equator, however, the biomass burning NO2 is located closer to the ground, reducing its visibility in the UV.While not enabling a full retrieval of the vertical NO2 profile shape in the troposphere, our results can help to constrain the vertical profile of NO2 in the lower troposphere and, when analysed together with simulated NO2 fields, can help to better interpret the model output.

Evaluation of the use of a commercially available cavity ringdown absorption spectrometer for measuring NO2 in flight, and observations over the Mid-Atlantic States, during DISCOVER-AQ

Real time, atmospheric NO2 column profiles over the Mid-Atlantic states, during the July 2011 National Aeronautics and Space Administration (NASA) Deriving Information on Surface Conditions from Column and Vertically Resolved Observations to Air Quality (DISCOVER AQ) flight campaign, demonstrated that a cavity ring down spectrometer with a light emitting diode light source (LED-CRD) is a suitable technique for detecting NO2 in the boundary layer (BL) and lower free troposphere (LFT). Results from a side-by-side flight between a NASA P3 aircraft and a University of Maryland (UMD) Cessna 402B aircraft show that NO2 concentrations in ambient air from 0.08 nmol /mol (or ppbv) to 1.3 nmol/mol were consistent with NO2 measurements obtained via laser induced fluorescence (LIF) and photolysis followed by NO chemiluminescence (P-CL). The current LED-CRD, commercially available by Los Gatos Research (LGR), includes the modifications added by Castellanos et al. (Rev. Sci. Instrum. 80:113107, 2009) to compensate for baseline drift and humidity through built in zeroing and drying. Because of laser instability in the initial instrument, the laser light source in the Castellanos et al. (Rev. Sci. Instrum. 80:113107, 2009) instrument has been replaced with a light emitting diode. Six independent calibrations demonstrated the instrument’s linearity up through 150 nmol/mol NO2 and excellent stability in calibration coefficient of 1.26 (± 3.7 %). The instrument detection limit is 80 pmol/mol. Aircraft measurements over the Mid-Atlantic are included showing horizontal and vertical distributions of NO2 during air quality episodes. During 23 research flights, NO2 profiles were measured west and generally upwind of the Baltimore/Washington, D.C. area in the morning and east (generally downwind) of the metropolitan region in the afternoon. Column contents (surface to 2,500 m altitude) were remarkably similar (≈3 × 1015 molecules/cm2) indicating that NO2 is widely distributed over the eastern US contributing to the regional (spatial scales of approximately1000 km) nature of smog events.

MAX-DOAS measurements of NO<sub>2</sub>, HCHO and CHOCHO at a rural site in Southern China

Abstract. We performed MAX-DOAS measurements during the PRIDE-PRD2006 campaign in the Pearl River Delta region (PRD), China, for 4 weeks in July 2006 at a site located 60 km north of Guangzhou. The vertical distributions of NO2, HCHO, and CHOCHO were independently retrieved by an automated iteration method. The NO2 mixing ratios measured by MAX-DOAS showed reasonable agreement with the simultaneous, ground based in-situ data. The tropospheric NO2 vertical column densities (VCDs) observed by OMI on board EOS-Aura satellite were higher than with those by MAX-DOAS. The 3-D chemical transport model CMAQ overestimated the NO2 VCDs as well as the surface concentrations by about 65%. From this observation, a reduction of NOx emission strength in CMAQ seems to be necessary in order to well reproduce the NO2 observations. The average mixing ratios of HCHO and CHOCHO were 7 ppb and 0.4 ppb, respectively, higher than in other rural or semirural environments. The high ratio of 0.062 between CHOCHO and HCHO corresponds to the high VOCs reactivity and high HOx turnover rate consistent with other observations during the campaign.

Vertical Distribution of NO2 in an Urban Area: Exposure Risk Assessment in Children

The aim of the study is to perform a potential health risk assessment on children in contracting respiratory symptoms due to inhaling traffic-generated nitrogen dioxide (NO2) in two typical high-rise naturally-ventilated residential building designs (slab and point block) located close to busy major expressways in a tropical climate. A total of six buildings were selected for the study. Ogawa passive samplers (PS-100) were used for NO2 measurements in each building over a period of 5 weeks during the predominant monsoon seasons. Health risk assessment showed children residing at the mid floors of the buildings had the highest health risk regardless of their age .i.e. infants, children (1 year and under), children (8-10 years)compared to those residents residing at the high and low floors. This was expected since the highest concentration of traffic-generated NO2 concentration occurred at the mid floors of the buildings. In a typical floor, children (1 year and under) had the highest followed by children (8-10 years) whilst new born infants had the least potential health risk in contracting respiratory symptoms. The reason might could be new born infants obtain passive immunity from their mothers and in children (1 year and under), the passive immunity fall during this age period as they are developing their very own immunity against respiratory symptoms. Children (8-10 years) had the their potential health risk to respiratory symptoms in between the other two age groups as these children could have developed more immunity against respiratory symptoms compared to the children (1 year and under) but less immunity compared to infants. Based on the mean overall HR values, children living in a slab block has about 1.27 times more risk in contracting a respiratory symptoms due to NO2 inhalation compared to those living in a point block.

Testing and improving OMI DOMINO tropospheric NO2 using observations from the DANDELIONS and INTEX‐B validation campaigns

We present a sensitivity analysis of the tropospheric NO2 retrieval from the Ozone Monitoring Instrument (OMI) using measurements from the Dutch Aerosol and Nitrogen Dioxide Experiments for Validation of OMI and SCIAMACHY (DANDELIONS) and Intercontinental Chemical Transport Experiment‐B (INTEX‐B) campaigns held in 2006. These unique campaigns covered a wide range of pollution conditions and provided detailed information on the vertical distribution of NO2. During the DANDELIONS campaign, tropospheric NO2 profiles were measured with a lidar in a highly polluted region of the Netherlands. During the INTEX‐B campaign, NO2 profiles were measured using laser‐induced fluorescence onboard an aircraft in a range of meteorological and polluted conditions over the Gulf of Mexico and the east Pacific. We present a comparison of measured profiles with a priori profiles used in the OMI tropospheric NO2 retrieval algorithm. We examine how improvements in surface albedo estimates improve the OMI NO2 retrieval. From these comparisons we find that the absolute average change in tropospheric columns retrieved with measured profiles and improved surface albedos is 23% with a standard deviation of 27% and no trend in the improved being larger or smaller than the original. We show that these changes occur in case studies related to pollution in the southeastern United States and pollution outflow in the Gulf of Mexico. We also examine the effects of using improved Mexico City terrain heights on the OMI NO2 product.

Measurements of stratospheric ozone and nitrogen dioxide at Èvora, Portugal

The Spectrometer for Atmospheric TRAcers Monitoring (SPATRAM) has been developed as a result of collaboration between the Geophysics Centre of Évora University (CGE-UE), the Institute for Atmospheric Sciences and Climate of the National Research Council (ISAC-CNR) in Italy and the Italian National Agency for New Technologies, Energy and the Environment (ENEA). SPATRAM is a multipurpose ultraviolet (UV)–visible scanning spectrometer (250–950 nm). It has been installed at the Observatory of the CGE, in Évora, since April 2004 and is currently used to carry out measurements of the zenith scattered radiation, the so-called ‘Passive mode’, to retrieve the vertical content and distribution of some atmospheric tracers such as ozone (O3) and nitrogen dioxide (NO2) using Differential Optical Absorption Spectroscopy (DOAS) methodology. The lack of such measurements taken automatically on a routine basis in southwestern European regions, specifically in Portugal, motivated the effort for its installation and constitutes a major driving force for the present work. For continuous NO2 monitoring the 425–455 nm spectral range is investigated. For O3 retrieval the spectral interval 320–340 nm is chosen. The measurements are in good agreement with the photochemical theory of NO2 (O3), showing maximum values during the summer (spring) and minimum values during the winter (autumn) seasons. Moreover, the application of sophisticated inversion schemes to the output of the DOAS program, using the Air Mass Factor (AMF) matrix as the kernel of the inversion algorithm, allowed for the determination of the vertical distribution of NO2 and O3 atmospheric compounds. In addition, the influence of desert dust aerosol absorption on ozone retrieval is assessed, revealing values of about 3.5% for an aerosol optical depth (AOD) of 1.0, in the case simulated. A correction factor is derived and applied whenever desert dust is detected. The ground-based results obtained for the ozone column content are compared with data from the satellite-borne Ozone Monitoring Instrument (OMI), and the two data sets are found to be in good agreement, with a correlation coefficient of 0.96.

Validation of MIPAS-ENVISAT NO<sub>2</sub> operational data

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument was launched aboard the environmental satellite ENVISAT into its sun-synchronous orbit on 1 March 2002. The short-lived species NO2 is one of the key target products of MIPAS that are operationally retrieved from limb emission spectra measured in the stratosphere and mesosphere. Within the MIPAS validation activities, a large number of independent observations from balloons, satellites and ground-based stations have been compared to European Space Agency (ESA) version 4.61 operational NO2 data comprising the time period from July 2002 until March 2004 where MIPAS measured with full spectral resolution. Comparisons between MIPAS and balloon-borne observations carried out in 2002 and 2003 in the Arctic, at mid-latitudes, and in the tropics show a very good agreement below 40 km altitude with a mean deviation of roughly 3%, virtually without any significant bias. The comparison to ACE satellite observations exhibits only a small negative bias of MIPAS which appears not to be significant. The independent satellite instruments HALOE, SAGE II, and POAM III confirm in common for the spring-summer time period a negative bias of MIPAS in the Arctic and a positive bias in the Antarctic middle and upper stratosphere exceeding frequently the combined systematic error limits. In contrast to the ESA operational processor, the IMK/IAA retrieval code allows accurate inference of NO2 volume mixing ratios under consideration of all important non-LTE processes. Large differences between both retrieval results appear especially at higher altitudes, above about 50 to 55 km. These differences might be explained at least partly by non-LTE under polar winter conditions but not at mid-latitudes. Below this altitude region mean differences between both processors remain within 5% (during night) and up to 10% (during day) under undisturbed (September 2002) conditions and up to 40% under perturbed polar night conditions (February and March 2004). The intercomparison of ground-based NDACC observations shows no significant bias between the FTIR measurements in Kiruna (68° N) and MIPAS in summer 2003 but larger deviations in autumn and winter. The mean deviation over the whole comparison period remains within 10%. A mean negative bias of 15% for MIPAS daytime and 8% for nighttime observations has been determined for UV-vis comparisons over Harestua (60° N). Results of a pole-to-pole comparison of ground-based DOAS/UV-visible sunrise and MIPAS mid-morning column data has shown that the mean agreement in 2003 falls within the accuracy limit of the comparison method. Altogether, it can be indicated that MIPAS NO2 profiles yield valuable information on the vertical distribution of NO2 in the lower and middle stratosphere (below about 45 km) during day and night with an overall accuracy of about 10–20% and a precision of typically 5–15% such that the data are useful for scientific studies. In cases where extremely high NO2 occurs in the mesosphere (polar winter) retrieval results in the lower and middle stratosphere are less accurate than under undisturbed atmospheric conditions.

NO 2 vertical profiles retrieved from ground-based measurements during spring 1999 in the Canadian Arctic

During the 1990s, Arctic stratospheric temperatures were lower and the breakup of the Arctic vortex occurred later than has been observed in earlier decades. These cold winters have been followed by significant ozone loss. A clear identification of all the processes involved in springtime Arctic ozone depletion is complicated by the strong coupling between transport, formation of solid and liquid aerosols, and halogen activation. One of the key chemical species in the photochemistry of ozone is NO 2. As the role of NO 2 is strongly dependent on altitude, it is desirable to know not only the NO 2 total column, but also its vertical distribution. We have a portable UV–Vis grating spectrometer that was deployed at Eureka, Canada (80.1°N, 86.4°W) in spring 1999, 2000, 2001, and 2003, and at Resolute Bay, Canada (74.7°N, 94.6°W) in spring 2002. Eureka is part of the Arctic primary station of the Network for the Detection of Stratospheric Change. Among other species, the spectrometer measures stratospheric NO 2 through observation of sunlight scattered from the zenith sky during twilight. Due to the scattering geometry, the NO 2 slant column increases with solar zenith angle (SZA), making it possible to retrieve information about the vertical distribution of NO 2 from the observed slant column variation with SZA. In this paper, we use the optimal estimation technique with a formal characterization of the errors to retrieve NO 2 concentration profiles from slant column observations made at Eureka during March and April 1999. Such measurements can also be used in the validation of NO 2 profile measurements made by satellite instruments.

Development of delta(15)N stratification of NO(3)(-) in soil profiles.

New evidence, obtained using a robust method for measuring the delta(15)N of NO(3)(-)-N in soil, is consistent with denitrification being the major determinant in the vertical distribution of NO(3)(-)-delta(15)N in soil profiles. These data also suggest that varying moisture regimes result in different effects of soil NO(3)(-)-N leaching on residual whole soil delta(15)N.

The vertical distribution of NO 3 in the atmospheric boundary layer

Recent urban measurements suggest that NO 3 concentrations vary significantly with altitude in the lowest few hundred metres of the atmosphere. Calculations using a one-dimensional boundary layer model show that NO 3 concentrations are small near the ground and increase with altitude to a maximum near the top of the nocturnal boundary layer (NBL). These results show that the NBL is not well mixed, and that where there are surface sources and sinks two-box models of the NBL are inadequate, and surface measurements are not representative and may lead to an underestimate of the oxidising capacity of the atmosphere.

Vertical distribution of nighttime stratospheric NO2 from balloon measurements: Comparison with models

Vertical distributions of NO2 and O3 mixing ratios and the aerosol extinction coefficient were measured by night between 18 and 32 km altitude (9–80 hPa) at mid latitudes using the balloon‐borne instrument AMON on March 24, 1994. The NO2 profile is compared with results of simulations involving the REPROBUS 3D Chemical‐Transport Model and a Lagrangian model in which the ozone mixing ratio and the aerosol surface area are initialized using AMON measurements (other mixing ratios are initialized with REPROBUS). As confirmed by Lidar observations, the surface areas were larger than the monthly and zonally averaged SAGE 2 data available for March 1994 at 45°N. The Lagrangian model shows relatively good agreement with AMON results for pressure higher than 40 hPa and smaller than 15 hPa, but the computed NO2 value is too high between 15 and 40 hPa. This seems to indicate that heterogeneous reactions involving the NOY species in the aerosol layer are still incompletely understood.

Nitrate production in nitrogen-saturated acid forest soils: Vertical distribution and characteristics

In order to study the vertical distribution, nature and pH-dependence of nitrate production in nitrogen-saturated acid forest soils, an incubation experiment was carried out using soil cores from five Dutch forests. Soil cores were left intact during incubation and divided into L, F, H horizons and top 5 cm of the mineral soil before analysis. One of the five forest soils revealed no significant NO 3 production during incubation, while in the other four sites different patterns with respect to vertical distribution of NO 3 production were found. The two coniferous sites snowed a dominance of NO 3 production in the ectorganic horizon (L + F + H), while in the two deciduous forests the production of NO 3 in the ectorganic horizon and in the top 5 cm of the mineral soil contributed about equally to total nitrification. At one of the deciduous sites, nitrification in the 5 cm top layer of the mineral soil was higher than the separate NO 3 productions in the L, F and H layer. Experiments with acetylene as a selective inhibitor indicated that the measured NO 3 production in the F layer of all four nitrifying forest sites was mainly carried out by chemolithotrophic bacteria. In the dominant nitrifying compartments, nitrification was mainly found to be acid-tolerant, which underlines the importance of this type of nitrification in N-saturated forest soils.

Stratospheric NO2 at night from stellar spectra in the 440 Nm region

NO2 was measured in the stratosphere at night by means of a balloon‐borne spectrometer, pointing at a bright rising star, in the 427 to 452 nm range. The vertical distribution of NO2 was deduced using the tangent ray technique between 23 and 38 km. The concentration was (6±1.5)×109 mol.cm−3 at 25 km, passing through a minimum of (2±1)×109 mol.cm−3 at 29 km and rising again up to (4±0.8)×109 mol.cm−3 at 32 km.

Vertical distribution of NO2 in the stratosphere as determined from balloon measurements of solar spectra in the 4500Å region

The stratospheric NO2 mixing ratio profile in the 20‐40 km altitude range is derived from balloon‐borne observations of the solar spectrum in the visible region. By comparisons of high sun and low sun spectra at float altitude (∼40 km), a number of NO2 features are identified. The resulting NO2 profile shows a gradual increase above 20 km to a peak value of 13 ppb(v) near 35 km, followed by a gradual decrease to 10.5 ppb(v) at 40 km.