Field measurements of NO and NO2 emissions from fertilized and unfertilized soils

Field measurements of NO and NO₂ emissions from soils have been performed in Finthen near Mainz (F.R.G.) and in Utrera near Seville (Spain). The applied method employed a flow box coupled with a chemiluminescent NO _x detector allowing the determination of minimum flux rates of 2 g N m^-2 h^-1 for NO and 3 g m^-2 h^-1 for NO₂.The NO and NO₂ flux rates were found to be strongly dependent on soil surface temperatures and showed strong daily variations with maximum values during the early afternoon and minimum values during the early morning. Between the daily variation patterns of NO and NO₂, there was a time lag of about 2 h which seem to be due to the different physico-chemical properties of NO and NO₂. The apparent activation energy of NO emission calculated from the Arrhenius equation ranged between 44 and 103 kJ per mole. The NO and NO₂ emission rates were positively correlated with soil moisture in the upper soil layer.The measurements carried out in August in Finthen clearly indicate the establishment of NO and NO₂ equilibrium mixing ratios which appeared to be on the order of 20 ppbv for NO and 10 ppbv for NO₂. The soil acted as a net sink for ambient air NO and NO₂ mixing ratios higher than the equilibrium values and a net source for NO and NO₂ mixing ratios lower than the equilibrium values. This behaviour as well as the observation of equilibrium mixing ratios clearly indicate that NO and NO₂ are formed and destroyed concurrently in the soil.Average flux rates measured on bare unfertilized soils were about 10 g N m^-2 h^-1 for NO₂ and 8 g N m^-2 h^-1 for NO. The NO and NO₂ flux rates were significantly reduced on plant covered soil plots. In some cases, the flux rates of both gases became negative indicating that the vegetation may act as a sink for atmospheric NO and NO₂.Application of mineral fertilizers increased the NO and NO₂ emission rates. Highest emission rates were observed for urea followed by NH₄Cl, NH₄NO₃ and NaNO₃. The fertilizer loss rates ranged from 0.1% for NaNO₃ to 5.4% for urea. Vegetation cover substantially reduced the fertilizer loss rate.The total NO _x emission from soil is estimated to be 11 Tg N yr^-1. This figure is an upper limit and includes the emission of 7 Tg N yr^-1 from natural unfertilized soils, 2 Tg N yr^-1 from fertilized soils as well as 2 Tg N yr^-1 from animal excreta. Despite its speculative character, this estimation indicates that NO _x emission by soil is important for tropospheric chemistry especially in remote areas where the NO _x production by other sources is comparatively small.     

Domestic Combustion of Biomass Fuels in Developing Countries: A Major Source of Atmospheric Pollutants

Biomass burning has important impacts on atmospheric chemistry and climate. Fires in tropical forests and savannas release large quantities of trace gases and particulate matter. Combustion of biofuels for cooking and heating constitutes a less spectacular but similarly widespread biomass burning activity. To provide the groundwork for a quantification of this source, we determined in rural Zimbabwe the emissions of CO₂, CO, and NO from more than 100 domestic fires fueled by wood, agricultural residues, and dung. The results indicate that, compared to open savanna fires, emissions from domestic fires are shifted towards products of incomplete combustion. A tentative global analysis shows that the source strength of domestic biomass burning is on the order of 1500 Tg CO₂–C yr^–1, 140 Tg CO–C yr^–1, and 2.5 Tg NO–N yr^–1. This represents contributions of about 7 to 20% to the global budget of these gases.     

青藏高原闪电与NO_2的分析及中国内陆地区LNO_x产量的估算

为了进一步认识闪电活动与对流层氮氧化物的关系及更准确地估算中国地区闪电产生的氮氧化物(LNOx)总量,选取人口稀疏,工业生产水平较低的青藏高原地区作为研究区域,基于LIS(Lightning Imaging Sensor)和GOME-2(The Global Ozone Monitoring Experiment-2)卫星探测仪资料,分析了青藏高原中部区域2009年1月至2012年2月闪电与对流层NO2垂直浓度(VCD)月均值资料的时空分布特性和相关性。在此基础上,结合Beirle et al.(2004)的LNOx估算方法,估算了中国内陆地区的LNOx产量。结果表明:青藏高原地区对流层NO2与闪电与在年际趋势、空间分布及季节变化上保持很好的一致性,闪电密度与NO2VCD的线性拟合相关系数为0.84,这表明青藏高原地区NOx受人为源影响小,是研究LNOx的理想区域。基于拟合结果,估算得到中国内陆地区LNOx的年均产量为0.15(0.03~0.38)Tg(N)a-1。这一结论进一步缩小了以往研究中中国地区LNOx产量估算的不确定范围,有助于更清楚地认识闪电在中国气候变化中的重要作用。     

Aqueous Phase Reaction of HNO4: The Impact on Tropospheric Chemistry

We use a global atmospheric chemistry transport model to study the possible influence of aqueous phase reactions of peroxynitric acid (HNO₄) on the concentrations and budgets of NO_x, SO_x, O₃ and H₂O₂. Laboratory studies have shown that the aqueous reaction of HNO_4aq withHSO^– _3aq, and the uni-molecular decomposition of the NO₄ ^– anion to form NO₂ ^– (nitrite) occur on a time scale of about a second. Despite a substantial contribution of the reaction of HSO^– _3aq with HNO_4aq to the overall in-cloud conversion of SO₂ to SO₄ ^2–, a simultaneous decrease of other oxidants (most notably H₂O₂) more than compensated the increase in SO₄ ^2– production. The strongest influence of heterogeneous HNO₄ chemistry was found in the boundary layer, where calculated monthly average ozone concentrations were reduced between 2% to 10% andchanges of H₂O₂ between –20% to +10%compared to a simulation which ignores this reaction. Furthermore, SO₂ was increased by 10% to 20% and SO₄ ^2–depleted by up to 10%. Since the resolution of our global model does not enable a detailed comparison with measurements in polluted regions, it is not possible to verify whether considering heterogeneous HNO₄ reactions results in a substantial improvement of atmospheric chemistry transport models. However, the conversion of HNO₄ in the aqueous phase seems to be efficient enough to warrant further laboratory investigations and more detailed model studies on this topic.     

Impact of Non-Methane Hydrocarbons on Tropospheric Chemistry and the Oxidizing Power of the Global Troposphere: 3-Dimensional Modelling Results

The impact of natural and anthropogenicnon-methane hydrocarbons (NMHC) on troposphericchemistry is investigated with the global,three-dimensional chemistry-transport model MOGUNTIA.This meteorologically simplified model allows theinclusion of a rather detailed scheme to describeNMHC oxidation chemistry. Comparing model resultscalculated with and without NMHC oxidation chemistryindicates that NMHC oxidation adds 40–60% to surfacecarbon monoxide (CO) levels over the continents andslightly less over the oceans. Free tropospheric COlevels increase by 30–60%. The overall yield of COfrom the NMHC mixture considered is calculated to beabout 0.4 CO per C atom. Organic nitrate formationduring NMHC oxidation, and their transport anddecomposition affect the global distribution of NO _x and thereby O₃ production. The impact of theshort-lived NMHC extends over the entire tropospheredue to the formation of longer-lived intermediateslike CO, and various carbonyl and carboxyl compounds.NMHC oxidation almost doubles the net photochemicalproduction of O₃ in the troposphere and leads to20–80% higher O₃ concentration inNO _x -rich boundarylayers, with highest increases over and downwind ofthe industrial and biomass burning regions. Anincrease by 20–30% is calculated for the remotemarine atmosphere. At higher altitudes, smaller, butstill significant increases, in O₃ concentrationsbetween 10 and 60% are calculated, maximizing in thetropics. NO from lightning also enhances the netchemical production of O₃ by about 30%, leading to asimilar increase in the global mean OH radicalconcentration. NMHC oxidation decreases the OH radicalconcentrations in the continental boundary layer withlarge NMHC emissions by up to 20–60%. In the marineboundary layer (MBL) OH levels can increase in someregions by 10–20% depending on season and NO _x levels.However, in most of the MBL OH will decrease by10–20% due to the increase in CO levels by NMHCoxidation chemistry. The large decreases especiallyover the continents strongly reduce the markedcontrasts in OHconcentrations between land and oceanwhich are calculated when only the backgroundchemistry is considered. In the middle troposphere, OHconcentrations are reduced by about 15%, although dueto the growth in CO. The overall effect of thesechanges on the tropospheric lifetime of CH₄ is a 15%increase from 6.5 to 7.4 years. Biogenic hydrocarbonsdominate the impact of NMHC on global troposphericchemistry. Convection of hydrocarbon oxidationproducts: hydrogen peroxides and carbonyl compounds,especially acetone, is the main source of HO _x in theupper troposphere. Convective transport and additionof NO from lightning are important for the O₃ budgetin the free troposphere.     

Estimation of Lightning-Generated NOx in the Mainland of China Based on Cloud-to-Ground Lightning Location Data

Lightning-generated nitrogen oxides(LNO_x) have a major influence on the atmosphere and global climate change.Therefore, it is of great importance to obtain a more accurate estimation of LNO_x. The aim of this study is to provide a reference for the accurate estimation of the total LNO_x in the mainland of China based on cloud-to-ground lightning(CG)location data from 2014 to 2018. The energy of each CG flash was based on the number of return strokes per CG flash, t...     

A global three-dimensional model of the tropospheric sulfur cycle

The tropospheric part of the atmospheric sulfur cycle has been simulated in a global three-dimensional model. The model treats the emission, transport, chemistry, and removal processes for three sulfur components; DMS (dimethyl sulfide), SO₂ and SO₄ ^2– (sulfate). These processes are resolved using an Eulerian transport model, the MOGUNTIA model, with a horizontal resolution of 10° longitude by 10° latitude and with 10 layers in the vertical between the surface and 100 hPa. Advection takes place by climatological monthly mean winds. Transport processes occurring on smaller space and time scales are parameterized as eddy diffusion except for transport in deep convective clouds which is treated separately. The simulations are broadly consistent with observations of concentrations in air and precipitation in and over polluted regions in Europe and North America. Oxidation of DMS by OH radicals together with a global emission of 16 Tg DMS-S yr^–1 from the oceans result in DMS concentrations consistent with observations in the marine boundary layer. The average turn-over times were estimated to be 3, 1.2–1.8, and 3.2–6.1 days for DMS, SO₂, and SO₄ ^2– respectively.     

Sink Potential of Canadian Agricultural Soils

Net greenhouse gas (GHG) emissions from Canadian crop and livestock production were estimated for 1990, 1996 and 2001 and projected to 2008. Net emissions were also estimated for three scenarios (low (L), medium (M) and high (H)) of adoption of sink enhancing practices above the projected 2008 level. Carbon sequestration estimates were based on four sink-enhancing activities: conversion from conventional to zero tillage (ZT), reduced frequency of summerfallow (SF), the conversion of cropland to permanent cover crops (PC), and improved grazing land management (GM). GHG emissions were estimated with the Canadian Economic and Emissions Model for Agriculture (CEEMA). CEEMA estimates levels of production activities within the Canadian agriculture sector and calculates the emissions and removals associated with those levels of activities. The estimates indicate a decline in net emissions from 54 Tg CO₂–Eq yr^–1 in1990 to 52 Tg CO₂–Eq yr^–1 in 2008. Adoption of thesink-enhancing practices above the level projected for 2008 resulted in further declines in emissions to 48 Tg CO₂–Eq yr^–1 (L), 42 TgCO₂–Eq yr^–1 (M) or 36 Tg CO₂–Eq yr^–1 (H). Among thesink-enhancing practices, the conversion from conventional tillage to ZT provided the largest C sequestration potential and net reduction in GHG emissions among the scenarios. Although rates of C sequestration were generally higher for conversion of cropland to PC and adoption of improved GM, those scenarios involved smaller areas of land and therefore less C sequestration. Also, increased areas of PC were associated with an increase in livestock numbers and CH₄ and N₂O emissions from enteric fermentation andmanure, which partially offset the carbon sink. The CEEMA estimates indicate that soil C sinks are a viable option for achieving the UNFCCC objective of protecting and enhancing GHG sinks and reservoirs as a means of reducing GHG emissions (UNFCCC, 1992).     

A three-dimensional model of the global ammonia cycle

Using a three-dimensional (3-D) transport model of the troposphere, we calculated the global distributions of ammonia (NH₃) and ammonium (NH ₄ ⁺ ), taking into account removal of NH₃ on acidic aerosols, in liquid water clouds and by reaction with OH. Our estimated global 10°×10° NH₃ emission inventory of 45 Tg N-NH₃ yr^– provides a reasonable agreement between calculated wet NH ₄ ⁺ deposition and measurements and of measured and modeled NH ₄ ⁺ in aerosols, although in Africa and Asia especially discrepancies exist.NH₃ emissions from natural continental ecosystems were calculated applying a canopy compensation point and oceanic NH₃ emissions were related to those of DMS (dimethylsulfide). In many regions of the earth, the pH found in rain and cloud water can be attributed to acidity derived from NO, SO₂ and DMS emissions and alkalinity from NH₃. In the remote lower troposphere, sulfate aerosols are calculated to be almost neutralized to ammonium sulfate (NH₄)₂SO₄, whereas in the middle and upper troposphere, according to our calculations, the aerosol should be more acidic, as a result of the oxidation of DMS and SO₂ throughout the troposphere and removal of NH₃ on acidic aerosols at lower heights. Although the removal of NH₃ by reaction with the OH radical is relatively slow, the intermediate NH₂ radical can provide a substantial annual N₂O source of 0.9 _–0.4 ^+0.9 Tg, thus contributing byca. 5% to estimated global N₂O production. The oxidation by OH of NH₃ from anthropogenic sources accounts for 10% of the estimated total anthropogenic sources of N₂O. This source was not accounted for in previous studies, and is mainly located in the tropics, which have high NH₃ and OH concentrations. Biomass burning plumes, containing high NO _x and NH₃ concentrations provide favourable conditions for gas phase N₂O production. This source is probably underestimated in this model study, due to the coarse resolution of the 3-D model, and the rather low biomass burning NH₃ and NO _x emissions adopted. The estimate depends heavily on poorly known concentrations of NH₃ (and NO _x ) in the tropics, and uncertainties in the rate constants of the reactions NH₂ + NO₂ N₂O + H₂O (R4), and NH₂ + O₃ NH₂O + O₂ (R7).     

Nitrous oxide emissions from fertilized and unfertilized soils in a subtropical region (Andalusia,Spain)

Field measurements of N₂O emission rates were carried out from August until October 1982 in a subtropical region in Europe, i.e. in Andalusia, Spain. The measurements were performed by using an automatic sampling and analysis technique allowing the semi-continuous determination of N₂O emission rates. The N₂O emission rates were positively correlated to the soil surface temperature and exhibited a diurnal rhythm with maximum rates in the afternoon and minimum rates in the early morning with average values of 1 g N₂O–N/m²/h for the grass lawn and 15 g N₂O–N/m²/h for cultivated land. Application of urea and ammonium nitrate resulted in elevated N₂O emission rates when compared to the unfertilized control. The loss of fertilizer-nitrogen as N₂O was 0.18% for urea and 0.04% for NH₄NO₃ which compares very well with data obtained in a temperate climate (Germany). The total source strength of fertilizer-derived N₂O is estimated to be 0.01–2.2 Tg N₂O–N per year. The N₂O flux from unfertilized natural soils may be as high as 4.5 Tg N₂O–N, indicating that the N₂O emission from soils contributes significantly to the global N₂O budget.     

Potential Soil C Sequestration on U.S. Agricultural Soils

Soil carbon sequestration has been suggested as a means to help mitigate atmospheric CO₂ increases, however there is limited knowledge aboutthe magnitude of the mitigation potential. Field studies across the U.S. provide information on soil C stock changes that result from changes in agricultural management. However, data from such studies are not readily extrapolated to changes at a national scale because soils, climate, and management regimes vary locally and regionally. We used a modified version of the Intergovernmental Panel on Climate Change (IPCC) soil organic C inventory method, together with the National Resources Inventory (NRI) and other data, to estimate agricultural soil C sequestration potential in the conterminous U.S. The IPCC method estimates soil C stock changes associated with changes in land use and/or land management practices. In the U.S., the NRI provides a detailed record of land use and management activities on agricultural land that can be used to implement the IPCC method. We analyzed potential soil C storage from increased adoption of no-till, decreased fallow operations, conversion of highly erodible land to grassland, and increased use of cover crops in annual cropping systems. The results represent potentials that do not explicitly consider the economic feasibility of proposed agricultural production changes, but provide an indication of the biophysical potential of soil C sequestration as a guide to policy makers. Our analysis suggests that U.S. cropland soils have the potential to increase sequestered soil C by an additional 60–70 Tg (10¹²g) C yr^{– 1}, over present rates of 17 Tg C yr^–1(estimated using the IPCC method), with widespread adoption of soil C sequestering management practices. Adoption of no-till on all currently annually cropped area (129Mha) would increase soil C sequestration by 47 Tg C yr^–1. Alternatively, use of no-till on 50% of annual cropland, with reduced tillage practices on the other 50%, would sequester less – about37 Tg C yr^–1. Elimination of summer fallow practices and conversionof highly erodible cropland to perennial grass cover could sequester around 20 and 28Tg C yr^–1, respectively. The soil C sequestration potentialfrom including a winter cover crop on annual cropping systems was estimated at 40Tg C yr^–1. All rates were estimated for a fifteen-yearprojection period, and annual rates of soil C accumulations would be expected to decrease substantially over longer time periods. The total sequestration potential we have estimated for the projection period (83 Tg C yr^–1) represents about 5% of 1999total U.S. CO₂ emissions or nearly double estimated CO₂ emissionsfrom agricultural production (43 Tg C yr^–1). For purposes ofstabilizing or reducing CO₂ emissions, e.g., by 7% of 1990 levels asoriginally called for in the Kyoto Protocol, total potential soil C sequestration would represent 15% of that reduction level from projected 2008 emissions(2008 total greenhouse gas emissions less 93% of 1990 greenhouse gasemissions). Thus, our analysis suggests that agricultural soil C sequestration could play a meaningful, but not predominant, role in helping mitigate greenhouse gas increases.     

Dynamics of Russian Forests and the Carbon Budget in 1961–1998: An Assessment Based on Long-Term Forest Inventory Data

Development trends of Russian forests and their impact on the global carbon budget were assessed at the national level on the basis of long-term forest inventory data (1961–1998). Over this period, vegetation of Russian forest lands are estimated as a carbon sink, with an annual average level of carbon sequestration in vegetational organic matter of 210 ± 30 Tg C · yr^–1 (soil carbon is not considered in this study), of which 153 Tg C · yr^–1 were accumulated in live biomass and 57 Tg C · yr^–1 in dead wood. The temporal variability of the sink is very large; for the five-year averages used in the analysis, the C sequestration varies from about 60 to above 300 Tg C· yr^–1. It is shown that long-term forest inventory data could serve as an important information base for assessing crucial indicators of full carbon accounting of forests.     

A model investigation of the impact of increases in anthropogenic NO x emissions between 1967 and 1980 on tropospheric ozone

Recent observations suggest that the abundance of ozone between 2 and 8 km in the Northern Hemisphere mid-latitudes has increased by about 12% during the period from 1970 to 1981. Earlier estimates were somewhat more conservative suggesting increases at the rate of 7% per decade since the start of regular observations in 1967. Previous photochemical model studies have indicated that tropospheric ozone concentrations would increase with increases in emissions of CO, CH₄ and NO _x . This paper presents an analysis of tropospheric ozone which suggests that a significant portion of its increase may be attributed to the increase in global anthropogenic NO _x emissions during this period while the contribution of CH₄ to the increase is quite small. Two statistical models are presented for estimating annual global anthropogenic emissions of NO _x and are used to derive the trend in the emissions for the years 1966–1980. These show steady increase in the emissions during this interval except for brief periods of leveling off after 1973 and 1978. The impact of this increase in emissions on ozone is estimated by calculations with a onedimensional (latitudinal) model which includes coupled tropospheric photochemistry and diffusive meridional transport. Steady-state photochemical calculations with prescribed NO _x emissions appropriate for 1966 and 1980 indicate an ozone increase of 8–11% in the Northern Hemisphere, a result which is compatible with the rise in ozone suggested by the observations.     

Reactive Nitrogen Compounds at Spitsbergen in the Norwegian Arctic

Simultaneousindependent measurements of NO_y and NO_x(NO_x= NO + NO₂) by high-sensitivitychemiluminescence systems and of PAN (peroxyacetylnitrate) and PPN (peroxypropionyl nitrate) by GC-ECDwere made at Spitsbergen in the Norwegian Arcticduring the first half year of 1994. The average mixingratio of the sum of PAN and PPN (denoted PANs)increased from around 150 pptv in early winter to amaximum of around 500 pptv in late March, whereasepisodic peak values reached 800 pptv. This occurredsimultaneously with a maximum in ozone which increasedto 45–50 ppbv in March–April. The average NO_xmixing ratio was 27 pptv and did not show any cyclethrough the period. The NO_y mixing ratio showeda maximum in late March, while the difference betweenNO_y and PAN decreased during spring. This is anindication of the dominance of PAN in the NO_ybudget in the Arctic, but possible changes in theefficiency of the NO_y converter could alsocontribute to this. Although most PAN in theArctic is believed to be due to long range transport,the observations indicate local loss and formationrates of up to 1–2 pptv h^-1 in April–May.Measurements of carbonyl compounds suggest thatacetaldehyde was the dominant, local precursor ofPAN.Now at 1.     

Simultaneous measurements of peroxy and nitrate radicals at Schauinsland

We present simultaneous field measurements of NO₃ and peroxy radicals made at night in a forested area (Schauinsland, Black Forest, 48° N, 8° N, 1150 ASL), together with measurements of CO, O₃, NO _x , NO _y , and hydrocarbons, as well as meteorological parameters. NO₂, NO₃, HO₂, and (RO₂) radicals are detected with matrix isolation/electron spin resonance (MIESR). NO₃ and HO₂ were found to be present in the range of 0–10 ppt, whilst organic peroxy radicals reached concentrations of 40 ppt. NO₃, RO₂, and HO₂ exhibited strong variations, in contrast to the almost constant values of the longer lived trace gases. The data suggest anticorrelation between NO₃ and RO₂ radical concentrations at night.The measured trace gas set allows the calculation of NO₃ and peroxy radical concentrations, using a chemical box model. From these simulations, it is concluded that the observed anthropogenic hydrocarbons are not sufficient to explain the observed RO₂ concentrations. The chemical budget of both NO₃ and RO₂ radicals can be understood if emissions of monoterpenes are included. The measured HO₂ can only be explained by the model, when NO concentrations at night of around 5 ppt are assumed to be present. The presence of HO₂ radicals implies the presence of hydroxyl radicals at night in concentrations of up to 10⁵ cm^–3.     

中国地区闪电和对流层上部NOx的时空分布特征及其相关性分析

为了解闪电对对流层上部NO_x的贡献,本文利用美国全球水资源和气候中心（GHRC）提供的1995年4月~2005年12月的闪电卫星格点资料及高层大气研究卫星 (UARS) 上的卤素掩星试验装置 (HALOE) 1991年10 月~2005 年11月的观测资料,分析了中国地区闪电与对流层上部NO_x体积混合比的时空分布特征及两者的相关性.结果表明:中国地区闪电和对流层上部的NO_x在季节分布、年际分布和空间分布上保持很好的一致性,闪电是对流层上部NO_x的重要来源;NO极值高度在350 hPa左右,云闪直接产生的NO是极值产生的主要原因,NO₂的极值高度在250 hPa左右,因为闪电产生的NO在传输过程中会被氧化成NO₂并通过雷暴的垂直输送作用抬升到更高高度;强对流活动有利于NO_x的传输,而人类活动产生的NO_x一般较难输送到对流层上部,因此闪电多发区的NO_x极值较大,所在的高度也较高.     

Active Nitrogen in Surface Ozone Depletion Events at Alert during Spring 1998

Measurements of NO_x,y were made at Alert, Nunavut, Canada (82.5° N, 62.3° W) during surface layer ozone depletion events. In spring 1998, depletion events were rare and occurred under variable actinic flux, ice fog, and snowfall conditions. NO_y changed by less than 10% between normal, partially depleted, and nearly completely depleted ozone air masses. The observation of a diurnal variation in NO_x under continuous sunlight supports a source from the snowpack but with rapid conversion to nitrogen reservoirs that are primarily deposited to the surface or airborne ice crystals. It was unclear whether NO_x was reduced or enhanced in different stages of the ozone depletion chemistry because of variations in solar and ambient conditions. Because ozone was depleted from 15–20 ppbv to less than 1 ppbv in just over a day in one event it is apparent that the surface source of NO_x did not grossly inhibit the removal of ozone. In another case ozone was shown to be destroyed to less than the 0.5 ppbv detection limit of the instrument. However, simple model calculations show that the rate of depletion of ozone and its final steady-state abundance depend sensitively on the strength of the surface source of NO_x due to competition from ozone production involving NO_x and peroxy radicals. The behavior of the NO/NO₂ ratio was qualitatively consistent with enhanced BrO during the period of active ozone destruction. The model is also used to emphasize that the diurnal partitioning of BrO_x during ozone depletion events is sensitive to even sub ppbv variations in O₃.     

Automobile Emissions of Acetonitrile: Assessment of its Contribution to the Global Source

Based on an estimated global fuel consumption of 2.57 × 10¹⁵g(C) y^–1 and the assumption thatthe fossil fuel burned in Austria is globallyrepresentative, an upper limit of 0.021 (+150%, –50%)Tg y^–1 for global CH₃CN emission dueto fossil fuel burning was obtained from the relativeenhancement of the concentrations of toluene, benzene,and acetonitrile (methyl cyanide) during strong,short-term traffic pollution. This is less than 6% ofthe total global budget of CH₃CN, which is dominatedby an emission rate of 0.8 Tg y^–1 from biomassburning.     

A Three-Dimensional Global Model Study of Carbonyl Sulfide in the Troposphere and the Lower Stratosphere

Global distributions of carbonyl sulfide and carbon disulfide have been calculated with a three-dimensional global model of the atmospheric general circulation (ECHAM). The model calculates a global sink strength for carbonyl sulfide of 0.3 Tg S yr^-1, with vegetation uptake being the largest sink. With this sink strength, the sources have to be close to the lower limit of the present estimate in the literature. The calculated mixing ratios are higher in the Southern Hemisphere than in the Northern Hemisphere. This interhemispheric gradient is the opposite of what is observed demonstrating that the present knowledge of the distribution of sinks and sources is not fully adequate. The model calculations support the idea that the open oceans could act as a net sink of carbonyl sulfide. The calculated stratospheric photolysis of carbonyl sulfide constitutes about 4% of the total sink of carbonyl sulfide. A stratospheric production of sulfate from carbonyl sulfide of 0.013 Tg S yr^-1 is obtained, which is 3 to 12 times less than what is needed to maintain the stratospheric sulfate aerosol layer. Although these results are associated with uncertainties, due to the low upper boundary and coarse vertical resolution of the model, they support recent findings of a low stratospheric production of sulfate from carbonyl sulfide. Instead, sulfur dioxide transported from the troposphere is calculated to be the most important precursor for the stratospheric sulfate aerosol layer.     

Latitudinal variation of measured NO2 photolysis frequencies over the Atlantic Ocean between 50° N and 30° S

Using a filter radiometer, the meridional profile of the NO₂ photolysis frequency, J(NO₂), was measured between 50° N and 30° S during the cruise ANTVII/1 September/October 1988 of the research vessel Polarstern on the Atlantic Ocean. Simultaneously, global broadband irradiance and acrosol were monitored. Clean marine background air with low aerosol loads (b _sp=(1–2)×10^-5 m^-1) was encountered at the latitudes 25° N–30° N and 18° S–27° S, respectively. Under these conditions and an almost cloudless sky J(NO₂) reached 7.3×10^-3 s^-1 (2 sr) for a zenith angle of 30°. Between 30° N and 30° S, the latitudinal variation of the J(NO₂) noontime maxima was less than ± 10%, while the mean value at noon was 7.8×10^-3 s^-1. For the set of all data between 50° N and 30° S, a nearly linear correlation of J(NO₂) vs. global broadland irradiance was found. The slope of (8.24±0.03)×10^-5 s^-1/mW cm^-2 agrees within 10% with observations in Jülich (51° N, 6.2° E).