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1.
Gaseous nitric acid and ammonia were sampled with annular denuders at a forest savannah site from April to December 1987. The analysis of the extract was made spectrophotometrically and by a selective electrode for NO3 – and NH4 +, respectively. Higher concentrations were observed during the vegetation burning period at the end of the dry season. In the studied savannah area, large soil emissions of NO occur during the rainy season, although very low concentrations of HNO3 (0.035 ppb) and also of particulate NO3 – (0.43 g m-3) were observed; it is likely that NOx are lost by fast vertical transport to the upper troposphere. During the nonburning period, the average concentration of NH3 was 2.7 ppb, which is much lower than values given in the literature for the tropical America atmosphere. The concentrations of HNO3 and NH3 were always below the values needed to produce ammonium nitrate aerosols.  相似文献   

2.
Dimethylsulfide (DMS), sulfur dioxide (SO2), methanesulfonate (MSA), nonsea-salt sulfate (nss-SO4 2–), sodium (Na+), ammonium (NH4 +), and nitrate (NO3 –) were determined in samples collected by aircraft over the open ocean in postfrontal maritime air masses off the northwest coast of the United States (3–12 May 1985). Measurements of radon daughter concentrations and isentropic trajectory calculations suggested that these air masses had been over the Pacific for 4–8 days since leaving the Asian continent. The DMS and MSA profiles showed very similar structures, with typical concentrations of 0.3–1.2 and 0.25–0.31 nmol m–3 (STP) respectively in the mixed layer, decreasing to 0.01–0.12 and 0.03–0.13 nmol m–3 (STP) at 3.6 km. These low atmospheric DMS concentrations are consistent with low levels of DMS measured in the surface waters of the northeastern Pacific during the study period.The atmospheric SO2 concentrations always increased with altitude from <0.16–0.25 to 0.44–1.31 nmol m–3 (STP). The nonsea-salt sulfate (ns-SO4 2–) concentrations decreased with altitude in the boundary layer and increased again in the free troposphere. These data suggest that, at least under the conditions prevailing during our flights, the production of SO2 and nss-SO4 2– from DMS oxidation was significant only within the boundary layer and that transport from Asia dominated the sulfur cycle in the free troposphere. The existence of a sea-salt inversion layer was reflected in the profiles of those aerosol components, e.g., Na+ and NO3 –, which were predominantly present as coarse particles. Our results show that long-range transport at mid-tropospheric levels plays an important role in determining the chemical composition of the atmosphere even in apparently remote northern hemispheric regions.  相似文献   

3.
In this study bulk airborne aerosol composition measured by the PILS-IC (integration time of 3 min 24 s) during TRACE-P P3B Flight 10 are used to investigate the ionic chemical composition and mixing state of biomass burning particles. A biomass burning plume, roughly 3–4 days old, moderately influenced by urban pollution aerosols recorded in the Philippine Sea is investigated. Focusing on the fine particle NO3−, SO42−, K+, NH4+, and water-soluble organics, the observed correlations and nearly 1-to-1 molar ratios between K+ and NO3− and between NH4+ and (SO42−+ inferred Organics) suggest the presence of fine-mode KNO3, (NH4)2SO4, and NH4(Organics) aerosols. Under the assumption that these ion pairs existed, and because KNO3 is thermodynamically less favored than K2SO4 in a mixture of NO3−, SO42−, K+, NH4+, and Organic anions, the measurements suggest that aerosols could be composed of biomass burning particles (KNO3) mixed to a large degree externally with the (NH4)2SO4 aerosols. A “closed-mode” thermodynamic aerosol simulation predicts that a degree of external mixing (by SO42− mass) of 60 to 100% is necessary to achieve the observed ionic associations in terms of the existence of KNO3. However, the degree of external mixing is most likely larger than 90%, based on both the presence of KNO3 and the amounts of NH4NO3. Calculations are also shown that the aerosol mixing state significantly impacts particle growth by water condensation/evaporation. In the case of P3B Flight #10, the internal mixture is generally more hygroscopic than the external mixture. This method for estimating particle mixing state from bulk aerosol data is less definitive than single particle analysis, but because the data are quantitative, it may provide a complementary method to single particle chemical analysis.  相似文献   

4.
Vertical distributions of dimethylsulfide (DMS), sulfur dioxide (SO2), aerosol methane-sulfonate (MSA), non-sea-salt sulfate (nss-SO4 2-), and other aerosol ions were measured in maritime air west of Tasmania (Australia) during December 1986. A few cloudwater and rainwater samples were also collected and analyzed for major anions and cations. DMS concentrations in the mixed layer (ML) were typically between 15–60 ppt (parts per trillion, 10–12; 24 ppt=1 nmol m–3 (20°C, 1013 hPa)) and decreased in the free troposphere (FT) to about <1–2.4 ppt at 3 km. One profile study showed elevated DMS concentrations at cloud level consistent with turbulent transport (cloud pumping) of air below convective cloud cells. In another case, a diel variation of DMS was observed in the ML. Our data suggest that meteorological rather than photochemical processes were responsible for this behavior. Based on model calculations we estimate a DMS lifetime in the ML of 0.9 days and a DMS sea-to-air flux of 2–3 mol m–2 d–1. These estimates pertain to early austral summer conditions and southern mid-ocean latitudes. Typical MSA concentrations were 11 ppt in the ML and 4.7–6.8 ppt in the FT. Sulfur-dioxide values were almost constant in the ML and the lower FT within a range of 4–22 ppt between individual flight days. A strong increase of the SO2 concentration in the middle FT (5.3 km) was observed. We estimate the residence time of SO2 in the ML to be about 1 day. Aqueous-phase oxidation in clouds is probably the major removal process for SO2. The corresponding removal rate is estimated to be a factor of 3 larger than the rate of homogeneous oxidation of SO2 by OH. Model calculations suggest that roughly two-thirds of DMS in the ML are converted to SO2 and one-third to MSA. On the other hand, MSA/nss-SO4 2- mole ratios were significantly higher compared to values previously reported for other ocean areas suggesting a relatively higher production of MSA from DMS oxidation over the Southern Ocean. Nss-SO4 2- profiles were mostly parallel to those of MSA, except when air was advected partially from continental areas (Africa, Australia). In contrast to SO2, nss-SO4 2- values decreased significantly in the middle FT. NH4 +/nss-SO4 2- mole ratios indicate that most non-sea-salt sulfate particles in the ML were neutralized by ammonium.  相似文献   

5.
A box model, involving simple heterogeneous reaction processes associated with the production of non-sea-salt sulfate (nss-SO 4 2– ) particles, is used to investigate the oxidation processes of dimethylsulfide (DMS or CH3SCH3) in the marine atmosphere. The model is applied to chemical reactions in the atmospheric surface mixing layer, at intervals of 15 degrees latitude between 60° N and 60° S. Given that the addition reaction of the hydroxyl radical (OH) to the sulfur atom in the DMS molecule is faster at lower temperature than at higher temperature and that it is the predominant pathway for the production of methanesulfonic acid (MSA or CH3SO3H), the results can well explain both the increasing tendency of the molar ratio of MSA to nss-SO 4 2– toward higher latitudes and the uniform distribution with latitude of sulfur dioxide (SO2). The predicted production rate of MSA increases with increasing latitude due to the elevated rate constant of the addition reaction at lower temperature. Since latitudinal distributions of OH concentration and DMS reaction rate with OH are opposite, a uniform production rate of SO2 is realized over the globe. The primary sink of DMS in unpolluted air is caused by the reaction with OH. Reaction of DMS with the nitrate radical (NO3) also reduces DMS concentration but it is less important compared with that of OH. Concentrations of SO2, MSA, and nss-SO 4 2– are almost independent of NO x concentration and radiation field. If dimethylsulfoxide (DMSO or CH3S(O)CH3) is produced by the addition reaction and further converted to sulfuric acid (H2SO4) in an aqueous solution of cloud droplets, the oxidation process of DMSO might be important for the production of aerosol particles containing nss-SO 4 2– at high latitudes.  相似文献   

6.
Gaseous nitrogen compounds (NO x , NO y , NH3, N2O) were measured at ground level in smoke plumes of prescribed savanna fires in Lamto, in the southern Ivory Coast, during the FOS/DECAFE experiment in January 1991. During the flaming phase, the linear regression between [NO x ] and [CO2] (differences in concentration between smoke plumes and atmosheric background) results volumic emission ratio [NO x ]/[CO2]=1.37×10–3 with only slight differences between heading and backing fires. Nearly 90% of the nitrogen oxides are emitted as NO. Average emission ratios of other compounds are: 1.91, 0.047, and 0.145×10–3 for NO y , NH3 and N2O, respectively. The emission ratios obtained during this field experiment are compred with corresponding values measured during former experiments with the same plant species in combustion chambers. An accurate determination of both the biomass actually burned and of the plant nitrogen content, allows an assessment of emission fluxes of N-compounds from Guinean savanna burns. Preliminary results dealing with the influence of fire on biogenic emissions from soils are also reported.  相似文献   

7.
The chemical compositions (Na+, NH4 +, K+, Mg2+, Ca2+, Cl?, NO2 ?, NO3 ?, SO4 2?, HCO3 ?) of wet precipitation and nitrogen isotope compositions δ15N(NH4 +) were studied from January to December 2010 in Wroc?aw (SW Poland). Results of a principle component analysis show that 82 % of the data variability can be explained by three main factors: 1) F1 (40 %) observed during vegetative season (electrical conductivity, HCO3 ?, NO3 ?, NO2 ?, NH4 + and SO4 2?), mainly controlling rainwater mineralization; 2) F2 (26 %) observed during vegetative and heating seasons (K+, Ca2+ and Mg2+), probably representing a combination of two processes: anthropogenic dusts and fertilizers application in agricultural fields, and 3) F3 (16 %) reported mainly during heating season (Na+ and Cl?) probably indicating the influence of marine aerosols. Variations of δ15N(NH4 +) from ?11.5 to 18.5?‰ identify three main pathways for the formation of NH4 +: 1) equilibrium fractionation between NH3 and NH4 +; 2) kinetic exchange between NH3 and NH4 +; 3) NH4 + exchange between atmospheric salts particles and precipitation. The coupled chemical/statistical analysis and δ15N(NH4 +) approach shows that while fossil fuels burning is the main source of NH4 + in precipitation during the heating season, during the vegetative season NH4 + originates from local sewage irrigation fields in Osobowice or agricultural fertilizers.  相似文献   

8.
FOS/DECAFE 91 (Fire of Savannas/Dynamique et Chimie Atmosphérique en Forêt Equatoriale) was the first multidisciplinary experiment organized in Africa to determine gas and aerosol emissions by prescribed savanna fires. The humid savanna of Lamto in Ivory Coast was chosen for its ecological characteristics representative of savannas with a high biomass density (900 g m–2 dry matter). Moreover the vegetation and the climate of Lamto have been studied for more than twenty years. The emission ratios (X/CO2) of the carbon compounds (CO2, CO, NMHC, CH4, PAH, organic acids and aerosols), nitrogen compounds (NOx, N2O, NH3 and soluble aerosols) and sulfur compounds (SO2, COS and aerosols) were experimentally determined by ground and aircraft measurements. To perform this experiment, 4 small plots (100×100 m) and 2 large areas (10×10 km) were prepared and burnt in January 1991 during the period of maximum occurrence of fires in this type of savanna. The detailed ecological study shows that the carbon content of the vegetation is constant within 1% (42 g C for 100 g of vegetal dry matter), the nitrogen content (0.29 g N for 100 g of dry matter) may vary by 10% and the sulfur content (0.05 g S/100 d.m.) by 20%. These variations of the biomass chemical content do not constitute an important factor in the variation of the gas and particle emission levels. With the emission ratios characteristic of humid savanna and flaming conditions (CO/CO2 of 6.1% at the ground and 8% for airborne measurements), we propose a set of new emission factors, taking into account the burning efficiency which is about 80%: 74.4% of the carbon content of the savanna biomass is released to the atmosphere in the form of CO2, 4.6% as CO, 0.2% as CH4, 0.5% as NMHC and 0.7% as aerosols. 17.2% of the nitrogen content of the biomass is released as NOx, 3.5% as N2O, 0.6% as NH3 and 0.5% as soluble aerosols.  相似文献   

9.
Daily rainwater samples collected at Lijiang in 2009 were analyzed for pH, electrical conductivity, major ion (SO4 2?, Cl?, NO3 ?, Na+, Ca2+, Mg2+, and NH4 +) concentrations, and δ18O. The rainwater was alkaline with the volume-weighted mean pH of 6.34 (range: 5.71 to 7.11). Ion concentrations and δ18O during the pre-monsoon period were higher than in the monsoon. Air mass trajectories indicated that water vapor from South Asia was polluted with biomass burning emissions during the pre-monsoon. Precipitation during the monsoon was mainly transported by flow from the Bay of Bengal, and it showed high sea salt ion concentrations. Some precipitation brought by southwest monsoon originated from Burma; it was characterized by low δ18O and low sea salt, indicating that the water vapor from the region was mainly recycled monsoon precipitation. Water vapor from South China contained large quantities of SO4 2?, NO3 ?, and NH4 +. Throughout the study, Ca2+ was the main neutralizing agent. Positive matrix factorization analysis indicated that crustal dust sources contributed the following percentages of the ions Ca2+ 85 %, Mg2+ 75 %, K+ 61 %, NO3 ? 32 % and SO4 2? 21 %. Anthropogenic sources accounted for 79 %, 68 %, and 76 % of the SO4 2?, NO3 ? and NH4 +, respectively; and approximately 93 %, 99 %, and 37 % of the Cl?, Na+, and K+ were from a sea salt source.  相似文献   

10.
We investigated the acidity and concentrations of water-soluble ions in PM2.5 aerosol samples collected from an urban site in Beijing and a rural site in Gucheng, Hebei Province from November 2016 to January 2017 to gain an insight into the formation of secondary inorganic species. The average SO42–, NO3–, and NH4+ concentrations were 8.3, 12.5, and 14.1 μg m–3, respectively, at the urban site and 14.0, 14.2, and 24.2 μg m–3, respectively, at the rural site. The nitrogen and sulfur oxidation ratios in urban Beijing were correlated with relative humidity (with correlation coefficient r = 0.79 and 0.67, respectively) and the aerosol loadings. Based on a parameterization model, we found that the rate constant of the heterogeneous reactions for SO2 on polluted days was about 10 times higher than that on clear days, suggesting that the heterogeneous reactions in the aerosol water played an essential role in haze events. The ISORROPIA II model was used to predict the aerosol pH, which had a mean (range) of 5.0 (4.9–5.2) and 5.3 (4.6–6.3) at the urban and rural site, respectively. Under the conditions with this predicted pH value, oxidation by dissolved NO2 and the hydrolysis of N2O5 may be the major heterogeneous reactions forming SO42– and NO3– in haze. We also analyzed the sensitivity of the aerosol pH to changes in the concentrations of SO42–, NO3–, and NH4+ under haze conditions. The aerosol pH was more sensitive to the SO42– and NH4+ concentrations with opposing trends, than to the NO3– concentrations. The sensitivity of the pH was relatively weak overall, which was attributed to the buffering effect of NH3 partitioning.  相似文献   

11.
This study elucidates the characteristics of ambient PM2.5 (fine) and PM1 (submicron) samples collected between July 2009 and June 2010 in Raipur, India, in terms of water soluble ions, i.e. Na+, NH 4 + , K+, Mg2+, Ca2+, Cl?, NO 3 ? and SO 4 2? . The total number of PM2.5 and PM1 samples collected with eight stage cascade impactor was 120. Annual mean concentrations of PM2.5 and PM1 were 150.9?±?78.6 μg/m3 and 72.5?±?39.0 μg/m3, respectively. The higher particulate matter (PM) mass concentrations during the winter season are essentially due to the increase of biomass burning and temperature inversion. Out of above 8 ions, the most abundant ions were SO 4 2? , NO 3 ? and NH 4 + for both PM2.5 and PM1 aerosols; their average concentrations were 7.86?±?5.86 μg/m3, 3.12?±?2.63 μg/m3 and 1.94?±?1.28 μg/m3 for PM2.5, and 5.61?±?3.79 μg/m3, 1.81?±?1.21 μg/m3 and 1.26?±?0.88 μg/m3 for PM1, respectively. The major secondary species SO 4 2? , NO 3 ? and NH 4 + accounted for 5.81%, 1.88% and 1.40% of the total mass of PM2.5 and 11.10%, 2.68%, and 2.48% of the total mass of PM1, respectively. The source identification was conducted for the ionic species in PM2.5 and PM1 aerosols. The results are discussed by the way of correlations and principal component analysis. Spearman correlation indicated that Cl? and K+ in PM2.5 and PM1 can be originated from similar type of sources. Principal component analysis reveals that there are two major sources (anthropogenic and natural such as soil derived particles) for PM2.5 and PM1 fractions.  相似文献   

12.
A photochemical box model is used to simulate seasonal variations in concentrations of sulfur compounds at latitude 40° S. It is assumed that the hydroxyl radical (OH) addition reaction to sulfur in the dimethyl sulfide (DMS) molecule is the predominant pathway for methanesulfonic acid (MSA) production, and that the rate constant increases as the air temperature decreases. Concentration of the nitrate radical (NO3) is a function of the DMS flux, because the reaction of DMS with NO3 is the most important loss mechanism of NO3. While the diurnally averaged concentration of OH in winter is a factor of about 8 smaller than in summer, due to the weak photolysis process, the diurnally averaged concentration of NO3 in winter is a factor of about 4–5 larger than in summer, due to the decrease of DMS flux. Therefore, at middle and high latitudes in winter, atmospheric DMS is mainly oxidized by the reaction with NO3. The calculated ratio of the MSA to SO2 production rates is smaller in winter than in summer, and the MSA to non-sea-salt sulfate (nssSO4 2-) molar ratio varies seasonally. This result agrees with data on the seasonal variation of the MSA/nssSO4 2- molar ratio obtained at middle and high latitudes. The calculations indicate that during winter the reaction of DMS with NO3 is likely to be a more important sink of NOx (NO+NO2) than the reaction of NO2 with OH, and to serve as a significant pathway of the HNO3 production. If dimethyl sulfoxide (DMSO) is produced through the OH addition reaction and is heterogeneously oxidized in aqueous solutions, half of the nssSO4 2- produced in summer may be through the oxidation process of DMSO. It is necessary to further investigate the oxidation products by the reaction of DMS with OH, and the possibility of the reaction of DMS with NO3 during winter.  相似文献   

13.
During 18–23 July 1990, 31 smoke samples were collected from an aircraft flying at low altitudes through the plumes of tropical savanna fires in the Northern Territory, Australia. The excess (above background) mixing ratios of 17 different trace gases including CO2, CO, CH4, several non-methane hydrocarbons (NMHC), CH3CHO, NO x (– NO + NO2), NH3, N2O, HCN and total unspeciated NMHC and sulphur were measured. Emissionratios relative to excess CO2 and CO, and emissionfactors relative to the fuel carbon, nitrogen or sulphur content are determined for each measured species. The emission ratios and factors determined here for carbon-based gases, NO x , and N2O are in good agreement with those reported from other biomass burning studies. The ammonia data represent the first such measurements from savanna fires, and indicate that NH3 emissions are more than half the strength of NO x emissions. The emissions of NO x , NH3, N2O and HCN together represent only 27% of the volatilised fuel N, and are primarily NO x (16%) and NH3 (9%). Similarly, only 56% of the volatilised fuel S is accounted for by our measurements of total unspeciated sulphur.  相似文献   

14.
A global 3-D Lagrangian chemistry-transport model STOCHEM is used to describe the tropospheric distributions of four components of the secondary atmospheric aerosol: nitrate, sulphate, ammonium and organic compounds. The model describes the detailed chemistry of the formation of the acid precursors from the oxidation of SO2, DMS, NOx, NH3 and terpenes and their uptake into the aerosol. Model results are compared in some detail with the available surface observations. Comparisons are made between the global budgets and burdens found in other modelling studies. The global distributions of the total mass of secondary aerosols have been estimated for the pre-industrial, present day and 2030 emissions and large changes have been estimated in the mass fractions of the different secondary aerosol components.  相似文献   

15.
This study reports comparisonsbetween model simulations, based on current sulfurmechanisms, with the DMS, SO2 and DMSOobservational data reported by Bandy et al.(1996) in their 1994 Christmas Island field study. For both DMS and SO2, the model results werefound to be in excellent agreement with theobservations when the observations were filtered so asto establish a common meteorological environment. Thisfiltered DMS and SO2 data encompassedapproximately half of the total sampled days. Basedon these composite profiles, it was shown thatoxidation of DMS via OH was the dominant pathway withno more than 5 to 15% proceeding through Cl atoms andless than 3% through NO3. This analysis wasbased on an estimated DMS sea-to-air flux of 3.4 ×109 molecs cm-2 s-1. The dominant sourceof BL SO2 was oxidation of DMS, the overallconversion efficiency being evaluated at 0.65 ± 0.15. The major loss of SO2 was deposition to theocean's surface and scavenging by aerosol. Theresulting combined first order k value was estimated at 1.6 × 10-5 s-1. In contrast to the DMSand SO2 simulations, the model under-predictedthe observed DMSO levels by nearly a factor of 50. Although DMSO instrument measurement problems can notbe totally ruled out, the possibility of DMSO sourcesother than gas phase oxidation of DMS must beseriously considered and should be explored in futurestudies.  相似文献   

16.
Secondary aerosol formation was studied at Allahabad in the Indo-Gangetic region during a field campaign called Land Campaign-II in December 2004 (northern winter). Regional source locations of the ionic species in PM10 were identified by using Potential Source Contribution Function (PSCF analysis). On an average, the concentration of water soluble inorganic ions (sum of anions and cations) was 63.2 μgm−3. Amongst the water soluble ions, average NO3− concentration was the highest (25.0 μgm−3) followed by SO42− (15.8 μgm−3) and NH4+ (13.8 μgm−3) concentrations. These species, contributed 87% of the total mass of water soluble species, indicating that most of the water soluble PM10 was composed of NH4NO3 and (NH4)2SO4/NH4HSO4 or (NH4)3H(SO4)2 particles. Further, the concentrations of SO42−, NO3−, and NH4+ aerosols increased at high relative humidity levels up to the deliquescence point (∼63% RH) for salts of these species suggesting that high humidity levels favor the conversion and partitioning of gaseous SO2, NOx, and NH3 to their aerosol phase. Additionally, lowering of ambient temperature as the winter progressed also resulted in an increase of NO3− and NH4+ concentrations, probably due to the semi volatile nature of ammonium nitrate. PSCF analysis identified regions along the Indo-Gangetic Plain (IGP) including Northern and Central Uttar Pradesh, Punjab, Haryana, Northern Pakistan, and parts of Rajasthan as source regions of airborne nitrate. Similar source regions, along with Northeastern Madhya Pradesh were identified for sulfate.  相似文献   

17.
合肥市降水化学组成成分分析   总被引：5，自引：1，他引：4  
为研究合肥市降水的化学组成成分,于2010年4—9月在合肥市国家基本气象站设立了采样点,进行降水的采集,对降水化学组成成分进行了测定,并系统分析了化学组成成分的特点。结果表明:合肥降水中阴离子主要为SO24-,阳离子主要为NH4+和Ca2+,[SO24-]/[NO3-]当量浓度比值范围为1.23～6.33,大部分样本的比值<3,说明酸雨类型以硝硫混合型为主。降水的酸度与单一离子当量浓度的相关性并不明显,应该是受多种离子综合影响的结果,SO24-与NO3-,Ca2+与Mg2+,NH4+与SO24-,NH4+与NO3-均表现出较好的相关性。  相似文献   

18.
Our long-term study provides an unequivocal evidence for near-quantitative (80–100%) depletion of chloride from sea-salts in the marine atmospheric boundary layer (MABL) of tropical Bay of Bengal. During the late NE-monsoon (Jan-Mar), continental outflow from south and south-east Asia dominate the wide-spread dispersal of pollutants over the Bay of Bengal. Among anthropogenic constituents, SO 4 2? (range: 0.6–35 μg m?3) is the most dominant. The non-sea-salt SO 4 2? (nss-SO 4 2? ) constitutes a major fraction (55–65%) of the aerosol water-soluble ionic composition (WSIC), whereas contribution of NO 3 ? is relatively minor. The magnitude of Cl-deficit (with respect to its sea-salt proportion) exhibits linear increase with the excess-nss-SO 4 2? (excess over NH 4 + ). We propose that displacement of HCl from sea-salt aerosols by H2SO4 is a dominant reaction mechanism for the chloride-depletion. These results also suggest that sea-salts could serve as a potential sink for anthropogenic SO2 in the downwind polluted marine environment. Furthermore, loss of hydrogen chloride, representing a large source of reactive chlorine, has implications to the oxidant chemistry in the MABL (oxidation of hydrocarbons and dimethyl sulphide).  相似文献   

19.
A one-dimensional photochemical model was used to explore the role of chlorine atoms in oxidizing methane and other nonmethane hydrocarbons (NMHCs) in the marine troposphere and lower stratosphere. Where appropriate, the model predictions were compared with available measurements. Cl atoms are predicted to be present in the marine troposphere at concentrations of approximately 103 cm-3, mostly as a consequence of the reaction of OH with HCl released from sea spray. Despite this low abundance, our results indicate that 20 to 40% of NMHC oxidation in the troposphere (0–10 km) and 40 to 90% of NMHC oxidation in the lower stratosphere (10–20 km) is caused by Cl atoms. At 15 km, NMHC-Cl reactions account for nearly 80% of the PAN produced.The model was also used to test the longstanding hypothesis that NOCl is an intermediate to HCl formation from sea salt aerosols. It was found that the NOCl concentration required (10 ppt) would be incompatible with field observations of reactive nitrogen and ozone abundance. Chlorine nitrate (ClONO2) and methyl nitrate (CH3ONO2) were shown to be minor components of the total NO y abundance. Heterogeneous reactions that might enhance photolysis of halocarbons or convert ClONO2 to HOCl or Cl2 were determined to be relatively unimportant sources of Cl atoms. Specific and reliable measurements of HCl and other reactive chlorine species are needed to better assess their role in tropospheric chemistry.  相似文献   

20.
In view of the uncertainty of the origin of the secular increase of N2O, we studied heterogeneous processes that contribute to formation of N2O in an environment that comes as close as possible to exhaust conditions containing NO and SO2, among other constituents. The N2O formation was followed using electron capture gas chromatography (ECD-GC). The other reactants and intermediates (SO2, NO, NO2 and HONO) were monitored using gas phase UV-VIS absorption spectroscopy. Experiments were conducted at 298 and 368 K as well as at dry and high humidity (approaching 100% rh) conditions. There is a significant heterogeneous rate of N 2 O formation at conditions that mimic an exhaust plume from combustion processes.The simultaneous presence of NO, SO2, O2 in the gas phase and condensed phase water, either in the bulk liquid or adsorbed state has been confirmed to be necessary for the production of significant levels of N2O. The stoichiometry of the overall reaction is: 2 NO+SO2+H2O N2O+H2SO4. The maximum rate of N2O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. A significant rate of N2O formation at 368 K at 100% rh was also observed in the absence of a bulk substrate. The diffusion of both gas and liquid phase reactants is not rate limiting as the reaction kenetics is dominated by the rate ofN2O formation under the experimental conditions used in this work. The simultaneous presence of high humidity (90–100% rh at 368 K) and bulk condensed phase results in the maximum rate and final yield of N2O approaching 60% and 100% conversion after one hour in the presence of amorphous carbon and fly-ash, respectively.Work performed in partial fulfillment of the requirements of Dr ès Sciences at EPFL.  相似文献   

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