Mass transfer at the air/water interface: Mass accommodation coefficients of SO2, HNO3, NO2 and NH3

We present here experimental determinations of mass accommodation coefficients using a low pressure tube reactor in which monodispersed droplets, generated by a vibrating orifice, are brought into contact with known amounts of trace gases. The uptake of the gases and the accommodation coefficient are determined by chemical analysis of the aqueous phase.We report in this article measurements of _exp=(6.0±0.8)×10^–2 at 298 K and with a total pressure of 38 Torr for SO₂, (5.0±1.0)×10^–2 at 297 K and total pressure of 52 Torr for HNO₃, (1.5±0.6)×10^–3 at 298 K and total pressure of 50 Torr for NO₂, (2.4±1.0)×10^–2 at 290 K and total pressure of 70 Torr for NH₃.These values are corrected for mass transport limitations in the gas phase leading to =(1.3±0.1)×10^–1 (298 K) for SO₂, (1.1±0.1)×10^–1 (298 K) for HNO₃, (9.7±0.9)×10^–2 (290 K) for NH₃, (1.5±0.8)×10^–3 (298 K) for NO₂ but this last value should not be considered as the true value of for NO₂ because of possible chemical interferences.Results are discussed in terms of experimental conditions which determine the presence of limitations on the mass transport rates of gaseous species into an aqueous phase, which permits the correction of the experimental values.     

In-cloud scavenging of gases and aerosols at a mountain site in Central Switzerland

An in-cloud scavenging case study of the major ions (NH₄ ⁺, SO₄ ^2- and NO₃ ^-) determining the cloudwater composition at a mountain site (1620 m.a.s.l.) is presented. A comparison between in-cloud measurements of the cloudwater composition, liquid water content, gas concentrations and aerosol concentrations and pre-cloud gas and aerosol concentrations yields the following results. Cloudwater concentrations resulted from scavenging of about half of the available NH₃, aerosol NH₄ ⁺, aerosol NO₃ ^-, and aerosol SO₄ ^2-. Approximately a third of the SO₂ was scavenged by the cloudwater and oxidized to SO₄ ^2-. Cloud acidity during the first two hours of cloud interception (pH 3.24) was determined mostly by the scavenged gases (NH₃, SO₂, and HNO₃); aerosol contributions to the acidity were found to be small. Observations of gas and aerosol concentrations at three elevations prior to several winter precipitation events indicated that NH₃ concentrations are typically half (12–80 %) of the total (gas and aerosol) N (-III) concentrations. HNO₃ typically is present at much lower concentrations (1–55 %) than aerosol NO₃ ^-. Concentrations of SO₂ are a substantial component of total sulfur, with concentrations averaging 60 % (14–76 %) of the total S (IV and VI).     

Gaseous and Particulate Oxidized and Reduced Nitrogen Species in the Atmospheric Boundary Layer in Scandinavia in Spring

Observations of the concentration of several nitrogen containing compounds at five rural Scandinavian sites during March–June 1993 are reported. Total nitrate (NO ₃ ^- + HNO₃) and total ammonium (NH ₄ ⁺ + NH₃) were measured by denuder and filter pack. In general the methods agree well. At all sites the particulate fraction dominated, with the largest fraction of NO ₃ ^- and the lowest of NH ₄ ⁺ at the sites which were closest to the emission sources. The fraction of NO ₃ ^- of total nitrate increased with increasing NO₂ concentrations, indicating that the nighttime conversion of NO₂ to NO ₃ ^- is an important route of formation for NO ₃ ^- . A positive correlation was found between HNO₃ and O₃ in June at all sites, while no correlation was found early in the spring. Model calculations were made with a lagrangian boundary layer photooxidant model for the whole period, and compared to the measured concentrations. The calculated ratio between mean observed and modelled daily maximum concentrations of ozone over the measurement period were within +/–10% at all sites. The models ability to describe the daily ozone maximum concentration was satisfactory with an average deviation of 19–22% from the observed concentrations. HNO₃ was underestimated by over 50% at all sites except the one closest to the emission sources. The correlation between modelled and observed concentrations was generally best for the sites with shortest transport distance from the sources of emission.     

Particle-size distribution of inorganic water soluble ions in the venezuelan savannah atmosphere during burning and nonburning periods

The results presented are the first complete analysis of inorganic soluble ions in a tropical savannah region. Atmospheric particles were collected in six rural Venezuelan savannah sites. Concentrations and size distribution of NO₃ ^–, SO₄ ^2-, CI^–, PO₄ ^3-, NH₄ ⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺ were determined in samples collected with Hi Vol samplers equipped with five-stage cascade impactors. Concentrations were higher in the dry season, with a maximum during the burning periods. Using Na⁺ as a reference, the results show a deficit of Cl^– and, with the exception of Mg²⁺, an enrichment of all other ions with respect to marine aerosols. Significant variations were observed in particle-size distribution between different periods. Various pairs of ions present similar size distributions: SO₄ ^2- and NH₄ ⁺; Cl^– and Na⁺; PO₄ ^3- and K⁺; Ca²⁺, Mg²⁺ and NO₃ ^–; indicating that the ions were produced by the same source and/or were involved in similar atmospheric processes. Possible primary sources, the gas-to-particle atmospheric process, environmental implication of long-range transport of nutrients during dry seasons, etc., are discussed.     

Modelling of the vertical fluxes of nitric acid,ammonia, and ammonium nitrate

Results from numerical investigations regarding the exchange of HNO₃, NH₃, and NH₄NO₃ between the atmosphere and the biosphere are presented. The investigations were performed with a modified inferential method which is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these nitrogen compounds. This modified inferential method calculates the micrometeorological quantities (such as the friction velocity and the fluxes of sensible and latent heat), the height-invariant fluxes of the composed chemically conservative trace species with group concentrationsc ₁=[HNO₃]+[NH₄NO₃] (total nitrate),c ₂=[NH₃]+[NH₄NO₃] (total ammonia), andc ₃=[HNO₃]-[NH₃] as well as the fluxes of the individual nitrogen compounds. The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The modified inferential method requires only the data of wind velocity, temperature, humidity and concentrations (HNO₃, NH₃, and NH₄NO₃) measured at a reference height by stations of a monitoring network.     

A modified profile method for determining the vertical fluxes of NO,NO2, ozone,and HNO3 in the atmospheric surface layer

A modified profile method for determining the vertical deposition (or/and exhalation) fluxes of NO, NO₂, ozone, and HNO₃ in the atmospheric surface layer is presented. This method is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these trace gases. The analysis (aerodynamic profile method) includes a detailed determination of the micrometeorological quantities (such as the friction velocity, the fluxes of sensible and latent heat, the roughness length and the zero plane displacement), and of the height-invariant fluxes of the composed chemically conservative trace gases with group concentrations c ₁=[NO]+[NO₂]+[HNO₃], c ₂=[NO₂]+[O₃]+3/2·[HNO₃], and c ₃=[NO]–[O₃]–1/2·[HNO₃]. The fluxes of the individual species are finally determined by the numerical solution of a system of coupled nonlinear ordinary differential equations for the concentrations of ozone and HNO₃ (decoding method). The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The model requires only the vertical profile data of wind velocity, temperature and humidity and concentrations of NO, NO₂, ozone, and HNO₃.The method has been applied to vertical profile data obtained at Jülich (September 1984) and collected in the BIATEX joint field experiment LOVENOX (Halvergate, U.K., September 1989).     

The effects of below-cloud aerosol on the acidification process of rain

Using a model of the acidification process of rain, we calculate and analyze the effects and contributions of a below-cloud aerosol in its different concentrations and acidities on the pH and ion components of rain (SO ₄ ^2– , H⁺, NO ₃ ^– , NH ₄ ⁺ , etc.) under the conditions of different concentrations of pollution gases. The results show that the aerosol has an acidification or alkalization effect on the rain which changes the pHs of rain and aerosol. As acidifying pollution gas concentrations (SO₂, HNO₃) are low, the acid aerosol has important effects on the pH and H⁺ of rain, but as the gas concentrations are high, the acid aerosol has very little effect. The alkalizing aerosol makes the pH of rain increase by between 0.3 and 0.5 and neutralizes about 60% of H⁺ in the rain. As alkalizing pollution gas NH₃ exists, the acid aerosol has important effects on the pH and H⁺ of rain. But the alkalizing aerosol has very little effect, especially as the NH₃ concentration is high. The percentage contribution of the aerosol to SO ₄ ^2– in rain is generally 7–15%, the contribution of the aerosol to NO ₃ ^– is nearly the same as that of HNO₃=1 ppb, and the contribution of the aerosol to NH ₄ ⁺ is nearly the same as that of NH₃, from 5 to 7 ppb, and is an important source of NH ₄ ⁺ in rain. Finally, according to the actual conditions of typical regions in the south and north of China (Chongqing and Beijing), we analyze the effects of aerosol and pollution gases on the ion components of rain.     

Vertical distribution of dimethylsulfide,sulfur dioxide,aerosol ions,and radon over the Northeast Pacific Ocean

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.     

Laboratory spectroscopic studies of atmospherically important radicals using fourier transform spectroscopy

This paper describes laboratory experiments designed to obtain the infrared spectra of some atmospherically important radical species and related compounds. A Fourier transform spectrometer was used that was capable of yielding resolutions as great as 0.0024 cm^-1, and optical paths of up to 512 m were employed. The objective of the experiments was to obtain the spectra for subsequent application to remote sounding measurements in the atmosphere.Radicals were generated by a variety of chemical reactions involving atoms or other highly reactive precursors. Spectra of the ₃ band of NO₃, at ca. 1500 cm^-1, were obtained with up to 0.005 cm^-1 resolution using the reaction between NO₂ and O₃ to produce the radical. The most satisfactory source of ClO was found to be the reaction between Cl and O₃, and the (1-0) vibration-rotation band in the region 829–880 cm^-1 was recorded at a resolution of 0.02 cm^-1. We were unable to observe infrared absorption of HO₂ with any of the radical sources that we tested. High-resolution survey spectra were obtained of compounds used as reactants, or formed as side-products in the radical-generating processes. These compounds included N₂O₅, HNO₃, ClONO₂, FNO₂, Cl₂O, HO₂NO₂, and probably FO₂.The ability to monitor concentrations of the NO₃ radical in the visible region of the spectrum as well as the concentrations of reactants and other products in the infrared region allowed us to undertake a study of the time-dependent interactions occurring when NO₂ reacts with O₃. The results indicate the importance of heterogeneous processes, especially when traces of water are present, and lend credence to suggestions that heterogeneous mechanisms in the NO₃–N₂O₅–H₂O system might be a viable source of HNO₃ in the atmosphere.     

Carboxylic Acids: Seasonal Variation and Relation to Chemical and Meteorological Parameters

Formic and acetic acid measured as daily averages in 1993–1994show equal and highly correlated concentrations up to 3 ppb in the summer(May–August). In the winter (October–March) the formicacid/acetic acid ratio was 0.6 and the formic acid concentrations wereusually below 1 ppb. In winter the carboxylic acids correlate withO_x, NO_y, SO₂ and particulatesulphur. The main sources are suggested to be ozonolysis of anthropogenicalkenes and reactions between peroxyacetyl radicals and RO₂radicals. In spring–summer the carboxylic acids correlate withO₃, O_x, HNO₃, PAN,NO_y, SO₂, particulate sulphur and temperature.In addition to the sources of the winter a contribution from ozonolysis ofbiogenic alkenes is likely. Quite similar formic acid/acetic acid ratios forall wind directions suggest that the source(s) are atmospheric oxidationprocesses distributed over large areas. The highest concentrations occurringfor winds from east to south and the correlation with e.g., particulatesulphur indicate chemical production in polluted air masses during longrange transport.     

On the Exchange of NO3 Radicals with Aqueous Solutions: Solubility and Sticking Coefficient

The exchange of NO₃ radicals with the aqueous-phase was investigated at room temperature (293 K) in a series of wetted denuders. From these experiments, the uptake coefficient of NO₃ was determined on 0.1 M NaCl solutions and was found to be (NO₃) 2 × 10^-3 in good agreement with recent studies. The Henry coefficient of NO₃ was estimated to be K_H(NO₃) = 1.8 M · atm^-1, with a (2) uncertainty of ±3 M · atm^-1. From the upper limit for the Henry coefficient (K_H = 5 M · atm^-1) and available thermodynamic data, the redox potential of dissolved NO₃/NO ₃ ^– is estimated to be in the range of 2.3 to 2.5 V. This range is at the lower boundary of earlier estimates. The results are discussed in the light of a recent publication. Based on our data and a model of the transport and chemistry in the liquid film, an upper limit is derived for the product of the Henry coefficient K_H and the rate coefficient k ₁₀ of the potential reaction NO₃ + H₂O HNO₃ + OH. For K_H = 0.6 M · atm^-1, we find k ₁₀ < 0.05 s^-1 · atm^-1, i.e., about 100 times smaller than what was suggested by Rudich and co-workers. Because of its small solubility, heterogeneous removal of NO₃ is only important under conditions where the dissolved NO₃ is removed quickly from equilibrium, for example by reactions with Cl^– or HSO ₃ ^– ions in the liquid-phase. Otherwise, heterogenous removal should mainly proceed via N₂O₅.     

Studies on Intercorrelation between Ions Co-Occurring in Precipitation in the Gdańsk-Sopot-Gdynia Tricity (Poland)

The paper presents monitoring results and environmental pollution assessment for the Gdask-Sopot-Gdynia Tricity (Poland), based onanalysis of precipitation. Precipitation samples were collected over a period of 12 months (January–December 1998) at ten locations in the Tricity. The following selected ions were determined in the samples:SO₄ ^2–, F^–, Cl^–, NO₃ ^–,PO₄ ^3–, NH₄ ⁺, Na⁺,Mg²⁺, Ca²⁺, K⁺. The results were subjected to full statistical evaluation. Values of the parameters determined were correlated with each other. An attempt was made to explain co-occurrences of certain ions and the significance of their mutual effects. Pollutant concentrations and loads in precipitation were also correlated with data on wind direction and temperature in the region.Deposition of pollutants was very high in spring due to the prevailing air circulation patterns and low temperatures. Analysis of the correlations between co-occurring ions confirmed the significant impact of the location (sea coast) on the composition of rain water. Ionic ratios in rainwater were similar to those observed for sea salt samples. In addition, heavy traffic was most probably responsible for high concentrations of various forms of nitrogen and sulphates in the vicinity of major highways.     

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.     

In Situ Trace Gas and Particle Measurements in the Summer Lower Stratosphere during STREAM II: Implications for O3 Production

In situ aircraft measurements of O₃, CO,HNO₃, and aerosol particles are presented,performed over the North Sea region in the summerlower stratosphere during the STREAM II campaign(Stratosphere Troposphere Experiments by AircraftMeasurements) in July 1994. Occasionally, high COconcentrations of 200-300 pbbv were measured in thelowermost stratosphere, together with relatively highHNO₃ concentrations up to 1.6 ppbv. The particlenumber concentration (at standard pressure andtemperature) between 0.018-1 m decreased acrossthe tropopause, from >1000 cm^-3 in the uppertroposphere to <500 cm^-3 in the lowermoststratosphere. Since the CO sources are found in thetroposphere, the elevated CO mixing ratios areattributed to mixing of polluted tropospheric air intothe lowermost extratropical stratosphere. Further wehave used a chemical model to illustrate that nitrogenoxide reservoir species (mainly HNO₃) determinethe availability of NO_x (=NO + NO₂) andtherefore largely control the total net O₃production in the lower kilometers of thestratosphere. Model simulations, applying additionalNO_x perturbations from aircraft, show that theO₃ production efficiency of NO_x is smallerthan previously assumed, under conditions withrelatively high HNO₃ mixing ratios, as observedduring STREAM II. The model simulations furthersuggest a relatively high O₃ productionefficiency from CO oxidation, as a result of therelatively high ambient HNO₃ and NO_xconcentrations, implying that upward transport of COrich air enhances O₃ production in the lowermoststratosphere. Analysis of the measurements and themodel calculations suggest that the lowermoststratosphere is a transition region in which thechemistry deviates from both the upper troposphere andlower stratosphere.     

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).     

Atmospheric deposition of nitrogen, sulfur and chloride in Thessaloniki, Greece

In the present study, the wet and dry depositions of particulate NO₃⁻, SO₄²⁻, Cl⁻ and NH₄⁺ were measured using a wet/dry sampler as a surrogate surface. Gas phase compounds of nitrogen, sulfur and chloride (HNO₃, NH₃, SO₂ and HCl) were measured by an annular denuder system (ADS) equipped with a back up filter for the collection of particles with diameter ≤ 5 μm. Ambient concentrations of NO, NO₂ and SO₂ were also taken into consideration. Sampling was conducted at an urban site in the center of the city of Thessaloniki, northern Greece. The presence of the aerosol species was examined by cold/warm period and the possible compounds in dry deposits were also considered. Dry deposition fluxes were found to be well correlated with ambient particle concentrations in order to be used for the calculation of particle deposition velocity. Average particulate deposition velocities calculated were 0.36, 0.20, 0.20 and 0.10 cm s^− 1 for Cl⁻, NO₃⁻, SO₄²⁻ and NH₄⁺, respectively. Total dry deposition fluxes (gas and particles) were estimated at 3.24 kg ha^− 1 year^− 1 for chloride (HCl + p-Cl), 9.97 kg ha^− 1 year^− 1 for nitrogen oxidized (NO + NO₂ + HNO₃ + p-NO₃), 5.32 kg ha^− 1 year^− 1 for nitrogen reduced (NH₃ + p-NH₄) and 15.77 kg ha^− 1 year^− 1 for sulfur (SO₂ + p-SO₄). 70–90% total dry deposition was due to gaseous species deposition. The contribution of dry deposition to the total (wet + dry) was at the level of 60–70% for sulfur and nitrogen (oxidized and reduced), whereas dry chloride deposition contributed 35% to the total. The dry-to-wet deposition ratio of all the studied species was found to be significantly associated with the precipitation amount, with nitrogen species being better and higher correlated. Wet, dry and total depositions measured in Thessaloniki, were compared with other countries of Europe, US and Asia.     

Particulate matter and gaseous pollutants during a tropical storm and air pollution episode in Southern Taiwan

Gaseous pollutants and PM_2.5 aerosol particles were investigated during a tropical storm and an air pollution episode in southern Taiwan. Field sampling and chemical analysis of particulate matter and gaseous pollutants were conducted in Daliao and Tzouying in the Kaohsiung area, using a denuder-filter pack system during the period of 22 October to 3 November 2004. Sulfate, nitrate and ammonium were the major ionic species in the PM_2.5, accounting for 46 and 39% of the PM_2.5 for Daliao and Tzouying, respectively. Higher PM_2.5, Cl^?, NO₃^? and NH₄⁺, HNO₂ and NH₃ concentrations were found at night in both stations, whereas higher HNO₃ was found during the day. In general, higher PM_2.5, HCl, NH₃, SO₂, Cl^?, NO₃^?, SO₄^2? and NH₄⁺ concentrations were found in Daliao. The synoptic weather during the experiment was first influenced by Typhoon NOCK-TEN, which resulted in the pollutant concentrations decreasing by about two-thirds. After the tropical thunderstorm system passed, the ambient air quality returned to the previous condition in 12 to 24 h. When there was a strong subsidence accompanied by a high-pressure system, a more stable environment with lower wind speed and mixing height resulted in higher PM_2.5, as well as HNO₂, NH₃, SO₄^2?, Cl^?, NO₃^?, NH₄⁺ and K⁺ concentrations during the episode days. The rainfall is mainly a scavenger of air pollutants in this study, and the stable atmospheric system and the high emission loading are the major reasons for high air pollutant concentrations.     

Rainwater Chemistry and Wet Deposition over the Equatorial Forested Ecosystem of Zoétélé (Cameroon)

Within the framework of IDAF (IGAC DEBITS AFRICA: International GlobalAtmospheric Chemistry/DEposition of Biogeochemically Important TraceSpecies/Africa) network, data analysis is realised on precipitation chemical composition collected in Zoétélé, in Southern Cameroon. This station, located atabout 200 km from the Atlantic Ocean, is representative of a so-called `Evergreen Equatorial Forest' ecosystem. An automatic wet-only precipitation collector was operated at the station from 1996 to 2000. The rainfall regime, associated with eastward advection of moist and cool monsoon air masses, amounts to an average of 1700 mm/year. Inorganic and organic content of the precipitation were determined by IC in 234 rainfall events, representing a total 4,583 mm of rainfall from an overall of 7,100 mm.The mean annual precipitation chemistry and wet deposition fluxes characteristic of an African equatorial forest are quantified. Typical atmospheric gases and particles sources influence the precipitation chemical content and the associated deposition of chemical species. Indeed, hydrogen concentration is the highest (12.0 eq.L^–1) of the IDAF measurements, leading to acid rains with a low mean pH 4.92. The mineral species are dominated by nitrogenous compounds (NH₄ ⁺:10.5 and NO₃ ^–: 6.9 eq.L^–1), Ca²⁺ (8.9 eq.L^–1) and SO₄ ^{2 –} 5.1 eq.L^–1. Relationship between Ca^{2 +} and SO₄ ^{2 –} indicated aterrigeneous particulate source and an additional SO₄ ^{2 –} contributionprobably due to swamps and volcano emissions. Na⁺ and Cl^–concentrations, around 4.0 eq.L^–1, seem very low for this site,accounting for the marine source. Besides, strong correlations between NH₄ ⁺/K⁺/Cl^– indicate the biomass burning originof these species. Accordingly, precipitation chemistry in Zoétéléis influenced by three major sources: biogenic emissions from soil and forest ecosystems, biomass burning from savannah, and terrigenous signature from particles emissions of arid zones; and three minor sources: marine, volcano and anthropogenic. In spite of the relatively low concentration of all these elements, the wet deposition is quite significant due to the high precipitation levels, with for example a nitrogenous compounds deposition of 34 mmol.m^–2.yr^–1.     

Nitrogen compound emission from biomass burning in tropical African savanna FOS/DECAFE 1991 experiment (Lamto,Ivory Coast)

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.     

A discharge flow mass-spectrometric study of the reaction between the NO3 radical and isoprene

The kinetics and mechanism of the reactionNO₃+CH₂=C(CH₃)–CH=CH₂productswere studied in two laboratories at 298 K in the pressure range 0.7–3 torr using the discharge-flow mass-spectrometric method. The rate constant obtained under pseudo-first-order conditions with excess of either NO₃ or isoprene was: k ₁=(7.8±0.6)×10^–13 cm³ molecule^–1 s^–1. The product analysis indicated that the primary addition of NO₃ occurred on both -bonds of the isprene molecule.