Photochemical Effects in the Heterogeneous Reaction of Soot with Ozone at Low Concentrations

The soot-ozone reaction at low concentrations (ppm O₃) hasbeen examined todetermine any influence of solar radiation on its products and kinetics. Theeffect of simulatedsolar radiation is to change the product distribution towardsCO₂(g), CO (g) and H₂O(g) at theexpense of soot surface functional groups formation. Little effect on theextent or rate ofdiminution of O₃ through this rapid reaction is observed. Theinitial rate laws for formation ofall products remain the same under simulated solar radiation, with changes inthe rate constants(and thus in the relative importance of mechanistic pathways) responsible forthe differingproduct distributions. Decarboxylation of soot surface functionalities hasbeen shown to be onepossible mechanism underlying these effects. Atmospheric soot, particularlythat emitted nearthe tropopause by increasing numbers of subsonic and supersonic aircraft, mayplay a role inozone depletion; the rapid diminution of ozone in soot's presence isunaffected by solarradiation.     

The influence of light intensity,SO2, NOx and C3H6 on sulfate aerosol formation

The photochemical oxidation of SO₂ in the presence of NO and C₃H₆ was studied in a 18.2 liter pyrex reactor. When light intensity, irradiation time and SO₂ concentration were constant, SO₄ ^2- concentration, derived from the total volume of aerosol produced, peaked when [C₃H₆]/[NO] was approximately 6.0. Another increase im SO₄ ^2- formation was reached at very high ratios (>50). The experimental observations are consistent with the two SO₂ oxidation mechanisms. At low [C₃H₆]/[NO] ratios, the processes proceed via the HO–SO₂ reaction, while at high ratios the O₃–C₃H₆ adduct is assumed to oxidize SO₂ to produce SO₄ ^2- aerosols.     

Numerical Investigation of Gas Scavenging by Weak Precipitation

A one-dimensional cloud model with size-resolved microphysics and size-resolved aqueous-phase chemistry, driven by prescribed dynamics, has been used to study gas scavenging by weak precipitation developed from low-level, warm stratiform clouds. The dependence of the gas removal rate on the physical and chemical properties of precipitation has been explored under controlled initial conditions. It is found that the removal of four gaseous species (SO₂, NH₃, H₂O₂ and HNO₃) strongly depends on the total droplet surface area, regardless the mean size of droplets. The removal rates also correlate positively with the precipitation rate, especially for precipitation having a mean radius larger than 20 μm. The dependence of the scavenging coefficients on the total droplet surface area is stronger than on the precipitation rate. The removal rates of SO₂, NH₃ and H₂O₂ by precipitation strongly depend on the others' initial concentrations. When NH₃ (or H₂O₂) concentration is much lower than that of SO₂, the removal rate of SO₂ is then controlled by the concentration of H₂O₂ (or NH₃). The removal of NH₃ (or H₂O₂) also directly depends on the concentration of SO₂. NH₃ and H₂O₂ can also indirectly affect each other's removal rate through interaction with SO₂. The scavenging coefficient of SO₂ increases with the concentration ratio of NH₃ to SO₂ if the ratio is larger than 0.5, while the scavenging coefficient of NH₃ increases with the concentration ratio of SO₂ to NH₃ when the ratio is smaller than 1. The scavenging coefficient of H₂O₂ generally increases with the concentration ratio of SO₂ to H₂O₂. Although the Henry's law equilibrium approach seems to be able to simulate gas scavenging by cloud droplets, it causes large errors when used for simulating the scavenging of soluble gas species by droplets of precipitating sizes.     

Chemical ionisation mass spectrometer for measurements of OH and Peroxy radical concentrations in moderately polluted atmospheres

A new version of an atmospheric pressure chemical ionisation mass spectrometer has been developed for ground based in situ atmospheric measurements of OH and total peroxy (HO₂ + organic peroxy) radicals. Based on the previously developed principle of chemical conversion of OH radicals to H₂SO₄ in reaction with SO₂ and detection of H₂SO₄ using an ion molecule reaction with NO₃^─, the new instrument is equipped with a turbulent chemical conversion reactor allowing for measurements in moderately polluted atmosphere at NO concentrations up to several ppb. Unlike other similar devices, where the primary NO₃^─ ions are produced using radioactive ion sources, the new instrument is equipped with a specially developed corona discharge ion source. According to laboratory measurements, the overall accuracy and detection limits are estimated to be, respectively, 25% and 2 × 10⁵ molecule cm^-3 for OH and 30% and 1 × 10⁵ molecule cm^-3 for HO₂ at 10 min integration times. The detection limit for measurements of OH radicals under polluted conditions is 5 × 10⁵ molecules cm^-3 at 10 min integration times. Examples of ambient air measurements during a field campaign near Paris in July 2007 are presented demonstrating the capability of the new instrument, although with reduced performance due to the employment of non isotopic SO₂.     

Study of metal aerosol systems as a sink for atmospheric SO2

The chemical removal of SO₂ in the presence of different aerosol systems has been investigated in laboratory experiments using a dynamic flow reactor. The aerosols consisted of wetted particles containing one of the following substances: MnCl₂, Mn(NO₃)₂, MnSO₄, CuCl₂, Cu(NO₃)₂, CuSO₄, FeCl₃, NaCl. The SO₂ removal rate R was measured as a function of the SO₂ gas phase concentration (SO₂)_g, the spatial metal concentration C_Me, and the relative humidity rH in the reactor. A first-order dependence with regard to (SO₂)_g was observed for each type of aerosol. For the Mn(II) and Cu(II) aerosols R was found to be a non-linear function of C_Me except for MnSO₄ and Cu(NO₃)₂ particles. The removal rate showed a significant increase with the relative humidity particularly when rH was close to the deliquescence point of the wetted particles. Among the Mn(II) and Cu(II) aerosols investigated Mn(NO₃)₂ was found to be most efficient for the chemical removal of SO₂ at atmospheric background conditions, especially in haze and fog droplets. The results further indicate that the catalytic oxidation of S(IV) in such aerosol systems may be as efficient as its oxidation by H₂O₂ in cloud water.     

Model studies of the impact of chemical inhomogeneity on SO2 oxidation in warm stratiform clouds

A one-dimensional, time-dependent model of the physics and chemistry of a warm stratiform cloud is used to study the possible impact of chemical inhomogeneity among cloud and raindrops on the oxidation of SO₂ in clouds. The effects of chemical inhomogeneity are examined using two contrasting models: In Model 1 a bulk-solution parameterization is adopted which effectively treats all cloud and raindrops as if they are chemically homogeneous; in Model 2 we allow the cloud and raindrops to have a dichotomous distribution. The dichotomous distribution in Model 2 is simulated by assuming that the two groups of cloud droplets nucleate from two chemically distinct populations of condensation nuclei; one being acidic and the other being alkaline. While the two models yield essentially identical results when the ambient levels of H₂O₂ are greater than the ambient levels of SO₂, the rate of conversion of SO₂ to sulfuric acid and the amount of sulfate removed in the precipitation can be significantly enhanced in Model 2 over that of Model 1 under conditions of oxidant limitation (i.e., H₂O₂ < SO₂). This enhancement is critically dependent upon the fraction of alkaline nuclei assumed to be present in Model 2 and arises from the rapid increase in the aqueous-phase reaction between O₃+S_IV at high pH. Our results suggest that cloud models which adopt a bulk-solution parameterization for cloud droplet chemistry, may underestimate the amount of in-cloud SO₂ oxidation under oxidant-limited conditions.     

The heterogeneous formation of N2O over bulk condensed phases in the presence of SO2 at high humidities

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.     

Uncertainty and sensitivity analyses of OH-initiated dimethyl sulphide (DMS) oxidation kinetics

A numerical experiment has been conducted on the OH-initiated tropospheric oxidation of DMS. This involved the selection of a set of reactions describing the OH-initiated oxidation kinetics and the conversion of the present level of uncertainty of the system into uncertainty ranges and distributions for the relevant system parameters (kinetic constants and initial concentrations). Uncertainties have been propagated through the model onto the output variables of interest. This has allowed (a) the uncertainty in model prediction to be quantified and compared with observations (uncertainty analysis) and (b) the relative importance of each input parameter in determining the output uncertainty to be quantified (sensitivity analysis). Output considered were the ratio of MSA/(SO₂ + H₂SO₄) concentration at a given time, the ratio SO₂/H₂SO₄, the total peroxynitrate species concentrations and the relative fraction of SO₂ and H₂SO₄ formed through the various pathways. Conditional upon the model and data assumptions underlying the experiment, the following main conclusions were drawn:

1. The possibility of direct formation of SO₃ without SO₂ as intermediate as suggested by Bandyet al. (1992) and Yinet al. (1990), involving direct thermal decomposition of CH₃SO₃ · does not seem to play a major role in the overall generation of sulphate. This is relevant to the issue of gas to particle conversion over remote areas.
2. Reaction of CH₃SOO · intermediate may be the most important pathway to the formation of SO₂.
3. The dominating peroxynitrate is CH₃S(O)₂O₂NO₂.

Through sensitivity analysis the kinetic constants have been identified which — because of their uncertainty and of their impact on the output — mostly contribute to the output uncertainty.     

Growth of monodisperse,submicron aerosol particles exposed to SO2, H2O2, and NH3

The growth of monodisperse particles (0.07 to 0.5 µm) exposed to SO₂ (0–860 ppb), H₂O₂ (0–150 ppb) and sometimes NH₃ (0–550 ppb) in purified air at 22 °C at relative humidities ranging from 25 to 75% were measured using the Tandem Differential Mobility Analyzer technique. The experiments were performed in a flow reactor with aqueous (NH₄)₂SO₄ and Na₂SO₄ droplets. For (NH₄)₂SO₄ droplets the fractional diameter growth was independent of size above 0.3 µm but decreased with decreasing size below that. When NH₃ was added the fractional growth increased with decreasing size. Measurements were compared with predictions of a model that accounts for solubility of the reactive gases, the liquid phase oxidation of SO₂ by H₂O₂, and ionic equilibria. Agreement between measured and predicted droplet growth is reasonable when the ionic strength effects are included. Theory and experiments suggest that NH₃ evaporation is responsible for the decrease in relative growth rates for small aqueous ammonium sulfate particles. The observed droplet growth rates are too slow to explain observed growth rates of secondary atmospheric sulfate particles.     

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.     

Numerical study of the oxidation process of dimethylsulfide in the marine atmosphere

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.     

Meteorology and haze structure during AGASP-II,Part 1: Alaskan Arctic flights, 2–10 April 1986

The second Arctic Gas and Aerosol Sampling Program (AGASP-II) was conducted across the Alaskan and Canadian Arctic in April 1986, to study the in situ aerosol, and the chemical and optical properties of Arctic haze. The NOAA WP-3D aircraft, with special instrumentation added, made six flights during AGASP-II. Measurements of wind, pressure, temperature, ozone, water vapor, condensation nuclei (CN) concentration, and aerosol scattering extinction (b_sp) were used to determine the location of significant haze layers. The measurements made on the first three flights, over the Arctic Ocean north of Barrow and over the Beaufort Sea north of Barter Island, Alaska are discussed in detail in this report of the first phase of AGASP II. In the Alaskan Arctic the WP-3D detected a large and persistent region of haze between 960 and 750 mb, in a thermally stable layer, on 2, 8, and 9 April 1986. At its most dense, the haze contained CN concentrations >10,000 cm^–3 and b_sp of 80×10^–6 m^–1 suggesting active SO₂ to H₂SO₄ gas-to-particle conversion. Calculations based upon observed SO₂ concentrations and ambient relative humidities suggest that 10⁴–10⁵ small H₂SO₄ droplets could have been produced in the haze layers. High concentrations of sub-micron H₂SO₄ droplets were collected in haze. Ozone concentrations were 5–10 ppb higher in the haze layers than in the surrounding troposphere. Outside the regions of haze, CN concentrations ranged from 100 to 400 cm^–3 and b_sp values were about (20–40)×10^–6 m^–1. Air mass trajectories were computed to depict the air flow upwind of regions in which haze was observed. In two cases the back trajectories and ground measurements suggested the source to be in central Europe.     

中国实施铁路电气化的节能减排量估算

基于中国铁路部门逐年统计数据,计算了1975-2007年中国电气化铁路带来的逐年节能量和CO2、烟尘、SO2、CO、NOx与CnHm的直接减排量,并分析了其变化特点.结果表明,33年来电气化铁路使得中国铁路运输行业年均节省123.0万t标准煤的能源消耗,节能量年均增长13.9万t标准煤;CO2、烟尘、SO2、CO、NO...     

Photooxidation of dimethyl sulfide and dimethyl disulfide. II: Mechanism evaluation

The mechanisms for atmospheric photooxidation of CH₃SCH₃ and CH₃SSCH₃ developed in Part I are evaluated by a series of outdoor smog chamber experiments. Measured product yields, including SO₂, H₂SO₄, CH₃SO₃H and HCHO, are reported. The predictions of the mechanisms developed in Part I are found to be in substantial agreement with the measured concentrations from the smog chamber. By comparison of mechanism predictions and observations, critical uncertainties in the mechanism are identified.     

Measurements of H2S and SO2 over the Atlantic Ocean

Concentrations of sulfur gases H₂S and SO₂ have been measured in the marine atmosphere over the Atlantic Ocean at various sites. Mean values of 40 ng H₂S m^-3 STP and 209 ng SO₂ m^-3 STP are the results of the measurements. A diurnal variation of H₂S concentration was detected on the west coast of Ireland with nighttime concentrations of up to 200 ng H₂S m^-3 STP and values below detection limit (15 ng H₂S m^-3 STP) during daytime.     

合肥市降水化学组成成分分析

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。     

Solubility of HOCl in water and aqueous H2SO4 to stratospheric temperatures

Henry's law constants K_H (mol kg^–1 atm^–1) for the reaction HOCl_(g)=HOCl_(aq) near room temperature, literature data for the associated enthalpy change, and solubilities of HOCl in aqueous H₂SO₄ (46 to 60 wt%) at temperatures relevant to the stratosphere (200 KT230 K) are shown to be thermodynamically consistent. Effective Henry's law constants [H*=mHOCl/pHOCl, in mol kg^–1 atm^–1] of HOCl in aqueous H₂SO₄ are given by: ln(H*)=6.4946–mH₂SO₄(–0.04107+54.56/T)–5862 (1/T_o–1/T) where T(K) is temperature and T_o=298.15K. The activity coefficient of HOCl in aqueous H₂SO₄ has a simple Setchenow-type dependence upon H₂SO₄ molality.     

Tropospheric ozone chemistry. The impact of cloud chemistry

The effect of clouds and cloud chemistry on tropospheric ozone chemistry is tested out in a two-dimensional channel model covering a latitudinal band from 30 to 60° N. Three different methods describing how clouds affect gaseous species are applied, and the results are compared. The three methods are:

?A first order parameterization scheme for the removal of sulphur and other soluble gases by liquid droplets.

?A parameterization scheme for SO₂, O₃, and H₂O₂ removal is constructed. The scheme is based on the solubility of gases in liquid droplets, cycling times of air masses between clouds and cloud free areas and on the chemical interaction of SO₂ with H₂O₂ and O₃ in the liquid phase.

?Gas-aqueous-phase interactions and aqueous-phase chemical reactions are included in the reaction scheme for a number of components in areas where clouds are present.

In all three methods, a full gas-phase chemistry scheme is used. Particular emphasis is given to the study of how the ozone and hydrogen peroxide levels are affected. Significant changes in the distributions are found when aqueous-phase chemical reactions are included. The result is loss of ozone in the aqueous phase, with pronounced reductions in ozone levels in the middle and lower troposphere. Ozone levels are reduced by 10 to 30% with the largest reductions in the remote middle troposphere, bringing the values in better agreement with observations. Changes in H₂O₂ are harder to predict. Although, in one case study, hydrogen peroxide is produced within the aqueous phase, concentrations are mostly comparable or even lower than in the other cases. Hydrogen peroxide levels are, however, shown to be very pH sensitive. pH values around 5 seem to favour high H₂O₂ levels. High H₂O₂ concentrations may be found particularly in the upper part of the clouds under favourable conditions.