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1.
We examine the sensitivities of heterogeneous sulfate chemistry in a mid-latitude and tropical storm using a cloud resolving model. Both thermodynamic environments show unstable conditions favorable for development of intensive convection, with more CAPE in tropical compared to mid-latitude storm. Compared with the observed severe storms, modeled results show a relatively good agreement with the radar and surface chemical observations. Microphysical evaluation indicates that the accretion and autoconversion appear to be most important processes in such considered clouds. This sensitivity simulation is an upper bound for conversion of S (IV) to sulfate. The tropical convective storm produces for about 2.5 times more sulfate compared to mid-latitude storm and converts more SO2 to sulfate, increasing wet deposition of sulfur. The results for a midlatitude run indicate that aerosol nucleation and impact scavenging account for between 18.9% and 28.9% of the in-cloud sulfate ultimately deposited. As a result of greater rainfall efficiency, tropical storm shows about two times higher sub-cloud scavenging rate than mid-latitude storm. The oxidation of S (IV) to SO4 ?2 in cloud droplets and in precipitation is found to be dominant in both convective storms accounting almost with the same percentage contribution of 45.4% and 46.3% to sulfur deposition, respectively. In-cloud oxidation contribute a larger fraction of the total amount of sulfur deposited in tropical case (29.2%) when compared to the mid-latitude case (11.8), respectively. Neglecting aqueous-phase chemistry in ice-phase hydrometeors in both convective clouds led to overpredict deposition of about 40% to 33% relative to the base runs.  相似文献   

2.
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.  相似文献   

3.
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+SIV 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.  相似文献   

4.
Modelling studies were performed with the multiphase mechanism RACM-MIM2ext/CAPRAM 3.0i to investigate the tropospheric multiphase chemistry in deliquesced particles and non-precipitating clouds using the SPACCIM model framework. Simulations using a non-permanent cloud scenario were carried out for two different environmental conditions focusing on the multiphase chemistry of oxidants and other linked chemical subsystems. Model results were analysed by time-resolved reaction flux analyses allowing advanced interpretations. The model shows significant effects of multiphase chemical interactions on the tropospheric budget of gas-phase oxidants and organic compounds. In-cloud gas-phase OH radical concentration reductions of about 90 % and 75 % were modelled for urban and remote conditions, respectively. The reduced in-cloud gas-phase oxidation budget increases the tropospheric residence time of organic trace gases by up to about 30 %. Aqueous-phase oxidations of methylglyoxal and 1,4-butenedial were identified as important OH radical sinks under polluted conditions. The model revealed that the organic C3 and C4 chemistry contributes with about 38 %/48 % and 8 %/9 % considerably to the urban and remote cloud / aqueous particle OH sinks. Furthermore, the simulations clearly implicate the potential role of deliquescent particles to operate as a reactive chemical medium due to an efficient TMI/HOx,y chemical processing including e.g. an effective in-situ formation of OH radicals. Considerable chemical differences between deliquescent particles and cloud droplets, e.g. a circa 2 times more efficient daytime iron processing in the urban deliquescent particles, were identified. The in-cloud oxidation of methylglyoxal and its oxidation products is identified as efficient sink for NO3 radicals in the aqueous phase.  相似文献   

5.
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.  相似文献   

    6.
    Summary We have used a multi-phase cloud photochemistry model to investigate the influence of dissolved iron (Fe) and copper (Cu) on the in-cloud production and loss of ozone and ozone-related species. Comparison of the results of our simulations with and without Fe and Cu reactions for three different photochemical scenarios (marine, averaged continental and polluted continental) indicate that Fe and Cu reactions, depending upon the scenario considered, can either increase or decrease the predicted rate of loss of ozone and ozone related species. For the marine and averaged continental scenarios the rate of loss of ozone in the aqueous-phase was decreased by as much as 45% and 70%, respectively, when Fe and Cu reactions were considered. For polluted continental conditions, the rate of loss of ozone in the aqueous phase increased with a factor 2 for low metal concentrations up to a factor 20 for high metal concentrations. In all three scenarios inclusion of the Fe and Cu reactions results in cloud droplets becoming more efficient sinks for gas-phase HO2 and also enhances OH production. The net effect of the decreased losses of ozone from the aqueous phase and the effect of the cloud droplets on HO2 and OH determine the overall impact on ozone and ozone related species, for each of the situations considered. Overall, when Fe and Cu reactions were included the marine cloud was found to be a less efficient sink for ozone, and averaged continental and polluted continental clouds were more efficient sinks for ozone (O3 losses doubled in the averaged continental scenario). The higher OH flux in the aqueous phase also enhances the rate at which organic compounds, such as formaldehyde and formic acid, are oxidized in the cloud.With 4 Figures  相似文献   

    7.
    Four case studies are described, from a three-site field experiment in October/November 1991 using the Great Dun Fell flow-through reactor hill cap cloud in rural Northern England. Measurements of total odd-nitrogen nitrogen oxides (NO y ) made on either side of the hill, before and after the air flowed through the cloud, showed that 10 to 50% of the NO y , called NO z , was neither NO nor NO2. This NO z failed to exhibit a diurnal variation and was often higher after passage through cloud than before. No evidence of conversion of NO z to NO3 - in cloud was found. A simple box model of gas-phase chemistry in air before it reached the cloud, including scavenging of NO3 and N2O5 by aerosol of surface area proportional to the NO2 mixing ratio, shows that NO3 and N2O5 may build up in the boundary layer by night only if stable stratification insulates the air from emissions of NO. This may explain the lack of evidence for N2O5 forming NO3 - in cloud under well-mixed conditions in 1991, in contrast with observations under stably stratified conditions during previous experiments when evidence of N2O5 was found. Inside the cloud, some variations in the calculated total atmospheric loading of HNO2 and the cloud liquid water content were related to each other. Also, indications of conversion of NO x to NO z were found. To explain these observations, scavenging of NO x and HNO2 by cloud droplets and/or aqueous-phase oxidation of NO2 - by nitrate radicals are considered. When cloud acidity was being produced by aqueous-phase oxidation of NO x or SO2, NO3 - which had entered the cloud as aerosol particles was liberated as HNO3 vapour. When no aqueous-phase production of acidity was occurring, the reverse, conversion of scavenged HNO3 to particulate NO3 -, was observed.  相似文献   

    8.
    The chemistry of cloud multiphase systems was studied within the Kleiner Feldberg Cloud Experiment 1990. The clouds encountered during this experimental campaign could be divided into two categories according to the origin of air masses in which the clouds formed. From the chemical point of view, clouds passing the sampling site during the first period of the campaign (26 October-4 November) were characterized by lower pollutant loading and higher pH, as compared to clouds during the final period of the experimental campaign (10–13 November). The study of multiphase partitioning of the main chemical constituents of the cloud systems and of atmospheric acidity within the multiphase systems themselves (gas + interstitial aerosol + liquid droplets) are presented in this paper. A general lack of gaseous NH3 was found in these cloud systems, which caused a lack of buffer capacity toward acid addition. Evidence supports the hypothesis that the higher acidity of the cloud systems during this final period of the campaign was due to input of HNO3. Our measurements, however, could not determine whether the observed input was due to scavenging of gaseous HNO3 from the air feeding into the cloud, or to heterogeneous HNO3 formation via NO2 oxidation by O3 to NO3 and N2O5. Sulfate in cloud droplets mainly originated from aerosol SO 4 2– scavenging, since S(IV) to S(VI) liquid phase conversion was inhibited due to both lack of H2O2 and low pH of cloud droplets, which made O3 and metal catalyzed S(IV) oxidation inefficient.  相似文献   

    9.
    The airflow, cloud microphysics and gas- and aqueous-phase chemistry on Kleiner Feldberg have been modelled for the case study of the evening of 1 November 1990, in order to calculate parameters that are not easily measured in the cloud and thus to aid the interpretation of the GCE experimental data-set. An airflow model has been used to produce the updraught over complex terrain for the cloud model, with some care required to ensure realistic modelling of the strong stable stratification of the atmosphere. An extensive set of measurements has been made self-consistent and used to calculate gas and aerosol input parameters for the model. A typical run of the cloud model has calculated a peak supersaturation of 0.55% which occurs about 20 s after entering cloud where the updraught is 0.6 m s–1. This figure has been used to calculate the efficiency with which aerosol particles were scavenged; it is higher than that calculated by other methods, and produces a cloud with slightly too many droplets. A broad cloud droplet size spectrum has been produced by varying the model inputs to simulate turbulent mixing and fluctuations in cloud parameters in space and time, and the ability of mixing processes near cloud-base to produce a lower peak supersaturation is discussed. The scavenging of soluble gases by cloud droplets has been observed and departures from Henry's Law in bulk cloud-water samples seen to be caused by variation of pH across the droplet spectrum and the inability of diffusion to adjust initial distributions of highly soluble substances across the spectrum in the time available. Aqueous-phase chemistry has been found to play a minor role in the cloud as modelled, but circumstances in which these processes would be more important are identified.  相似文献   

    10.
    Cloud droplet chemistry is modelled for the first 150 m of rise in a wintertime, mid-latitude, marine stratus cloud using observations made at and near the Cape Grim Baseline Station as a source of input parameters. The emphasis in this work was to study the variation in droplet chemistry as a function of both droplet size and nucleus composition, with a particular focus on the way in which oxidation of dissolved sulfur dioxide varied.At 150 m above the condensation level, solute concentration as a function of droplet size was found to increase by as much as 2 to 3 orders of magnitude for only a factor of 2 increase in droplet radius, primarily as a consequence of the 1/r dependence in the droplet growth equation. This type of size dependence exists at all levels in the model cloud, and has a significant influence on oxidation rate of sulfur dioxide in droplets growing on sulfate nuclei, oxidation by ozone being favoured in the smallest droplets, but oxidation by hydrogen peroxide being favoured in the larger droplets. Oxidation by ozone is favoured at all sizes in droplets formed on sea-salt nuclei as a result of the initially high alkalinity of these droplets, and in the cloud overall is calculated to be the more important oxidation pathway. Although based on a simplified chemical scheme, these results suggest that both size-dependent and nucleus-dependent chemistry of cloud droplets may need to be considered explicitly in cloud modelling work.Volume-weighted mean pH values in the range 5 to 6 were predicted from sensitivity studies in which input variables were varied over reasonable ranges, in agreement with two sets of bulk cloud-water pH data obtained by aircraft near Cape Grim.  相似文献   

    11.
    During March and April of 1995 a major international field project was conducted at the UMIST field station site on Great Dun Fell in Cumbria, Northern England. The hill cap cloud which frequently envelopes this site was used as a natural flow through reactor to examine the sensitivity of the cloud microphysics to the aerosol entering the cloud and also to investigate the effects of the cloud in changing the aerosol size distribution, chemical composition and associated optical properties. To investigate these processes, detailed measurements of the cloud water chemistry (including the chemistry of sulphur compounds, organic and inorganic oxidised nitrogen and ammonia), cloud microphysics and properties of the aerosol and trace gas concentrations upwind and downwind of the cap cloud were undertaken. It was found that the cloud droplet number was generally strongly correlated to aerosol number concentration, with up to 2000 activated droplets cm−3 being observed in the most polluted conditions. In such conditions it was inferred that hygroscopic organic compounds were important in the activation process. Often, the size distribution of the aerosol was substantially modified by the cloud processing, largely due to the aqueous phase oxidation of S(IV) to sulphate by hydrogen peroxide, but also through the uptake and fixing of gas phase nitric acid as nitrate, increasing the calculated optical scattering of the aerosol substantially (by up to 24%). New particle formation was also observed in the ultrafine aerosol mode (at about 5 nm) downwind of the cap cloud, particularly in conditions of low total aerosol surface area and in the presence of ammonia and HCl gases. This was seen to occur at night as well as during the day via a mechanism which is not yet understood. The implications of these results for parameterising aerosol growth in Global Climate Models are explored.  相似文献   

    12.
    本文将水汽在云滴上凝结增长的物理过程与气溶胶、气体的化学过程相结合，对气溶胶核化清除的化学效应进行了研究。 计算结果表明:气溶胶的核化清除造成了云滴化学成分随云滴大小分布的非均匀性，这种非均匀性又对云滴内发生的气体吸收、液相氧化产生影响。 本文还比较了不同污染状况下，不同大小的云滴内气溶胶核化清除与液相氧化对云滴化学的相对贡献的差异。 因此，这种云滴化学的非均匀性(云微化学)的研究对于云化学的野外观测及数值模拟都是重要的。  相似文献   

    13.
      相似文献   

    14.
    Concentration differences between small (r < 8.5 m) and large droplets(r > 8.5 m) were observed for formic acid, acetic acid and formaldehyde in fog droplets collected in California's Central Valley. The concentration ratios (large/small droplets) of these compounds were investigated by a stepwise model approach. Assuming thermodynamic equilibrium (KH eff) results in an overestimate of the concentration ratios. Considering the time dependence of gas phase diffusion and interfacial mass transport, it appears that the lifetime of fog droplets might be sufficiently long to enable phase equilibrium for formaldehyde and acetic acid, but not for formic acid (at pH 7). Oxidation by the OH radical has no effect on formaldehyde concentrations but reduces formic acid concentrations uniformly in all drop size classes. The corresponding reaction for acetic acid is less efficient so that only in large droplets, where replenishment is slowed because the uptake rate of acid from the gas phase is slower, is the acid concentration reduced leading to a smaller concentration ratio. Formaldehyde concentrations in fog can be higher than predicted by Henry's Law due to the formation of hydroxymethanesulfonate. Its formation is dependent on the sulfur(IV) concentration. At high pH values the uptake rate for sulfur(IV) is drop-size dependent. However, the observed concentration ratios for formaldehyde cannot be fully explained by the adduct formation. Finally, it is estimated that mixing effects, i.e., the combination of individual droplets into a bulk sample, have a minor influence (<15%) on the measured heterogeneities.  相似文献   

    15.
    Abstract

    Aqueous‐phase H2O2 production in a rainband and its possible effect on sulphate production are studied by means of a two‐dimensional numerical model. In‐cloud peroxide production is incorporated into this chemistry model and its simulation results are compared with those in which aqueous‐phase H2O2 came only from the dissolution of gaseous H2O2 from the cloud interstitial air.

    Results are presented for two different polluted situations ‐ Case 1 having initial SO2 and sulphate aerosol profiles representative of a moderately polluted air mass, and Case 2 having chemical profiles expected to increase the relative importance of oxidation to nucleation as a means of contributing sulphate to cloud and rain. Sulphate production increased in both cases, although in Case 1 the effect of this increase on the concentration of sulphate in rain is negligible because nucleation and scavenging of aerosol are the major processes by which sulphate enters cloud and rain. In Case 2, sulphate concentrations in rain increase by 5–10%. Under environmental conditions of low sulphate aerosol, where oxidation reactions are the dominant means for sulphate to enter cloud and rain, the neglect of sulphate produced by the additional H2O2 may lead to error. The usual uncertainties in the initial SO2 and sulphate aerosol vertical profiles, however, could be a more significant source of error in simulations of the chemistry of cloud and precipitation than the neglect of aqueous‐phase peroxide production during the lifetime of even a long‐lived system.  相似文献   

    16.
    Direct physical measurements of particle mass and number concentration indicate an increase in overall aerosol mass resulting from cloud processing, most likely through aqueous-phase chemistry (e.g., SO2 oxidation). Measurements conducted in the Pennines of Northern England reveal an average increase of 14 to 20% in dry aerosol mass (0.003<particle diameter<0.9 μm) after aerosol passage through an orographic cloud. The rate of in-cloud mass production is most sensitive to changes in upwind particle size distributions, SO2 concentration, and cloud water acidity. Newly-formed mass appears in size range between 200 and 600 nm and enhances the bimodality of the particle number distribution after cloud processing. Furthermore, the cloud-produced mass is estimated to increase total light scattering, bsp, by 18 to 24%. The scattering efficiency of the dry, cloud-generated aerosol is 5.0±0.3 m2 g−1 and increases to 7.4±0.7 m2 g−1 when adjusted to 90% relative humidity by incorporating particle hygroscopicity data.  相似文献   

    17.
    Using an equilibrium headspace technique, Henry's law coefficients were measured for methacrolein (H = 6.5 ± 0.7 M atm-1) and methylvinyl ketone (41 ± 7.0 M atm-1) in water at 25 °C. In addition, 2-methyl-3-buten-2-ol was studied at 30 °C in water and in an aqueous ionic solution representative of plant tissue. Similar values were found in deionized water (65 ± 3.5 M atm-1) and in a 0.05 mol kg-1 Ca2+, K+, NO3-, SO42- solution (62 ± 0.8 M atm-1). These Henry's Law coefficients are too small to allow for significant partitioning of methacrolein, methylvinyl ketone or methylbutenol into cloud water under equilibrium conditions.  相似文献   

    18.
    The effect of UV-visible light and natural sunlight on the Fe(III)-catalyzed oxidation of dissolved sulfur dioxide has been studied under the conditions representative for those of acidified atmospheric liquids. The experimental results have shown that both sunlight and UV-visible light enhance the rate of Fe(III)-catalyzed oxidation of aqueous sulfite with wavelength ranging from 300 to 575 nm. The light enhanced oxidation is mainly due to photochemical formation of OH radicals from Fe(OH)2+ complexes in the wavelength region below 420 nm and SO3•− free radicals from Fe(III) sulfite complexes above 420 nm in the absence of organic ligands. Like the Fe(III)-catalyzed thermal chemical oxidation, the Fe(III)-catalyzed photochemical oxidation is also first order with respect to sulfite ion concentration. The sunlight irradiation can increase the Fe(III)-catalyzed oxidation of S(IV) over 45%. The presence of organic complex ligands, such as oxalate, can completely inhibit the Fe-catalyzed oxidation of S(IV) in the dark. However, the photolysis of Fe(III)-oxalato complexes generates oxalate free radicals, leading to the formation of H2O2 and OH radicals and the oxidation of S(IV). The rate of Fe(III)-catalyzed oxidation of S(IV) species is found to increase with increasing light intensity. The effects of sunlight on the Fe(III)-catalyzed oxidation of S(IV) should be taken into account when predicting the daytime rates of sulfuric acid formation in atmospheric water droplets.  相似文献   

    19.
    The mathematical model presented in this paper describes in detail the gas-phase chemistry (22 reactions), gas-phase/liquid-phase equilibrium (18 equilibria) and liquid-phase chemistry (57 reactions and equilibria) in a stratiform cloud system. The model is used to analyze the influence of the liquid phase on the photooxidant formation and destruction for different gaseous SO2 concentrations with and without consideration of organic aqueous phase chemistry. It has been shown that for [SO2]>1 ppb the cloud is quantitatively a sink for H2O2, OH, HO2 and O3. The ozon destruction via O3+O2 -, which is most important in remote areas, is in polluted areas only significant at summer days. The role of organic components in cloud water consists in the transformation OH HO2 where HO2 is further transformed into H2O2.  相似文献   

    20.
    《Atmospheric Research》2007,83(3-4):698-708
    Airborne aerosol collections were performed over Wakasa bay (36°00′N, 135°30′E) in March and Kumano open sea (34°00′N, 136°50′E) and Seto (35°10′N, 137°10′E) in July 2001 at altitudes between 1.0 and 5.8 km. The particles were individually analyzed using transmission electron microscopy (TEM). Relatively large mineral-dust (mostly clay) particles were abundant in the March samples. They also dominated in July in the mid-troposphere higher than 4 km altitude, whereas sea salt and ammonium sulfate were more abundant at lower altitudes. Ca-coated grid samples show many traces of aqueous sulfate droplets. The proportions of former sulfate droplets to the total collected particles apparently increased with increasing relative humidity at the time of sampling. TEM analysis revealed that a significant fraction of these former droplets enclose mineral-dust particles as well as sea salt, soot, and fly ash. Some enclose mixtures of mineral-dust, sea-salt, soot, and fly ash particles. The results provide evidence that mineral dust from the Asian continent could acquire coatings of sulfate while being transported in the free troposphere. The mineral-dust particles probably acquired the sulfate coatings either through heterogeneous uptake of gaseous SO2 and subsequent oxidation or through coagulation with cloud or fog droplets. The presence of the mixed particles in sulfate droplets also indicates that aggregation of particles of different origins occurred through cloud processing. Such sulfate-coated dust particles would affect cloud formation, precipitation, and chemistry of the free troposphere.  相似文献   

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