Sulfate cloud composition

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

The chemical composition of fogs, clouds, and particles (see Chapter 9) varies as a function of particle size. For example, Figure 8.19 shows the concentrations of the major cations and anions measured in small and large cloud droplets at La Jolla peak in southern California (Collett et al., 1994, 1999). The large drops are enriched in soil and sea salt derived species such as Mg2+, Ca2+, Na+, and Cl whereas the smaller particles contain higher concentrations of sulfate and H+,... 

The seasonal cycle of CCN has also been shown to be correlated with that of cloud optical depth in one remote marine area (Boers et al., 1994), and the isotope composition of non-sea salt sulfate over remote regions of the southern Pacific Ocean has been shown to be consistent with a DMS source (Calhoun et al., 1991). 

These plots are called Kohler curves after their originator (Kohler, 1936). His assumptions that cloud condensation nuclei (CCN) are water-soluble materials is now widely accepted. In the past, it was often thought that NaCl particles from the ocean were the main CCN however, more recent studies have demonstrated the frequent dominance of sulfate particles with composition between H2SO4 and (NH4)2S04-... 

The composition of cloud and precipitation water was investigated by Petrenchuk and Drozdova (1966), among others they developed a special cloud water collector that worked at positive as well as at negative temperatures. Their results, obtained over the European parts of the U.S.S.R., are given in Table 28. It can be seen that over clean northern regions the difference between the sum of ions in cloud and precipitation elements is not great. In these areas the concentration of sulfate and nitrate ions is relatively small while the chloride content is great. This situation can be explained by maritime influences. In comparison, sulfate is the... 

More recently, Fricke et al. (1978) reported the results of a similar investigation. In this study, carried out over Bavaria, F.R.G., the composition of cloud water collected at the cloud base was compared with the composition of rainwater sampled at the surface. It was found that the concentration of heavy metals at cloud base was about twice the value at ground level in rainwater, in agreement with some of the results of Petrenchuk and Drozdova (1966). Fricke s study also indicated that the total sulfur content (sulfate + S02 + sulfite about 30 % of the total was S02 and sulfite) was doubled between the cloud base and the surface, probably due to S02 wash-out. 

Reactions taking place on the surface of solid or liquid particles and inside liquid droplets play an important role in the middle atmosphere, especially in the lower stratosphere where sulfate aerosol particles and polar stratospheric clouds (PSCs) are observed. The nature, properties and chemical composition of these particles are described in Chapters 5 and 6. Several parameters are commonly used to describe the uptake of gas-phase molecules into these particles (1) the sticking coefficient s which is the fraction of collisions of a gaseous molecule with a solid or liquid particle that results in the uptake of this molecule on the surface of the particle (2) the accommodation coefficient a which is the fraction of collisions that leads to incorporation into the bulk condensed phase, and (3) the reaction probability 7 (also called the reactive uptake coefficient) which is the fraction of collisions that results in reactive loss of the molecule (chemical reaction). Thus, the accommodation coefficient a represents the probability of reversible physical uptake of a gaseous species colliding with a surface, while the reaction probability 7 accounts for reactive (irreversible) uptake of trace gas species on condensed surfaces. This latter coefficient represents the transfer of a gas into the condensed phase and takes into account processes such as liquid phase solubility, interfacial transport or aqueous phase diffusion, chemical reaction on the surface or inside the condensed phase, etc. 

The condensation of water vapor and its precipitation from the atmosphere in the form of rain, snow, sleet, or hail are important not only for the water cycle, but also because they bring to the earth surface other atmospheric constituents, primarily those substances that have a pronounced affinity toward water in the condensed state. Cloud and precipitation elements may incorporate both aerosol particles and gases. The uptake mechanisms are discussed in this chapter, together with the inorganic composition of cloud and rain water that they determine. These processes are, in principle, well understood. Another subject requiring discussion is the occurrence of chemical reactions in the liquid phase of clouds. The oxidation of S02 dissolved in cloud water is considered especially important. As a result of laboratory studies, the conversion of S02 to sulfate is now known to proceed by several reaction pathways in aqueous solution. 

Processing of accumulation and coarse mode aerosols by clouds (Chapter 17) can also modify the concentration and composition of these modes. Aqueous-phase chemical reactions take place in cloud and fog droplets, and in aerosol particles at relative humidities approaching 100%. These reactions can lead to production of sulfate (Chapter 7) and after evaporation of water, a larger aerosol particle is left in the atmosphere. This transformation can lead to the formation of the condensation mode and the droplet mode (Hering and Friedlander 1982 John et al. 1990 Meng and Seinfeld 1994). 

The idealized calculation just presented shows what are thought to be the essential elements of the aqueous-phase chemistry of acid rain. Measurements of H2O2 in rain and cloudwater show a range of concentrations between approximately 10 and 1(X) /itM (Kok, 1980 Zika et al., 1982). Water with this composition is in equilibrium with between 0.1 and 1.0 ppb gas-phase H2O2. Kleinman (1984) has examined the question of whether H2O2 can account for the in-cloud oxidation of SO2 and found that under summertime conditions between 3 and 5 ppb of H2O2 would be required to account for estimated incloud sulfate formation. Seigneur et al. (1984) presented the results of simulations of atmospheric sulfate and nitrate formation by both gas- and aqueous-phase paths under... 

As a parcel of air is cooled toward saturation, the relative hnmidity approaches 100%, and water vapor begins to condense, or nucleate, on small particles of aiibome dnst, or cloud condensation nuclei (CCNs). These small particles nsnally contain a soluble component, often a salt such as sodium chloride or ammonium sulfate. CCNs of diiferent sizes and compositions are present at each position in the atmosphere. Some of the nuclei become wet at relative humidities below 100% and form haze, while the... 

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