Cloud droplet composition

Because of the mixture of VOCs in the atmosphere, the composition of smog reaction products and intermediates is extremely complex. formed via reaction 16, is important because when dissolved in cloud droplets it is an important oxidant, responsible for oxidising SO2 to sulfuric acid [7664-93-9] H2SO4, the primary cause of acid precipitation. The oxidation of many VOCs produces acetyl radicals, CH CO, which can react with O2 to produce peroxyacetyl radicals, CH2(C0)02, which react with NO2... 

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+,... 

Gurciullo, C. S., and S. N. Pandis, Effect of Composition Variations in Cloud Droplet Populations on Aqueous-Phase Chemistry, J. Geophys. Res., 102, 9375-9385 (1997). 

Venus is completely covered with dense clouds. The composition of the clouds has been the subject of much speculation for many years. It includes ice, carbon suboxide, sulfuric acid, hydrocarbons, mercuric chloride, ammonium chloride, and hydrated ferrous chloride. Recently, Young (1066a) has proposed, based on refractive index measurement, that the clouds are composed most probably of droplets of 75% H2S04. 

The composition of liquid-water clouds and processes responsible for this composition are of obvious current interest in conjunction with the so-called acid precipitation phenomenon since clouds constitute the immediate precursor of precipitation. Additionally, cloud composition is of interest because impaction of cloud droplets on surfaces may directly deliver dissolved substances onto natural or artificial materials. In-cloud processes also influence clear-air composition since dissolved substances resulting from such reactions are released into clear air as gases or aerosol particles upon cloud evaporation. It is thus desired to gain enhanced description of the composition of clouds and the mecha-... 

Model system Hydrometeor types in clouds Cloud droplet size distribution Cloud droplet number CCN/IDN composition CCN/IDN spectrum Cloud radiative properties... 

GCMOM with RH ZSR equation simulated MDRH Hysteresis is treated composition-resolved CCN/IDN based on Kohler theory activation, Nucl. scavenging (rainout), autoconversion for size-resolved cloud droplets or precip. rate dependent of aerosol size and composition hydrometeor coag. (washout), calculated precip. rate dependent of aerosol size and composition dependent sedimentation for all aerosols and hydrometeors... 

Both the number concentrations and sizes of aerosol particles directly affect many of their properties and effects. For example, the ability of particles to serve as nuclei for cloud droplet formation depends on their composition as a function of size, although their effectiveness in any given situation depends also on the number of particles present. Knowledge of these aerosol properties is required to evaluate the indirect effects (Section 4.04.7.3) of aerosol particles on climate, i.e., the effect of aerosol particles on cloud reflectivity and persistence. Therefore much attention has been and continues to be focused on determining particle number concentrations and size distributions. 

As a first step in assessing the potential importance of nanoparticle reactions, we compare the volume and surface areas of these particles with the same values from other condensed phases with known chemical effects. We first consider nanoparticle volumes. As an upper limit, we consider an urban air parcel containing 20-nm diameter nanoparticles at a number concentration of 10 cm. Under this scenario, the nanoparticle volume is a small fraction (10 of the total air parcel volume. Thus the nanoparticle reaction rate (in units of mol m -air s ) would have to be ca. 10 times as fast as the equivalent gas phase reaction (mol m -air s ) to have a comparable overall rate in the air parcel. For comparison, clouds typically have liquid water contents of 10 to 10 (volume fraction) and can have significant effects upon atmospheric chemistry (Seinfeld and Pandis 1998). For simplicity of argument, if the medium of the cloud droplets and nanoparticles are assumed similar (e.g., dilute aqueous), then the fundamental rate constants in each medium are similar. Under this condition, reactant concentrations in the nanoparticles would need to be enhanced by 10, as compared to the cloud droplets, to have equal rates. Based on this analysis, it appears unlikely that reactions occurring in the bulk of nanoparticles could affect the composition of the gas phase. 

Sea aerosols initially have an ionic composition of sea water but quickly loose water to attain equilibrium with water vapour in the surrounding air, simultaneously cumulating reactive trace gases and undergoing chemical reactions (Pszenny et al., 1998). Also cloud droplets may imdergo dehydration, depending on air humidity. The concentration of nitrate may reach very high values, a mole per (hir and over, when droplets evaporate almost to dryness. 

In addition to solute from CCN, clouds contain dissolved gases (e.g. SO2, NH3, HCHO, H2O2, HNO3, and many more). In turn, some of these may react in the cloud droplets to form other substances which subsequently can appear in rainwater. Finally, falling raindrops can collect other materials (e.g. large dust particles) on their way to the Earth s surface. Thus, rainwater composition does not uniquely reflect the chemistry of the CCN. 

Chameides, W. L., and Davis, D. D., The free radical chemistry of cloud droplets and its impact upon the composition of rain. J. Geophys. Res. 87, 4863 (1982). 

Fitzgerald, J. W. (1974). Effect of aerosol composition on cloud droplet size distribution. A numerical study. J. Atmos. Sci. 31, 1358-1367. 

The CCN behavior of ambient particles can be measured by drawing an air sample into an instrument in which the particles are subjected to a known supersaturation, a so-called CCN counter (Nenes et al. 2001). If the size distribution and chemical composition of the ambient particles are simultaneously measured, then the measured CCN behavior can be compared to that predicted by Kohler theory on the basis of their size and composition. Such a comparison can be termed a CCN closure, that is, an assessment of the extent to which measured CCN activation can be predicted theoretically [see, for example, VanReken et al. (2003), Ghan et al. (2006), and Rissman et al. (2006)]. The next level of evaluation is an aerosol-cloud drop closure, in which a cloud parcel model, which predicts cloud drop concentration using observed ambient aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against direct airborne measurements of cloud droplet number concentration as a function of altitude above cloud base. The predicted activation behavior can also be evaluated by independent measurements by a CCN instrument on board the aircraft. Such an aerosol-cloud drop closure was carried out by Conant et al. (2004) for warm cumulus clouds in Florida. 

FIGURE 17.19 Measured composition of the small and large cloud droplets collected in coastal stratus clouds at La Jolla Peak, California, in July 1993 (Collett et al. 1994). 

Particles are eventually removed from the atmosphere by two mechanisms deposition at the Earth s surface (dry deposition) and incorporation into cloud droplets during the formation of precipitation (wet deposition). Because wet and dry deposition lead to relatively short residence times in the troposphere, and because the geographical distribution of particle sources is highly nonuniform, tropospheric aerosols vary widely in concentration and composition over the Earth. Whereas atmospheric trace gases have lifetimes ranging from less than a second to a century or more, residence times of particles in the troposphere vary only from a few days to a few weeks. 

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