Cloud drop

Kiehl, J. T. (1994). Sensitivity of a GCM climate simulation to differences in continental versus maritime cloud drop size, /. Geophys. Res. 99, 23107-23115. 

All of these species are very soluble in a rain or cloud drop and are an important source of atmospheric aerosols. For ammonia and ammonium, the condensed phases (I and s) represent approximately two-thirds of the total atmospheric burden, whereas for nitric acid and nitrates, about two-thirds is in the gas phase (Soderlund and Svensson, 1976). 

The net enhancement factor for a droplet consisting of pure water can be as much as 1.6 (Madronich, 1987). Calculations by Ruggaber et al. (1997) suggest that the actinic flux inside cloud drops with a typical size distribution and dissolved particulate matter is more than a factor of two greater than in the cloud interstitial air. This effect of enhanced actinic flux inside droplets may be quite important for aqueous-phase photochemistry in fogs and clouds. 

Miller, D. F D. Lamb, and A. W. Gertler, S02 Oxidation in Cloud Drops Containing NaCI or Sea Salt as Condensation Nuclei, Atmos. Environ., 21, 991-993 (1987). 

An air mass has an O3 concentration of 60 ppb and cloud drops with pH 4.0. What must the concentration of gaseous H202 be so that the rate of oxidation of S(IV) in the cloud by these two oxidants is equal ... 

Collett, J. L., Jr., A. Bator, X. Rao, and B. B. Demoz, Acidity Variations across the Cloud Drop Size Spectrum and Their Influence on Rates of Atmospheric Sulfate Production, Geophys. Res. Lett., 21, 2393-2396 (1994). 

Effect of aerosol particles on cloud drop number concentrations and size distributions Clouds and fogs are characterized by their droplet size distribution as well as their liquid water content. Fog droplets typically have radii in the range from a few /an to 30-40 /an and liquid water contents in the range of 0.05-0.1 g m" Clouds generally have droplet radii from 5 /an up to 100 /im, with typical liquid water contents of 0.05-2.5 gin"5 (e.g., see Stephens, 1978, 1979). For a description of cloud types, mechanisms of formation, and characteristics, see Wallace and Hobbs (1977), Pruppacher (1986), Cotton and Anthes (1989), Heyms-field (1993), and Pruppacher and Klett (1997). 

Wurzler, S., A. I. Flossmann, H. R Pruppacher, and S. E. Schwartz, The Scavenging of Nitrate by Clouds and Precipitation. I. A Theoretical Study of the Uptake and Redistribution of NaNO, Particles and HNO, Gas by Growing Cloud Drops Using an Entraining Air Parcel Model, J. Atmos. Chem., 20, 259-280 (1995). 

In cloud physics, coagulation is generally used synonymously with accretion. Less frequently, it refers to any process by which a cloud s numerous small cloud drops are convened into a smaller number of larger precipitation particles. When so used, the term is employed in analogy In the coagulation of any colloidal state. (See 1 above.)... 

Figure 7 Transmission of the atmosphere containing a polluted stratus cloud with a varying amount of soot inclusion and disolved absorbers, normalised to clean cloudless conditions. Here f is the soot volume fraction in the cloud drops (from figure 7 of Erlick et al.l998)...

However, not every aerosol particle serves as CCN. Accumulation mode aerosols provide the nuclei for most cloud drops (Penner et al., 2001). As in the case of anthropogenic and natural sulphate particles, OAs can also serve as CCN (Ramanathan et al., 2001). Additionally, the presence of water-soluble organic compounds in the particles and the presence of soluble gases (HN03) in the atmosphere can amplify the CCN activity of the aerosols and further increase the concentration of cloud droplets and the indirect forcing (Charlson et al., 2001). Also, biomass... 

Interfacial mass transfer of trace gases into aqueous pnase is investigated in a UV absorption-stop flow apparatus. For the first time, the mass accommodation coefficients are determined for O3 (5.3x10" ) and for SO2 (>2x10 2) The results are incorporated into a simple model considering the coupled interfacial mass transfer and aqueous chemistry in cloud drops. It is shown that dissolution of O3 into a drop is fast compared with its subsequent oxidation of dissolved S02 In addition, the conversion rate of S(IV) to S(VI) in aqueous drops by ozone reactions is not limited by interfacial resistance. 

The impact of secondary aerosols on indirect radiative forcing is the most variable and is the least understood [3]. The reasons why the indirect effect of secondary aerosols is so difficult to describe is that it depends upon [1] (1) a series of molecular-microphysical processes that connect aerosol nucleation to cloud condensation nuclei to cloud drops and then ultimately to cloud albedo and (2) complex cloud-scale dynamics on scales of 100-1000 km involve a consistent matching of multiple spatial and time scales and are extremely difficult to parameterize and incorporate in climate models. Nucleation changes aerosol particle concentrations that cause changes in cloud droplet concentrations, which in turn, alter cloud albedo. Thus, macro-scale cloud properties that influence indirect forcing result from both micro-scale and large-scale dynamics. To date, the micro-scale chemical physics has not received the appropriate attention. 

FIGURE 20.2 Illustration of the size (below particle) and number of molecules (upper right) in atmospheric particles ranging from critical clusters up to cloud drops. The visible spectrum is shown to emphasize the size range (0.1 to 1.0 mm) relevant to aerosol radiative forcing. Nucieation creates new particles that evolve into larger particles relevant to aerosol radiative... 

First indirect effect - affect cloud drop size, number, reflectivity, and optical depth via cloud condensation nuclei (CCN) ... 

Key variables interstitial/activated fraction, CCN size/composition, cloud drop size/number/liquid water content (LWC), cloud optical depth (COD), and updraft velocity... 

Two quick questions about aqueous phase oxidation in addition to oxidation in cloud drops can aqueous phase oxidation in aerosols be important ... 

I think perhaps this is just a technical quibble on your use of homogeneous for the aqueous phase — there is an implication in the word homogeneous that the concentration is the same throughout, and it is my impression that when you are dealing with cloud drops, the concentrations are quite variable depending on how far into the drops you are. It is an inhomogeneous reaction in one phase. 

That is a good point there are certainly large chemical inhomogeneities in the cloud system — inhomogeneities from drop to drop as well as within a single drop. For instance for a species like OH in the aqueous phase, the lifetime is about 10" seconds. So that OH is destroyed chemically before it has a chance to really mix uniformly throughout most cloud drops. Studies of the effect of this non-uniformity have yet to be carried out, although I do... 

Rain-out of aerosol particles is begun at the moment of cloud formation, since at the supersaturations occurring in the atmosphere (< 1 %, which is equivalent to a relative humidity of 101 %) condensation takes place on aerosol particles, called nuclei. To understand the principle of this phenomenon (see Mason, 1957 Fletcher, 1962) let us consider a cooling air parcel. Because of the cooling the relative humidity in the parcel increases. After reaching the saturation level, cloud drops begin to form on aerosol particles in the updraft. Each particle becomes active in the... 

Because the mass of ammonium sulfate and sulfuric acid particles is mainly in the size range of active condensation nuclei, it is believed that this process provides a very effective removal mechanism for the tropospheric background aerosol. However, we have to emphasize that other processes are also operating in the cloud to remove small aerosol particles, of which the most important process is the coagulation of particles and cloud drops. As we have seen (Subsection 4.1.1), thermal coagulation is particularly effective in the range of very small particles inactive in condensation. To estimate this process, let us consider a cloud in which the number concentration of drops with radius rc is Nc. If the number concentration... 

The rain-out of reactive gases is determined by the transport of gas molecules to the droplet surface, their molecular diffusion into the liquid water, and by their rate of chemical transformation. It is shown (Beilke and Gravenhorst, 1978 Hales, 1978) that, in case of sulfbr dioxide removal, the rate of this latter process is much smaller than that of diffusion, since the equilibrium between S02 in the gas phase and sulfur(IV) in liquid water is reached within less than 1 s (in this form of notation, a Roman numeral in parentheses means the valence of the elements). For this reason the oxidation of sulfur(IV) in cloud drops is the rate-determining process in S02 removal. 

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