Cloudwater composition is very much a function of location, being dominated by availability of soluble ionic species. Principal ionic species present in cloudwater include sodium and chloride, from seawater, sulfate and nitrate anions, and ammonium and hydrogen ion as cations. In regions influenced by industrial emissions of sulfur and nitrogen oxides, cloudwater concentrations of H+ are commonly 10"4 mol/L (molar, M) and not uncommonly 10"3 M or higher (Daum et al., 1984).
The fact that cloud droplets form on existing aerosol particles has immediate implications for cloudwater composition. Consider an ammonium sulfate aerosol particle of dry diameter 0.1 pm that serves as a nucleus of a cloud droplet of 10 pm diameter. The volume of the particle is ~10-21 m3. For density ~ 1000 kg/rrT3 and molecular weight lOOg/mol (~0.1 kg/mol), the amount of ammonium sulfate contained in the particle is 10~17 mol. For this material dissolved in a 10-pm droplet (~10-15 m3) the solution concentration is ~10-2 mol/m~3 or ~10-5 M. This concentration is at the low end of the range of concentrations of sulfate in cloudwater (and also in precipitation) in regions influenced by industrial emissions (see Chapter 15). It should be stressed that this figure varies as the third power of the particle diameter, that is, an order of magnitude for a factor of 2 in particle diameter. Thus for the particle diameter 0.2 pm, the concentration is 10~4 M.
Consider the correspondence between aqueous-phase concentration and the equivalent mixing ratio of the material in air. For one thousand 0.1 -pm-diamctcr, unit-density particles per cm3, the corresponding mass loading is 1 pg/rrT3, a loading that is rather low in the context of industrialized regions (See Chapter 16), albeit still substantially greater than that characteristic of regions remote from industrial sources. For molecular weight 100, this corresponds to a molar mixing ratio relative to air, x % 0.3 nmol/mol(air) (ppb). For a substance S that dissolves entirely in cloudwater, the relation between the mixing ratio of the substance xs in air and concentration in cloudwater is
where [S] is aqueous concentration, /?atm is the atmospheric pressure, Rg is the universal gas constant, and T is the absolute temperature. In SI units p.dlm is in units of pascal and Rg = 8.3 J/mol/K. The resulting concentration [S] is in units moles per cubic meters. In practical units (concentration in mol/L and pressure in bar; 1 bar = 105 Pa)
In general the fractional uptake of soluble (ionic) aerosol species into cloudwater is fairly high, approaching unity at low aerosol loading and/or high updraft velocities leading to fairly high maximum supersaturation governing activation of aerosol particles (Leaitch et al., 1996). However, in the case of gases the uptake varies substantially depending on the solubility and/or reactivity of the gas in question.
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