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On the chemistry of inorganic free radicals in cloud water.

Posted on 27. March, 2014.

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Cloud water droplets are formed by the condensation of water on to aerosol particles, known as cloud condensation nuclei, which occurs when the relative humidity in an ascending mass of air exceeds the saturation level due to adiabatic cooling. The chemical composition of cloud droplets has been the subject of several field studies in which rainwater is collected and analysed [1,2].

The main anions in rainwater are sulphate, chloride and nitrate, and the principal cations include the alkali and alkaline earth metal ions (Na+, K+, Mg2+, Ca2+), ions of the transition metals iron, manganese and copper, and ammonium ion [3]. Organic species are also present such as carboxylic acids, aldehydes and their addition compounds with S(IV); for example formaldehyde forms hydroxymethanesulfonate [4].  Typical concentrations of the major constituents of cloud water are listed in Table 1, but are dependent on the environment, e.g. urban, remote or marine.

The pH of cloud water is governed both by physical and chemical processes. He former involve change in droplet size due to evaporation and condensation; the latter include formation of strong and weak acids by oxidation, e.g. SO2(aq) to H2SO4, aldehyde to carboxylic acid, and their partial neutralisation by ammonia. In the absence of pollutants the pH is governed by the equilibrium concentration of atmospheric carbon dioxide in water, i.e. pH 5-6.

Early studies were focused on the mechanisms by which SO2 is oxidised to H2SO4, producing “acid rain”. As a result of these experiments it is generally accepted that there are four main pathways for the oxidation of S(IV) in the aqueous phase [3,9]. 

These are:

Oxidation by ozone
Oxidation by hydrogen peroxide
Oxidation by oxygen in a chain reaction involving free radicals
Oxidation by oxygen catalysed by transition metal ions

It is the role of inorganic free radicals in the oxidative formation of acids to lower the pH to produce “acid rain” that is the focus of this review. These radicals include OH, HO2, O2•-, SOx•- (x = 3, 4 5), NO3, Cl, Cl2•-. The laboratory experiments described in later sections of this review have been invaluable for verifying and quantifying the details of mechanisms that occur in the atmosphere. The formulation of detailed computer models of cloud chemistry, for example, the CAPRAM model [10], requires the kind of detailed mechanistic information provided by laboratory experiments.

The majority of the kinetic and spectroscopic data have been obtained using laser flash photolysis and pulse radiolysis, which are convenient methods for generating free radicals in aqueous solution [11]. Final products have generally been measured in a complementary experiments using continuous photolysis and radiolysis.

Read the full article, free of charge in Progress in Reaction Kinetics and Mechanism, Volume 28, Number 3, 2003, pp. 257-297


water droplet image courtesy of Oleg Gekman/