Cloud H2O2 formation

Heikes et al. (1982) first suggested the idea of atmospheric aqueous phase H2O2 formation in cloud droplets. The in-droplet chemical formation of H2O2 occurs much faster than in the gas phase. However, one must consider the liquid volume fraction (LWC) ratio as well as the occurrence of clouds to assess the share of in-... 

A second pathway to the formation of sulfuric acid depends on the presence of hydrogen peroxide (H2O2) in clouds, fog, rain, and other forms of water in the atmosphere. Hydrogen peroxide is now known to form in such locations when hydroperoxyl radicals react with each other ... 

Techniques are at hand to evaluate the rates of aqueous-phase acid formation reactions in clouds. Such evaluations indicate that oxidation of SO2 by H2O2 and O3 can be important in-cloud reactions for assumed representative reagent concentrations and other conditions. Rapid aqueous-phase reactions do not appear to be indicated for oxidation of nitrogen oxides to nitric acid. 

The ambient atmosphere at the mobile instrument site in Hoboken, N.J. contained up to 4 pphm of H2O2 on a day with high solar radiation and apparent photochemical smog formation. Hydrogen peroxide was observed between 12 00 A.M. and 2 00 P.M. On days when solar radiation was low because of cloud cover, no H2O2 was observed. 

Another link between halogen and sulfur chemistry is the formation of S(VI) within particles. Aerosol particles grow, among other processes, by uptake of SO2 in cloud droplets where it is oxidized to sulfate. The most important aqueous phase oxidants for S(IV) are often thought to be H2O2 and O3 (e.g., Seinfeld and Pandis, 1998) with O3 being important only for pH > 6. Some authors state that oxidation by O3 is the dominant process for the formation of non-sea-salt sulfate in sea salt particles... 

The basic assumption in the closed system is that the total quantity of each species is fixed. Consider, as an example, the cloud formation (liquid water mixing ratio wj in an air parcel that has initially a H2O2 partial pressure and assume that no reactions take place. If we treat the system as open, then at equilibrium the aqueous-phase concentration of H2O2 will be given by... 

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... 

Recent investigations have shown that hydrogen peroxide (H2O2) Is a common constituent of the hydrosphere. Including clouds, rainwater, freshwater, and seawater (1-3). In Figure 1, we have summarized several of the processes likely to be Important In the formation and decay of H2O2 In natural water bodies. 

Under atmospheric conditions, the self-reaction of the HO2 radical according to Eq. (5.24) is the most important gas phase formation of H2O2 (reviews Gunz and Hoffmann 1990, Lee et al. 2000, Vione et al. 2003). H2O2 is rather stable (Finlay-son-Pitts and Pitts 1986) in the gas phase, i. e. it does not undergo fast photochemical and gas phase reactions. The only important sinks in the boundary layer are dry deposition and scavenging by clouds (with subsequent aqueous phase chemistry) and precipitation (wet deposition). 

The majority of airborne acid sulfate appears to be formed in cloud droplets ("aqueous-phase oxidation"). SO2 dissolves to form the bisulfite anion (HSO3"), which then reacts with hydrogen peroxide (H2O2) to form acid sulfate. The lower the pH the faster this reaction proceeds. At pHs above 5.0, reaction between HS03 and ozone (O3) becomes appreciable and may become the dominant pathway for acid formation. ... 

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