Detectability of acid producing reactions in natural clouds

Oxidation reactions of SO₂ and NO₂ in clouds are considered important atmospheric acid formation processes. This paper evaluates the feasibility of detecting the occurrence of these oxidation reactions in natural clouds by means of field measurements. This evaluation is performed by calculating the changes expected in reagent and product concentrations resulting from these reactions in representative cloud types, and comparing those changes with concentration differences detectable by available analytical methods in the context of typical atmospheric variability. Four in-cloud oxidation reactions are considered: aqueous-phase reactions of SO₂ with O₃ and with H₂O₂, and gas-phase reactions of NO₂ with OH radical and with O₃, the latter leading to acid formation by reaction of N₂O₅ with cloud liquid water. The cloud types considered are fog, stratus, cumulus and mountain lee wave. This evaluation indicates that oxidation of SO₂ by H₂O₂ should be detectable in a wide variety of cloud conditions, but that oxidation of SO₂ by O₃ is unlikely to be detected by field measurements. Oxidation of NO₂ may be detectable in fog and stratus clouds, which provide long in-cloud residence times. The paper includes discussion of factors which favor or hinder detection of acid producing reactions in clouds, and reviews evidence from published field studies on the occurrence of these reactions.