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Factors Governing the pH, Availability of H^+, and Oxidation Capacity of Rain

Morgan, J. J. (1982) Factors Governing the pH, Availability of H^+, and Oxidation Capacity of Rain. In: Atmospheric Chemistry: Report of the Dahlem Workshop on Atmospheric Chemistry. Dahlem Workshop Report. No.4. Springer Verlag , New York, NY, pp. 17-40. ISBN 9783642686405.

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The acidity of rain is coupled to redox reactions in air and in atmospheric water. The pH, an intensive quantity, needs to be distinguished from the base neutralizing capacity. For acidic rain observed at most locations, H_2SO_4, HNO_3, NH_3, and CaCO_3 are dominant components. Their local availability or production rates govern net acidity. pH is thus almost entirely determined by these major "strong" components imposed on a CO_2 background, with some influence by SO_2(aq), smaller concentrations of HNO_2 and weak organic acids and minor bases, e.g., Fe_2O_3, yielding acid aquo metal ions. Total global emissions to tfie atmosphere of H_2SO_4 precursors outweigh those of HNO_3 by a factor of 2-3 on an equivalent basis. In specific settings HNO_3 may be comparable to H_2SO_4 in rain. Total atmospheric acidity appears to be a useful quantity for estimating potential acidity of rain at different locations. There are indications in photochemical models of HNO_3 and H_2SO_4 that feedback among S and N species may be important. Heterogeneous oxidations of SO_2 in cloud, fog, and rain play important roles in the acidification process. "Background" acidities of rain appear to be highly variable; pH values are expected to range from below 5 to above 6. Present-day SO_2 and NO_x fluxes account for a pH lowering of ~ 0.5 to 1.5, depending upon source location and transport-conversion rates.

Item Type:Book Section
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Alternate Title:Factors Governing the pH, Availability of H+, and Oxidation Capacity of Rain
Additional Information:© 1982 Springer-Verlag. I thank H.M. Liljestrand of the University of Texas for his help and advice. I also thank J.N. Galloway and G. E. Likens for interesting me in acidic rain. The support of the Air Resources Board of California is acknowledged.
Group:Environmental Quality Laboratory
Funding AgencyGrant Number
California Air Resources BoardUNSPECIFIED
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Other Numbering System NameOther Numbering System ID
Environmental Quality LaboratoryA-166
Series Name:Dahlem Workshop Report
Issue or Number:4
Record Number:CaltechAUTHORS:20140923-085937246
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:49925
Deposited By: Ruth Sustaita
Deposited On:09 Oct 2014 20:28
Last Modified:10 Nov 2021 18:49

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