# Global modeling of cloud water acidity, precipitation acidity, and acid inputs to ecosystems

Cloud water acidity affects the atmospheric chemistry of sulfate and organic aerosol formation, halogen radical cycling, and trace metal speciation. Precipitation acidity including post-depositional inputs adversely affects soil and freshwater ecosystems. Here, we use the GEOS-Chem model of atmospheric chemistry to simulate the global distributions of cloud water and precipitation acidity as well as the total acid inputs to ecosystems from wet deposition. The model accounts for strong acids (inline-formulaH2SO4, inline-formulaHNO3, and HCl), weak acids (HCOOH, inline-formulaCH3COOH, inline-formulaCO2, and inline-formulaSO2), and weak bases (inline-formulaNH3 as well as dust and sea salt aerosol alkalinity). We compile a global data set of cloud water pH measurements for comparison with the model. The global mean observed cloud water pH is inline-formula5.2±0.9, compared to inline-formula5.0±0.8 in the model, with a range from 3 to 8 depending on the region. The lowest values are over East Asia, and the highest values are over deserts. Cloud water pH over East Asia is low because of large acid inputs (inline-formulaH2SO4 and inline-formulaHNO3), despite inline-formulaNH3 and dust neutralizing 70 % of these inputs. Cloud water pH is typically 4–5 over the US and Europe. Carboxylic acids account for less than 25 % of cloud water inline-formulaH+ in the Northern Hemisphere on an annual basis but 25 %–50 % in the Southern Hemisphere and over 50 % in the southern tropical continents, where they push the cloud water pH below 4.5. Anthropogenic emissions of inline-formulaSO2 and inline-formulaNOx (precursors of inline-formulaH2SO4 and inline-formulaHNO3) are decreasing at northern midlatitudes, but the effect on cloud water pH is strongly buffered by inline-formula $M17inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg8b20487e53d7ab6a3bf592e9df90e3eb acp-20-12223-2020-ie00001.svg24pt15ptacp-20-12223-2020-ie00001.png and carboxylic acids. The global mean precipitation pH is 5.5 in GEOS-Chem, which is higher than the cloud water pH because of dilution and below-cloud scavenging of inline-formulaNH3 and dust. GEOS-Chem successfully reproduces the annual mean precipitation pH observations in North America, Europe, and eastern Asia. Carboxylic acids, which are undetected in routine observations due to biodegradation, lower the annual mean precipitation pH in these areas by 0.2 units. The acid wet deposition flux to terrestrial ecosystems taking into account the acidifying potential of inline-formula $M19inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimg7248c728767abac31fc80ac33e5f4469 acp-20-12223-2020-ie00002.svg25pt16ptacp-20-12223-2020-ie00002.png and inline-formula $M20inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg730c3bacbcf80f0a2c8df5dbccbd0cf0 acp-20-12223-2020-ie00003.svg24pt15ptacp-20-12223-2020-ie00003.png in N-saturated ecosystems exceeds 50 inline-formula $M21inlinescrollmathmlunit\mathrm{normal meq}\phantom{\rule{0ex}{0ex}}{\mathrm{normal m}}^{-normal 2}\phantom{\rule{0ex}{0ex}}{\mathrm{normal a}}^{-normal 1}$ 62pt15ptsvg-formulamathimg1fd1a98175102c22120585552b52f977 acp-20-12223-2020-ie00004.svg62pt15ptacp-20-12223-2020-ie00004.png in East Asia and the Americas, which would affect sensitive ecosystems. inline-formula $M22inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg44642da34e3da1fffc83c720c465c894 acp-20-12223-2020-ie00005.svg24pt15ptacp-20-12223-2020-ie00005.png is the dominant acidifying species in wet deposition, contributing 41 % of the global acid flux to continents under N-saturated conditions.

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Shah, Viral / Jacob, Daniel J. / Moch, Jonathan M. / et al: Global modeling of cloud water acidity, precipitation acidity, and acid inputs to ecosystems. 2020. Copernicus Publications.

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