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. Cloud water acidity affects the atmospheric chemistry ofsulfate and organic aerosol formation, halogen radical cycling, and tracemetal speciation. Precipitation acidity including post-depositional inputsadversely affects soil and freshwater ecosystems. Here, we use the GEOS-Chem model of atmospheric chemistry to simulate the global distributions ofcloud water and precipitation acidity as well as the total acid inputs toecosystems from wet deposition. The model accounts for strong acids( H2SO4 , HNO3 , and HCl), weak acids (HCOOH, CH3COOH , CO2 , and SO2 ), and weak bases ( NH3 as well as dust and sea salt aerosolalkalinity). We compile a global data set of cloud water pH measurements forcomparison with the model. The global mean observed cloud water pH is 5.2±0.9 , compared to 5.0±0.8 in the model, with a range from 3 to8 depending on the region. The lowest values are over East Asia, and the highestvalues are over deserts. Cloud water pH over East Asia is low because oflarge acid inputs ( H2SO4 and HNO3 ), despite NH3 and dustneutralizing 70 % of these inputs. Cloud water pH is typically 4–5 overthe US and Europe. Carboxylic acids account for less than 25 % ofcloud water H+ in the Northern Hemisphere on an annual basis but25 %–50 % in the Southern Hemisphere and over 50 % in the southerntropical continents, where they push the cloud water pH below 4.5.Anthropogenic emissions of SO2 and NOx (precursors of H2SO4 and HNO3 ) are decreasing at northern midlatitudes, butthe effect on cloud water pH is strongly buffered by NH 4 + andcarboxylic acids. The global mean precipitation pH is 5.5 in GEOS-Chem, which ishigher than the cloud water pH because of dilution and below-cloud scavengingof NH3 and dust. GEOS-Chem successfully reproduces the annual meanprecipitation pH observations in North America, Europe, and eastern Asia.Carboxylic acids, which are undetected in routine observations due tobiodegradation, lower the annual mean precipitation pH in these areas by 0.2units. The acid wet deposition flux to terrestrial ecosystems taking intoaccount the acidifying potential of NO 3 - and NH 4 + inN-saturated ecosystems exceeds 50  meq m - 2 a - 1 in East Asia and theAmericas, which would affect sensitive ecosystems. NH 4 + is thedominant acidifying species in wet deposition, contributing 41 % of theglobal acid flux to continents under N-saturated conditions.

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