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Preprints
https://doi.org/10.5194/acp-2020-485
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-485
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  22 Jun 2020

22 Jun 2020

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A revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Global modeling of cloudwater acidity, rainwater acidity, and acid inputs to ecosystems

Viral Shah1, Daniel J. Jacob1,2, Jonathan M. Moch2, Xuan Wang1,a, and Shixian Zhai1 Viral Shah et al.
  • 1Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
  • anow at: School of Energy and Environment, City University of Hong Kong, Hong Kong SAR, China

Abstract. Cloudwater acidity affects the atmospheric chemistry of sulfate and organic aerosol formation, halogen radical cycling, and trace metal speciation. Rainwater 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- and rainwater acidity, and the total acid inputs to ecosystems from wet deposition. The model accounts for strong acids (H2SO4, HNO3, HCl), weak acids (HCOOH, CH3COOH, CO2, SO2), and weak bases (NH3, dust and sea salt aerosol alkalinity). We compile a global dataset of cloudwater pH measurements for comparison with the model. The global mean observed cloudwater pH is 5.2 ± 0.9, compared to 5.0 ± 0.8 in the model, with a range of 3 to 8 depending on region. The lowest values are over East Asia and the highest values are over deserts. Cloudwater pH over East Asia is low because of large acid inputs (H2SO4, HNO3), despite NH3 and dust neutralizing 70 % of these inputs. Cloudwater pH is typically 4–5 over the US and Europe. Carboxylic acids account for less than 25 % of cloudwater H+ 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 cloudwater pH below 4.5. Anthropogenic emissions of SO2 and NOx (precursors of H2SO4 and HNO3) are decreasing at northern mid-latitudes, but the effect on cloudwater pH is strongly buffered by NH4+ and carboxylic acids. The global mean rainwater pH is 5.5 in GEOS-Chem, higher than the cloudwater pH because of dilution and below-cloud scavenging of NH3 and dust. GEOS-Chem successfully reproduces the rainwater pH observations in North America, Europe, and eastern Asia. Carboxylic acids, which are undetected in routine observations due to biodegradation, lower the annual mean rainwater pH in these areas by 0.2 units. The acid wet deposition flux to terrestrial ecosystems taking into account the acidifying potential of NO3 and NH4+ in N-saturated ecosystems exceeds 50 meq m−2 a−1 in East Asia and the Americas, which would affect sensitive ecosystems. NH4+ is the dominant acidifying species in wet deposition, contributing 41 % of the global acid flux to continents under N-saturated conditions.

Viral Shah et al.

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Viral Shah et al.

Viral Shah et al.

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Short summary
Cloudwater pH affects atmospheric chemistry and acid rain damages ecosystems. We simulate the global distributions of cloud- and rainwater pH and evaluate them with worldwide observations. Sulfuric acid, nitric acid and ammonia control the pH in the northern midlatitudes, but carboxylic acids and dust cations are more important in the tropics and subtropics. The acid inputs to many nitrogen-saturated ecosystems are high enough to cause acidification, with ammonium as the main acidifying species.
Cloudwater pH affects atmospheric chemistry and acid rain damages ecosystems. We simulate the...
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