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Discussion papers
https://doi.org/10.5194/acp-2019-889
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-889
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: review article 18 Oct 2019

Submitted as: review article | 18 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

The Acidity of Atmospheric Particles and Clouds

Havala O. T. Pye1, Athanasios Nenes2,3, Becky Alexander4, Andrew P. Ault5, Mary C. Barth6, Simon L. Clegg7, Jeffrey L. Collett Jr.8, Kathleen M. Fahey1, Christopher J. Hennigan9, Hartmut Herrmann10, Maria Kanakidou11, James T. Kelly12, I-Ting Ku8, V. Faye McNeill13, Nicole Riemer14, Thomas Schaefer10, Guoliang Shi15, Andreas Tilgner10, John T. Walker1, Tao Wang16, Rodney Weber17, Jia Xing18, Rahul A. Zaveri19, and Andreas Zuend20 Havala O. T. Pye et al.
  • 1Office of Research and Development, U.S.Environmental Protection Agency, Research Triangle Park, NC, 27711, USA
  • 2School of Architecture, Civil and Environmental Engineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne, 1015, Switzerland
  • 3Institute for Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, 26504, Greece
  • 4Department of Atmospheric Science, University of Washington, Seattle, WA, 98195, USA
  • 5Department of Chemistry, University of Michigan, Ann Arbor, MI, 48109-1055, USA
  • 6National Center for Atmospheric Research, Boulder, CO, 80307, USA
  • 7School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
  • 8Department of Atmospheric Science, Colorado State University, Fort Collins, CO, 80523, USA
  • 9Department of Chemical, Biochemical, and Environmental Engineering, University of Maryland Baltimore County, Baltimore, MD, 21250, USA
  • 10Leibniz Institute for Tropospheric Research (TROPOS), Atmospheric Chemistry Department (ACD), Leipzig, 04318, Germany
  • 11Department of Chemistry, University of Crete, Voutes, Heraklion Crete, 71003, Greece
  • 12Office of Air Quality Planning & Standards, U.S.Environmental Protection Agency, Research Triangle Park, NC, 27711, USA
  • 13Department of Chemical Engineering, Columbia University, New York, NY, 10027, USA
  • 14Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana-Champaign, Illinois, 61801, USA
  • 15State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, Nankai University, Tianjin, 300071, China
  • 16Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
  • 17School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332, USA
  • 18School of Environment, Tsinghua University, Beijing, 100084, China
  • 19Atmospheric Sciences & Global Change Division, Pacific Northwest National Laboratory, Richland, WA, 99352, USA
  • 20Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, H3A 0B9, Canada

Abstract. Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semi-volatile gases such as HNO3, NH3, and HCl, as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally-constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicates acidity may be relatively constant due to the semi-volatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

Havala O. T. Pye et al.
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Short summary
Acid rain is recognized for its impacts on human health and ecosystems, and programs to mitigate these effects have had implications for atmospheric acidity. Historical measurements indicate that cloud and fog droplet acidity has changed in recent decades in response to controls on emissions from human activity, while the limited trend data for suspended particles indicates acidity may be relatively constant. This review synthesizes knowledge on the acidity of atmospheric particles and clouds.
Acid rain is recognized for its impacts on human health and ecosystems, and programs to mitigate...
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