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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 30 Apr 2019

Submitted as: research article | 30 Apr 2019

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

Effects of Water-soluble Organic Carbon on Aerosol pH

Michael A. Battaglia Jr.1, Rodney J. Weber2, Athanasios Nenes2,3,4,5, and Christopher J. Hennigan1 Michael A. Battaglia Jr. et al.
  • 1Department of Chemical, Biochemical and Environmental Engineering, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
  • 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
  • 3Institute for Chemical Engineering Sciences, Foundation for Research and Technology – Hellas, Patras, 26504, Greece
  • 4Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Palea Penteli, Athens, 15236, Greece
  • 5Laboratory of Atmospheric Processes and their Impacts, School of Architecture, Civil and Environmental Engineering, Ecole Polytechnique Fédérale de Lausanne, CH-1015, Lausanne, Switzerland

Abstract. Water soluble organic carbon (WSOC) is a ubiquitous and significant fraction of fine particulate matter. Despite advances in aerosol thermodynamic equilibrium models, there is limited understanding on the comprehensive impacts of WSOC on aerosol acidity (pH). We address this limitation by studying submicron aerosol that represent the two extremes in acidity levels found in the atmosphere: strongly acidic aerosol from Baltimore, MD, and weakly acidic conditions characteristic of Beijing, China. These cases are then used to construct mixed inorganic/organic single-phase aqueous particles, and thermodynamically analyzed by the E-AIM and ISORROPIA models (in combination with activity coefficient model AIOMFAC) to evaluate the effects of WSOC on the H+ ion activity coefficients (γH+) and activity (pH). We find that addition of organic acids and non-acid organic species concurrently increases γH+ and aerosol liquid water. When allowed to modulate pH, these effects mostly offset each other, giving pH changes of < 0.6 pH units even if organics dominate aerosol dry mass (in excess of 60 %). Surprisingly, non-acidic WSOC compounds were found to have a larger effect on pH than organic acids owing to their stronger impacts on γH+. The model simulations were run at 70 %, 80 %, and 90 % relative humidity (RH) levels and the effect of WSOC was inversely related to RH. At 90 % RH, WSOC altered aerosol pH by up to ~ 0.2 pH units, though the effect was up to ~ 0.6 pH units at 70 % RH. The offsetting nature of these effects suggests that aerosol pH is sufficiently constrained by the inorganic constituents alone and under conditions where liquid-liquid phase separation is not anticipated to occur.

Michael A. Battaglia Jr. et al.
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Status: final response (author comments only)
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Michael A. Battaglia Jr. et al.
Michael A. Battaglia Jr. et al.
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Short summary
The effects of water-soluble organic carbon (WSOC) on aerosol pH were characterized for aqueous-phase particles containing a mixture of inorganics and organics. The ISORROPIA-II and E-AIM models were used in conjunction with AIOMFAC to quantify the effect of organics on aerosol pH through (1) changes to the aerosol liquid water content, and (2) changes to the hydrogen ion activity coefficient. The study included both organic acids and non-acids, at RH levels ranging from 70–90 %.
The effects of water-soluble organic carbon (WSOC) on aerosol pH were characterized for...