Preprints
https://doi.org/10.5194/acp-2021-368
https://doi.org/10.5194/acp-2021-368

  28 May 2021

28 May 2021

Review status: this preprint is currently under review for the journal ACP.

Urban aerosol chemistry at a land-water transition site during summer – Part 2: Aerosol pH and liquid water content

Michael A. Battaglia Jr.1,a, Nicholas Balasus1, Katherine Ball1, Vanessa Caicedo2, Ruben Delgado2, Annmarie G. Carlton3, and Christopher J. Hennigan1 Michael A. Battaglia Jr. et al.
  • 1Department of Chemical, Biochemical, and Environmental Engineering, University of Maryland, Baltimore County
  • 2Joint Center for Earth Systems Technology, University of Maryland, Baltimore County
  • 3Department of Chemistry, University of California, Irvine
  • acurrent affiliation: School of Earth and Atmospheric Sciences, Georgia Institute of Technology

Abstract. Particle acidity (aerosol pH) is an important driver of atmospheric chemical processes and the resulting effects on human and environmental health. Understanding the factors that control aerosol pH is critical when enacting control strategies targeting specific outcomes. This study characterizes aerosol pH at a land-water transition site near Baltimore, MD during summer 2018 as part of the second Ozone Water-Land Environmental Transition Study (OWLETS-2) field campaign. Inorganic fine mode aerosol composition, gas-phase NH3 measurements, and all relevant meteorological parameters were used to characterize the effects of temperature, aerosol liquid water (ALW), and composition on predictions of aerosol pH. Temperature, the factor linked to the control of NH3 partitioning, was found to have the most significant effect on aerosol pH during OWLETS-2. Overall, pH varied with temperature at a rate of −0.047 K−1 across all observations, though the sensitivity was −0.085 K−1 for temperatures > 293 K. ALW had a minor effect on pH, except at the lowest ALW levels (< 1 µg m−3) which caused a significant increase in aerosol acidity (decrease in pH). Aerosol pH was generally insensitive to composition (SO42− , SO42−:NH4+ , Tot-NH3 = NH3 + NH4+), consistent with recent studies in other locations. In a companion paper, the sources of episodic NH3 events (95th percentile concentrations, NH3 > 7.96 µg m−3) during the study are analyzed; aerosol pH was higher by only ~0.1–0.2 pH units during these events compared to the study mean. A case study was analyzed to characterize the response of aerosol pH to nonvolatile cations (NVCs) during a period strongly influenced by primary Chesapeake Bay emissions. Depending on the method used, aerosol pH was estimated to be either weakly (~0.1 pH unit change based on NH3 partitioning calculation) or strongly (~1.4 pH unit change based on ISORROPIA thermodynamic model predictions) affected by NVCs. The case study suggests a strong pH gradient with size during the event and underscores the need to evaluate assumptions of aerosol mixing state applied to pH calculations. Unique features of this study, including the urban land-water transition site and the strong influence of NH3 emissions from both agricultural and industrial sources, add to the understanding of aerosol pH and its controlling factors in diverse environments.

Michael A. Battaglia Jr. et al.

Status: open (until 10 Jul 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Michael A. Battaglia Jr. et al.

Michael A. Battaglia Jr. et al.

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Short summary
This study characterizes aerosol liquid water content and aerosol pH at a land-water transition site near Baltimore, Maryland. We characterize the effects of unique meteorology associated with the close proximity to the Chesapeake Bay and episodic NH3 events emitted from industrial and agricultural sources on aerosol chemistry during the summer. We also examine two events where primary Bay emissions underwent ageing in the polluted urban atmosphere.
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