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

Michael A Battaglia Jr,Ruben Delgado,Christopher J Hennigan,Nicholas Balasus,Annmarie G Carlton,Vanessa Caicedo,Katherine Ball

doi:10.5194/acp-21-18271-2021

Michael A Battaglia Jr, Ruben Delgado + Show 5 more

Open Access

https://doi.org/10.5194/acp-21-18271-2021

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Abstract

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+, total NH3 (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.

Highlights

The acidity, or pH, of atmospheric aerosols affects the chemical and physical properties of airborne particles and, their impacts on climate and health (Pye et al, 2020)
Single-particle studies of aerosol pH using Raman spectroscopy have been performed but are limited by the presence of both HSO−4 and SO−4 limiting their application to more acidic particles (Boyer et al, 2020; Rindelaub et al, 2016)
Colorimetric measurements of aerosol pH have been employed, but such techniques have been limited to laboratory studies with relatively simple aerosol compositions (Craig et al, 2018; Jang et al, 2020)

Summary

Introduction

The acidity, or pH, of atmospheric aerosols affects the chemical and physical properties of airborne particles and, their impacts on climate and health (Pye et al, 2020). The gas–particle partitioning of semi-volatile acidic and basic compounds – notably NH3, HNO3, HCl, and organic acids – depends in part on aerosol pH, which directly affects the particulate matter (PM) mass concentration (Nenes et al, 2020). The optical properties of light-absorbing organic compounds, known as brown carbon, can exhibit a strong pH dependence, which directly affects their climate impacts (Phillips et al, 2017). Given the importance of aerosol pH for atmospheric processes and the limitation in estimating acidity with proxies (e.g., ion balances), there has been increased effort in recent years to identify the factors that affect pH and to characterize temporal and spatial variations in the atmosphere (Hennigan et al, 2015)

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