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

<p>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 <span class="inline-formula">NH₃</span> 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 <span class="inline-formula">NH₃</span> partitioning, was found to have the most significant effect on aerosol pH during OWLETS-2. Overall, pH varied with temperature at a rate of <span class="inline-formula">−</span>0.047 <span class="inline-formula">K⁻¹</span> across all observations, though the sensitivity was <span class="inline-formula">−</span>0.085 <span class="inline-formula">K⁻¹</span> for temperatures <span class="inline-formula">&gt;</span> 293 <span class="inline-formula">K</span>. ALW had a minor effect on pH, except at the lowest ALW levels (<span class="inline-formula">&lt;</span> 1 <span class="inline-formula">µg m⁻³</span>), which caused a significant increase in aerosol acidity (decrease in pH). Aerosol pH was generally insensitive to composition (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="815783a157bc15e547bdd7a24388d96b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-18271-2021-ie00001.svg" width="29pt" height="17pt" src="acp-21-18271-2021-ie00001.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow><mo>:</mo><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="cf5b1b8a68e95e817603fea50cdf8bb1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-18271-2021-ie00002.svg" width="58pt" height="17pt" src="acp-21-18271-2021-ie00002.png"/></svg:svg></span></span>, total <span class="inline-formula">NH₃</span> (Tot-<span class="inline-formula">NH₃</span>) <span class="inline-formula">=</span> <span class="inline-formula">NH₃</span> <span class="inline-formula">+</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="7b888eb222795ca6a6dd2ceb535b59a0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-18271-2021-ie00003.svg" width="24pt" height="15pt" src="acp-21-18271-2021-ie00003.png"/></svg:svg></span></span>), consistent with recent studies in other locations. In a companion paper, the sources of episodic <span class="inline-formula">NH₃</span> events (95th percentile concentrations, <span class="inline-formula">NH₃</span> <span class="inline-formula">&gt;</span> 7.96 <span class="inline-formula">µg m⁻³</span>) during the study are analyzed; aerosol pH was higher by only <span class="inline-formula">∼</span> 0.1–0.2 <span class="inline-formula">pH</span> 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 (<span class="inline-formula">∼</span> 0.1 <span class="inline-formula">pH</span> unit change based on <span class="inline-formula">NH₃</span> partitioning calculation) or strongly (<span class="inline-formula">∼</span> 1.4 <span class="inline-formula">pH</span> 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 <span class="inline-formula">NH₃</span> emissions from both agricultural and industrial sources, add to the understanding of aerosol pH and its controlling factors in diverse environments.</p>