Fine Aerosol Acidity and Water during Summer in the Eastern North Atlantic

Aerosol pH governs many important atmospheric processes that occur in the marine boundary layer, including regulating halogen and sulfur chemistries, and nutrient fertilization of surface ocean waters. In this study, we investigated the acidity of PM₁ over the eastern North Atlantic during the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) aircraft campaign. The ISORROPIA-II thermodynamic model was used to predict PM₁ pH and water. We first investigated the sensitivities of PM₁ pH and water predictions to gas-phase NH₃ and HNO₃ concentrations. Our sensitivity analysis indicated that even though NH₃ and HNO₃ were present at very low concentrations in the eastern North Atlantic during the campaign, PM₁ pH calculations can still be sensitive to NH₃ concentrations. Specifically, NH₃ was needed to constrain the pH of populations of PM₁ that had low mass concentrations of NH₄⁺ and non-volatile cations (NVCs). We next assumed that gas-phase NH₃ and HNO₃ concentrations during the campaign were 0.15 and 0.09 µg m⁻³, respectively, based on previous measurements conducted in the eastern North Atlantic. Using the assumption that PM₁ were internally mixed (i.e., bulk PM₁), we determined that PM₁ pH ranged from 0.3–8.6, with a mean pH of 5.0 ± 2.3. The pH depended on both <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>H</mi><mrow><mi>a</mi><mi>i</mi><mi>r</mi></mrow><mo>+</mo></msubsup></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>W</mi><mi>i</mi></msub></mrow></semantics></math></inline-formula>. <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>H</mi><mrow><mi>a</mi><mi>i</mi><mi>r</mi></mrow><mo>+</mo></msubsup></mrow></semantics></math></inline-formula> was controlled primarily by the NVCs/SO₄²⁻ molar ratio, while <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>W</mi><mi>i</mi></msub></mrow></semantics></math></inline-formula> was controlled by the SO₄²⁻ mass concentration and RH. Changes in pH with altitude were driven primarily by changes in SO₄²⁻. Since aerosols in marine atmospheres are rarely internally mixed, the scenario where non-sea salt species and sea-salt species were present in two separate aerosol modes in the PM₁ (i.e., completely externally mixed) was also considered. Smaller pH values were predicted for the aerosol mode comprised only of non-sea salt species compared to the bulk PM₁ (difference of around 1 unit on average). This was due to the exclusion of sea-salt species (especially hygroscopic alkaline NVCs) in this aerosol mode, which led to increases in <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>H</mi><mrow><mi>a</mi><mi>i</mi><mi>r</mi></mrow><mo>+</mo></msubsup></mrow></semantics></math></inline-formula> values and decreases in <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>W</mi><mi>i</mi></msub></mrow></semantics></math></inline-formula> values. This result demonstrated that assumptions of aerosol mixing states can impact aerosol pH predictions substantially, which will have important implications for evaluating the nature and magnitude of pH-dependent atmospheric processes that occur in the marine boundary layer.