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

Theodora Nah,Jian Wang,Junwei Yang,Rodney J Weber,Amy P Sullivan

doi:10.3390/atmos12081040

Abstract

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 Hair+ and Wi. Hair+ was controlled primarily by the NVCs/SO42− molar ratio, while Wi 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 Hair+ values and decreases in Wi 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.

Highlights

Aerosol acidity is an important property that governs many atmospheric processes that transform the mass concentration and composition of atmospheric aerosols, which in turn have important implications for air quality, climate, and human and ecosystem health. Examples of these atmospheric processes include enhancing the formation of secondary organic aerosols (SOA) through acid-catalyzed reactions during the oxidation of volatile organic compounds (VOCs) [1,2,3], controlling the gas-aerosol partitioning of atmospheric semi-volatile basic and acidic species (e.g., ammonia (NH3), hydrochloric acid (HCl), nitric acid (HNO3)) [4,5,6], regulating the water solubilities of trace metals and nutrient species in aerosols [7,8,9], and modulating halogen and sulfur chemistries in the marine boundary layer [10,11]. pH is the parameter commonly used to characterize the acidity of atmospheric aerosols
Organics, NH4+, SO42−, and NO3− measured by an Aerodyne High-Resolution Time-ofFlight Aerosol Mass Spectrometer during the summer intense operating periods (IOP) were low, with the mean mass concentrations of these species ranging from 0.01–0.55 μg m−3 [47]
The pH of aerosols in remote marine atmospheres influences many important atmospheric processes that occur in the marine boundary layer, including regulating halogen and sulfur chemistries, and nutrient fertilization of surface ocean waters [11,14]

Summary

Introduction

Aerosol acidity is an important property that governs many atmospheric processes that transform the mass concentration and composition of atmospheric aerosols, which in turn have important implications for air quality, climate, and human and ecosystem health. Significant mass concentrations of chloride (Cl−) and non-volatile cations (NVCs) such as sodium (Na+), calcium (Ca2+), potassium (K+), and magnesium (Mg2+) can be present in atmospheric aerosols. This is especially the case for aerosols found in marine atmospheres and/or areas affected by severe dust events [14]. Battaglia Jr. et al (2019) showed that water-soluble inorganic ions alone adequately constrain the aerosol pH under conditions where liquid–liquid phase separation is not expected to occur [19]

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Results

Conclusion