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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-339
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-339
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  31 Aug 2020

31 Aug 2020

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This preprint is currently under review for the journal AMT.

Aerosol pH Indicator and Organosulfate Detectability from Aerosol Mass Spectrometry Measurements

Melinda K. Schueneman1, Benjamin A. Nault1, Pedro Campuzano-Jost1, Duseong S. Jo1,2, Douglas A. Day1, Jason C. Schroder1,a, Brett B. Palm1, Alma Hodzic2, Jack E. Dibb3, and Jose L. Jimenez1 Melinda K. Schueneman et al.
  • 1Department of Chemistry, and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
  • 2Atmospheric Chemistry Observations and Modeling, National Center for Atmospheric Research, Boulder, CO 80301, USA
  • 3Earth Systems Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, USA
  • anow at: Air Pollution Control Division, Colorado Department of Public Health and the Environment, Denver, CO, USA

Abstract. Aerosol sulfate is a major component of submicron particulate matter (PM1). Sulfate can be present as inorganic (mainly ammonium sulfate, AS) or organic sulfate (OS). Although OS are thought to be a smaller fraction of total sulfate in most cases, recent literature argues that this may not be the case in more polluted environments. Aerodyne Aerosol Mass Spectrometers (AMS) measure total submicron sulfate, but it has been difficult to apportion AS vs. OS as the detected ion fragments are similar. Recently, two new methods have been proposed to quantify OS separately from AS with AMS data. We use observations collected during several airborne field campaigns covering a wide range of sources and airmass ages (spanning the continental US, marine remote troposphere, and Korea) and targeted laboratory experiments to investigate the performance and validity of the proposed OS methods. Four chemical regimes are defined to categorize the factors impacting sulfate fragmentation (Fig. shown in abstract). In polluted areas with high ammonium nitrate concentrations and in remote areas with high aerosol acidity, the decomposition and fragmentation of sulfate in the AMS is influenced by multiple complex effects, and estimation of OS does not seem possible with current methods. In regions with lower acidity (pH>0) and ammonium nitrate (fraction<0.3), the proposed OS methods might be more reliable, although application of these methods often produced nonsensical results. However, the fragmentation of ambient neutralized sulfate varies somewhat within studies, adding uncertainty, possibly due to variations in the effect of organics. Under highly acidic conditions, sulfate fragment ratios show a clear relationship with acidity (pH and ammonium balance). The measured ammonium balance (and to a lesser extent, the HySOx+/SOx+ AMS ratio) is a promising indicator for rapid estimation of aerosol pH < 0, including when gas-phase NH3 and HNO3 are not available. These results allow an improved understanding of important intensive properties of ambient aerosols.

Melinda K. Schueneman et al.

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Melinda K. Schueneman et al.

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Short summary
This work focuses on two important properties of the aerosol, acidity and sulfate composition, which are important for our understanding of aerosol health and environmental impacts. We explore different methods to understand the composition of the aerosol with measurements from a specific instrument, and apply those methods to a large dataset. These measurements are confounded by other factors, making it challenging to predict aerosol sulfate composition. pH estimations, however, show promise.
This work focuses on two important properties of the aerosol, acidity and sulfate composition,...
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