Aerosol pH indicator and organosulfate detectability from aerosol mass spectrometry measurements
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Published:2021-03-23
Issue:3
Volume:14
Page:2237-2260
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ISSN:1867-8548
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Container-title:Atmospheric Measurement Techniques
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language:en
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Short-container-title:Atmos. Meas. Tech.
Author:
Schueneman Melinda K., Nault Benjamin A.ORCID, Campuzano-Jost PedroORCID, Jo Duseong S.ORCID, Day Douglas A.ORCID, Schroder Jason C., Palm Brett B.ORCID, Hodzic Alma, Dibb Jack E., Jimenez Jose L.ORCID
Abstract
Abstract. Aerosol sulfate is a major component of submicron particulate matter
(PM1). Sulfate can be present as inorganic (mainly ammonium sulfate,
AS) or organosulfate (OS). Although OS is 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 (AMSs) 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 air mass 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. 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 of total mass < 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 (when calculated pH < 0 and ammonium balance < 0.65), sulfate fragment ratios show a clear relationship with acidity. The
measured ammonium balance (and to a lesser extent, the
HySOx+ / SOx+ AMS ratio) is a promising indicator of 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.
Funder
Earth Sciences Division
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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