# Aerosol pH indicator and organosulfate detectability from aerosol mass spectrometry measurements

Aerosol sulfate is a major component of submicron particulate matter (PMinline-formula1). 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 inline-formula> 0) and ammonium nitrate (fraction of total mass inline-formula< 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 inline-formula< 0 and ammonium balance inline-formula< 0.65), sulfate fragment ratios show a clear relationship with acidity. The measured ammonium balance (and to a lesser extent, the Hinline-formulaySOinline-formula $M7inlinescrollmathml{}_{x}^{+}$ 8pt14ptsvg-formulamathimg939a0fd4ac8a32f0b74ccb09ff5e6502 amt-14-2237-2021-ie00001.svg8pt14ptamt-14-2237-2021-ie00001.png inline-formula $M8inlinescrollmathml/$ 8pt14ptsvg-formulamathimgfb147fccdcf98a9911cf3d26a8f6dc33 amt-14-2237-2021-ie00002.svg8pt14ptamt-14-2237-2021-ie00002.png  SOinline-formula $M9inlinescrollmathml{}_{x}^{+}$ 8pt14ptsvg-formulamathimgc89109ffea58260f8e30ed77e7b90fb9 amt-14-2237-2021-ie00003.svg8pt14ptamt-14-2237-2021-ie00003.png AMS ratio) is a promising indicator of rapid estimation of aerosol pH inline-formula< 0, including when gas-phase NHinline-formula3 and HNOinline-formula3 are not available. These results allow an improved understanding of important intensive properties of ambient aerosols.

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Schueneman, Melinda K. / Nault, Benjamin A. / Campuzano-Jost, Pedro / et al: Aerosol pH indicator and organosulfate detectability from aerosol mass spectrometry measurements. 2021. Copernicus Publications.

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