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.
Highlights
PM1, or submicron particulate matter, have important impacts on visibility, climate, and environmental and human health (Dockery et al, 1996; Lighty et al, 2000; Lohmann et al, 2004; IPCC, 2013)
WINTER and KORUS-AQ were airborne campaigns that focused on urbanized regions; the campaigns had appreciable mass concentrations of ammonium nitrate due to anthropogenic emissions of NOx and the subsequent production of HNO3 that partitions into the aerosol with ammonia (Seinfeld and Pandis, 2006)
The presence of organosulfates in particles is a topic of much recent interest, but there is a lack of online methods to quantify them
Summary
PM1, or submicron particulate matter, have important impacts on visibility, climate, and environmental and human health (Dockery et al, 1996; Lighty et al, 2000; Lohmann et al, 2004; IPCC, 2013). In order to quantify the impacts of PM1 and their evolution with changes in emissions, chemistry, and climate, PM1 sources, chemistry, and composition must be understood. Field measurements are critical to that goal, and one tool used extensively in field studies since the early 2000s is the Aerodyne aerosol mass spectrometer (AMS) and more recently its simplified version, the aerosol chemical speciation monitor (ACSM) (Jayne et al, 2000; DeCarlo et al, 2006; Canagaratna et al, 2007; Ng et al, 2011a). Schueneman et al.: Aerosol pH and organosulfate detectability from the aerosol mass spectrometer
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