Aerosol pH indicator and Organosulfate Detectability from Aerosol Mass Spectrometry Measurements

Abstract:

Aerosol sulfate is a major component of submicron particulate matter (PM1). Sulfate can be present as inorganic (mainly ammonium sulfate or 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 some 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 major 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 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 variable, and the proposed literature methods cannot retrieve OS concentrations. In regions with lower acidity (pH>0) and ammonium nitrate (fraction in aerosol phase<0.3), the proposed OS methods may be more reliable. 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 HySOx+/SOx+ AMS ratio and measured ammonium balance are promising for rapid estimation of aerosol pH < 0, which is expected in ⅔ of the troposphere. These results allow an improved understanding of important intensive properties of ambient aerosols.