Abstract. Measurements of organosulfates in ambient aerosols provide insight to the extent of secondary organic aerosol (SOA) formation from mixtures of biogenic gases and anthropogenic pollutants. Organosulfates have, however, proved analytically challenging to quantify, due to lack of authentic standards and the complex sample matrix in which organosulfates are observed. This study presents a sensitive and accurate new analytical method for the quantification of organosulfates based upon ultra-performance liquid chromatography (UPLC) with negative electrospray ionization mass spectrometry (MS) with the aid of synthesized organosulfate standards. The separation is based upon hydrophilic interaction liquid chromatography (HILIC) with an amide stationary phase that provides excellent retention of carboxy-organosulfates and isoprene-derived organosulfates. The method is validated using six model compounds: methyl sulfate, ethyl sulfate, benzyl sulfate, hydroxyacetone sulfate, lactic acid sulfate and glycolic acid sulfate. A straightforward protocol for synthesis of highly pure organosulfate potassium salts for use as quantification standards is presented. This method is used to evaluate the efficiency and precision of two methods of ambient PM2.5 sample extraction. Spike recoveries averaged 98 ± 8% for extraction by ultra-sonication and 98 ± 10% for extraction by rotary shaking. Ultra-sonication was determined to be a better method due to its higher precision compared to rotary shaking. Analysis of ambient PM2.5 samples collected on 10–11 July 2013 in Centreville, AL, USA during the Southeast Atmosphere Study (SAS) confirms the presence of hydroxyacetone sulfate in ambient aerosol for the first time. Lactic acid sulfate was the most abundant compound measured (9.6–19 ng m−3), followed by glycolic acid sulfate (8–14 ng m−3) and hydroxyacetone sulfate (2.7–5.8 ng m−3). Trace amounts of methyl sulfate were detected, while ethyl sulfate and benzyl sulfate were not. Application of this HILIC separation method to ambient aerosol samples further demonstrates its utility in resolving additional biogenic organosulfates.

Atmospheric brown carbon (BrC) is an important contributor to the radiative forcing of climate by organic aerosols. Because of the molecular diversity of BrC compounds and their dynamic transformations, it is challenging to predictively understand BrC optical properties. OH radical and O3 reactions, together with photolysis, lead to diminished light absorption and lower warming effects of biomass burning BrC. The effects of night-time aging on the optical properties of BrC aerosols are less known. To address this knowledge gap, night-time NO3 radical chemistry with tar aerosols from wood pyrolysis was investigated in a flow reactor. This study shows that the optical properties of BrC change because of transformations driven by reactions with the NO3 radical that form new absorbing species and lead to significant absorption enhancement over the ultraviolet–visible (UV-vis) range. The overnight aging increases the mass absorption coefficients of the BrC by a factor of 1.3–3.2 between 380 nm and 650 nm. Nitrated organic compounds, particularly nitroaromatics, were identified as the main products that contribute to the enhanced light absorption in the secondary BrC. Night-time aging of BrC aerosols represents an important source of secondary BrC and can have a pronounced effect on atmospheric chemistry and air pollution.

Abstract. Organosulfates are secondary organic aerosol (SOA) products that form from reactions of volatile organic compounds (VOC), such as isoprene, in the presence of sulfate that is primarily emitted by fossil fuel combustion. This study examines the anthropogenic influence on biogenic organosulfate formation at an urban site in Atlanta, Georgia (GA) in the southeastern United States (US). Organosulfates were analyzed in fine particulate matter (PM2.5) collected during August 2015 in Atlanta using hydrophilic interaction liquid chromatography (HILIC), tandem mass spectrometry (MS/MS), and high-resolution time-of-flight (ToF) mass spectrometry. By their MS/MS response, 32 major organosulfate species were identified, selected species were quantified, and other species were semi-quantified using surrogate standards. Organosulfates accounted for 16.5 % of PM2.5 organic carbon (OC). Isoprene-derived organosulfates were the most abundant, dominated by methyltetrol sulfate which accounted for 12.6 % of PM2.5 OC. Together, the isoprene-derived organosulfates accounted for the majority of the isoprene-derived SOA that had been previously observed in Atlanta, but had not been identified at the molecular level. Other major species included seven monoterpene-derived organosulfates, five diesel and/or biodiesel-derived organosulfates, and three new organosulfates that are also expected to derive from isoprene. Organosulfate species and concentrations in Atlanta were compared to those in a rural forested site in Centreville, Alabama (AL) during summer 2013, which were also dominated by isoprene-derived organosulfates. In Atlanta, isoprene-derived organosulfate concentrations were 2–6 times higher and accounted for twice as much OC. The greatest enhancement in concentration was observed for 2-methylglyceric acid sulfate whose formation is enhanced in the presence of nitrogen oxides (NO and NO2; NOx) and is a tracer for isoprene high-NOx SOA. The isoprene-derived organosulfates indicated a stronger influence of NOx in Atlanta compared to Centreville. Overall, these results suggest that SOA in the southeastern US can be reduced by controlling NOx and SO2 emissions from fossil fuel combustion. This study gives insights into the major organosulfate species that should be targets for future measurements in urban environments and standard development.

Abstract. Organosulfates are components of secondary organic aerosols (SOA) that form from oxidation of volatile organic compounds (VOCs) in the presence of sulfate. In this study, the composition and abundance of organosulfates were determined in fine particulate matter (PM2.5) collected from Centreville, AL, during the Southern Oxidant and Aerosol Study (SOAS) in summer 2013. Six organosulfates were quantified using hydrophilic interaction liquid chromatography (HILIC) with triple quadrupole mass spectrometry (TQD) against authentic standards. Among these, the three most abundant species were glycolic acid sulfate (0.5–52.5 ng m−3), lactic acid sulfate (0.5–36.7 ng m−3), and hydroxyacetone sulfate (0.5–14.3 ng m−3). These three species were strongly inter-correlated, suggesting similar precursors and/or formation pathways. Further correlations with sulfate, isoprene, and isoprene oxidation products indicate important roles for these precursors in organosulfate formation in Centreville. Positive filter sampling artifacts associated with these organosulfates due to gas adsorption or reaction of gas phase precursors of organosulfates with sulfuric acid were assessed for a subset of samples and were less than 7.8 % of their PM2.5 concentrations. Together, the quantified organosulfates accounted for  <  0.3 % of organic carbon mass in PM2.5. To gain insights into other organosulfates in PM2.5 collected from Centreville, semi-quantitative analysis was employed by way of monitoring characteristic product ions of organosulfates (HSO4− at m∕z 97 and SO4− ⋅  at m∕z 96) and evaluating relative signal strength by HILIC–TQD. Molecular formulas of organosulfates were determined by high-resolution time-of-flight (TOF) mass spectrometry. The major organosulfate signal across all samples corresponded to 2-methyltetrol sulfates, which accounted for 42–62 % of the total bisulfate ion signal. Conversely, glycolic acid sulfate, the most abundant organosulfate quantified in this study, was 0.13–0.57 % of the total bisulfate ion signal. Precursors of m∕z 96 mainly consisted of nitro-oxy organosulfates. Organosulfates identified were mainly associated with biogenic VOC precursors, particularly isoprene and to a lesser extent monoterpenes and 2-methyl-3-buten-2-ol (MBO). While a small number of molecules dominated the total organosulfate signal, a large number of minor species were also present. This study provides insights into the major organosulfate species in the southeastern US, as measured by tandem mass spectrometry that should be targets for future standard development and quantitative analysis.

This study provides molecular insights into the light absorption properties of biomass burning (BB) brown carbon (BrC) through the chemical characterization of tar condensates generated from heated wood pellets at oxidative and pyrolysis conditions. Both liquid tar condensates separated into “darker oily” and “lighter aqueous” immiscible phases. The molecular composition of these samples was investigated using reversed-phase liquid chromatography coupled with a photodiode array detector and a high-resolution mass spectrometer. The results revealed two sets of BrC chromophores: (1) common to all four samples and (2) specific to the “oily” fractions. The common BrC chromophores consist of polar, monoaromatic species. The oil-specific BrC chromophores include less-polar and nonpolar polyaromatic compounds. The most-light-absorbing pyrolysis oily phase (PO) was aerosolized and size-separated using a cascade impactor to compare the composition and optical properties of the bulk versus the aerosolized BrC. The mass absorption coefficient (MAC300–500 nm) of aerosolized PO increased compared to that of the bulk, due to gas-phase partitioning of more volatile and less absorbing chromophores. The optical properties of the aerosolized PO were consistent with previously reported ambient BB BrC measurements. These results suggest the darkening of atmospheric BrC following non-reactive evaporation that transforms the optical properties and composition of aged BrC aerosols.