Organic aerosol is a major constituent of atmospheric fine particulates (PM2.5), which adversely affect human health and change the Earth’s radiative energy balance. Primary organic aerosol is directly emitted from sources and secondary organic aerosol (SOA) is formed in the atmosphere following oxidation of volatile organic compounds (VOC) from anthropogenic and biogenic sources. Biogenic SOA is enhanced by anthropogenic pollutants such as sulfate and NOx that mainly come from fossil fuel combustion. However, the extent to which the anthropogenic pollutants enhance biogenic SOA in different environments is unknown. The central hypothesis of this thesis is that organosulfates, organic compounds containing a sulfate ester group, are useful as tracers for anthropogenically-influenced biogenic SOA. This research aims to provide a better understanding of the sources of PM2.5 organic carbon (OC), particularly secondary organic carbon (SOC), through the inclusion of organosulfates in an organic tracer-based source apportionment model. The specific objectives of this research include 1) development of a highly sensitive and accurate method to quantify highly polar organosulfates in atmospheric aerosols, 2) identification and quantification of major organosulfate species in the ambient air, and 3) determination of anthropogenic and biogenic sources and their contributions to PM2.5 OC using an organic tracer-based positive matrix factorization (PMF) model.
A highly sensitive and accurate method was developed and validated for the quantification of highly polar organosulfates using hydrophilic interaction liquid chromatography (HILIC) and tandem mass spectrometry (MS/MS). The developed method shows excellent retention of carboxylic acid and hydroxyl containing organosulfates. The HILIC-MS/MS method was applied to PM2.5 samples collected in summer 2013 at a rural site in Centreville, AL. Quantified organosulfates accounted for approximately 0.3% of PM2.5 OC. Other major organosulfates, for which standards are not available, were monitored by their fragmentation to the bisulfate anion and/ or sulfate ion radical. The major organosulfates were determined to be 2-methyltetrol sulfate and other isoprene-derived organosulfates. Eight sources of the PM2.5 OC in Centreville, AL were identified using PMF model through the application of organosulfates and commonly used organic tracers measured in samples collected during the daytime and nighttime: vehicle emissions (8%), prescribed burning (11%), isoprene SOC formed under low-NOx (13%) and high-NOx conditions (11%), SOC formed by photochemical reactions (9%), oxidatively aged biogenic SOC (6%), sulfuric acid-influenced SOC (21%), and monoterpene SOC formed under high-NOx conditions (21%). The organosulfates enabled organic tracer-based PMF to resolve sulfuric acid-influenced SOC, while the daytime and nighttime measurements enabled organic tracer-based PMF to resolve SOC formation pathways with diurnal variations (e.g. SOC formed by photochemical reactions). The PM2.5 OC in Centreville was mainly secondary in origin (81%) and was influenced by NOx, ozone (a product of photochemical reactions of NOx and VOC), and sulfuric acid. Together, primary and secondary OC influenced by the fossil fuel use was 76%. Thus, the majority of the PM2.5 OC in Centreville during summer can be controlled by the reduction of fossil fuel use.
The HILIC-MS/MS method was also applied to daily PM2.5 samples collected from an urban site in Atlanta, GA during August 2015. The major organosulfate species identified in Atlanta were dominated by 2-methyltetrol sulfate and other isoprene-derived organosulfates, similar to Centreville. They contributed 16% of PM2.5 OC and accounted for the majority of the isoprene-derived SOA that had not previously been identified at the molecular level. The concentrations of the major isoprene-derived organosulfates in Atlanta were two to six times higher than in Centreville. The greatest enhancement was obtained for 2-methylglyceric acid sulfate, a known isoprene SOA tracer formed under high-NOx conditions, reflecting the 15 times higher average NOx concentration in Atlanta during August 2015 compared to Centreville in summer 2013. These results indicate that NOx had a stronger influence on isoprene-derived organosulfate formation in urban Atlanta compared to rural Centreville.
Overall, these results indicate that organosulfates are useful tracers for anthropogenically-influenced biogenic SOA. Thus, it is important to quantify them for use in organic tracer-based PMF modeling to determine the anthropogenically-influenced biogenic SOC in PM2.5 OC.