Citation:

Chen, X., X. Mingjie, M. Hays, E. Edgerton, D. Schwede, AND Johnt Walker. Characterization of organic nitrogen in aerosols at a forest site in the southern Appalachian Mountains. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 18(9):6829-6846, (2018). https://doi.org/10.5194/acp-18-6829-2018

Impact/Purpose:

Organic nitrogen plays a role in new particle formation and aerosol characteristics such as hygroscopicity and is a large component of atmospheric reactive nitrogen deposition. However, the composition of organic nitrogen in aerosols and rainfall remains poorly characterized. This study examines the contribution of water soluble organic nitrogen to total nitrogen in PM2.5 at a remote forest site in southern Appalachia. New advanced chemical methods are used to speciate contributions to water soluble organic N from organosulfates and nitro-aromatics associated with biomass burning.

Description:

This study investigates the composition of organic particulate matter in a remote montane forest in the southeastern U.S., focusing on the role of organic nitrogen (N) in sulfur-containing secondary organic aerosol (nitrooxy-organosulfates) and aerosols associated with biomass burning (nitro-aromatics). Bulk water soluble organic N (WSON) represented  14% w/w of water soluble total N (WSTN) in PM2.5, on average, across seasonal measurement campaigns conducted in the spring, summer, and fall of 2015. Largest contributions of WSON to WSTN were observed in spring (~ 18% w/w) and lowest in the fall (~10% w/w). On average, identified nitro-aromatic and nitrooxy-organosulfate compounds accounted for a small fraction of WSON, ranging from  1% in spring to  4% in fall, though were observed to contribute as much as 28% w/w of WSON in individual samples. Highest concentrations of oxidized organic N species occurred during summer (average of 0.65ngN/m3) along with a greater relative abundance of higher generation oxygenated terpenoic acids, indicating an association with more aged aerosol. Highest concentrations of nitro-aromatics (eg. nitrocatechol and methyl-nitrocatechol), levoglucosan, and aged SOA tracers were observed during fall, associated with aged biomass burning plumes. Nighttime nitrate radical chemistry is the most likely formation pathway for nitrooxy-organosulfates observed at this low NOx site (generally <1ppb). Isoprene derived organosulfate (MW216, 2-methyltetrol derived), which is formed from isoprene epoxydiols (IEPOX) under low NOx conditions, was the most abundant individual organosulfate. Concentration weighted average N/C ratios for nitro-aromatics + organosulfates + terpenoic acids were one order of magnitude lower than the overall aerosol N/C ratio, indicating the presence of other uncharacterized higher N content species. Although nitrooxy organosulfates and nitroaromatics contributed a small fraction of WSON, our results provide new insight into the atmospheric formation processes and sources of these largely uncharacterized components of atmospheric organic N.

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