Characterization of organic nitrogen in aerosols at a forest site in the southern Appalachian Mountains May 2018

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.