Organic nitrogen in aerosols at a forest site in southern Appalachia

John Walker1, Xi Chen2, Mingjie Xie3, Michael Hays4 and Eric Edgerton5

This study investigates the composition of organic particulate matter (PM2.5) in a remote montane forest in the southeastern U.S., focusing on the role of organic nitrogen (N) in sulfur-containing secondary organic aerosol (SOA) and aerosols associated with biomass burning.  Measurements targeted four groups of compounds: 1) nitro-aromatics associated with biomass burning; 2) organosulfates and nitrooxy organosulfates produced from biogenic SOA precursors (i.e., isoprene, monoterpenes and unsaturated aldehydes); 3) terpenoic acids formed from monoterpene oxidation; and 4) organic molecular markers including methyltetrols, C-5 alkene triols, 2-methylglyceric acid, 3-hydroxyglutaric acid and levoglucosan.  Terpenoic acids and organic markers were included to assist in characterizing the extent of biogenic compound oxidation and atmospheric processing (i.e., aerosol aging) as well as contributions from biomass burning sources.

The average fraction of WSON in water soluble total nitrogen (WSTN) exhibited a pronounced seasonal pattern, ranging from ~18% w/w in the spring to ~10% w/w in the fall. Nitro-aromatic and nitrooxy-organosulfate compounds accounted for as much as 28% w/w of WSON. Oxidized organic nitrogen species showed a maximum concentration in summer (average of 0.65ngN/m3, maximum of 1.83ngN/m3) consistent with greater relative abundance of aged biogenic SOA tracers (higher generation oxygenated terpenoic acids). Highest concentrations of nitro-aromatics (eg. Nitrocatechol and methyl-nitrocatechol) were observed during the fall season associated with aged biomass burning plumes.  Isoprene derived organosulfate (MW216, 2-methyltetrol derived), which is formed from isoprene epoxydiols (IEPOX) under low Nox conditions, was the most abundant individual organosulfate. Although nitro-aromatics and nitrooxy organosulfates account for a small fraction (seasonal averages of 1.0 to 4.4%) of WSON, our results provide insight into atmospheric formation processes and sources of these largely uncharacterized organic nitrogen species.


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