Chen Xi, Xie Mingjie, Hays Michael D, Edgerton Eric, Schwede Donna, Walker John T
National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711, USA.
Oak Ridge Institute for Science and Education (ORISE), National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711, USA.
Atmos Chem Phys. 2018 May 16;18(9):6829-6846. doi: 10.5194/acp-18-6829-2018.
This study investigates the composition of organic particulate matter in PM in a remote montane forest in the southeastern US, 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% of water-soluble total N (WSTN) in PM on average across seasonal measurement campaigns conducted in the spring, summer, and fall of 2015. The largest contributions of WSON to WSTN were observed in spring (~ 18% ) and the lowest in the fall (~ 10% ). 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% of WSON in individual samples that were impacted by local biomass burning. The highest concentrations of oxidized organic N species occurred during summer (average of 0.65 ng N m) along with a greater relative abundance of higher-generation oxygenated terpenoic acids, indicating an association with more aged aerosol. The highest concentrations of nitro-aromatics (e.g., 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 NO site (generally < 1 ppb). Isoprene-derived organosulfate (MW216, 2-methyltetrol derived), which is formed from isoprene epoxydiols (IEPOX) under low NO conditions, was the most abundant individual organosulfate. Concentration-weighted average WSON / WSOC ratios for nitro-aromatics + organosulfates + terpenoic acids were 1 order of magnitude lower than the overall aerosol WSON / WSOC ratio, indicating the presence of other uncharacterized higher-N-content species. Although nitrooxy-organosulfates and nitro-aromatics 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, which also helps to advance the atmospheric models to better understand the chemistry and deposition of reactive N.
本研究调查了美国东南部偏远山区森林中细颗粒物（PM）中有机颗粒物的组成，重点关注有机氮（N）在含硫二次有机气溶胶（硝基氧代有机硫酸盐）以及与生物质燃烧相关的气溶胶（硝基芳烃）中的作用。在2015年春、夏、秋三季进行的季节性测量活动中，水溶性有机氮（WSON）平均占PM中水溶性总氮（WSTN）的约14%。WSON对WSTN的贡献在春季最大（约18%），秋季最小（约10%）。平均而言，已鉴定出的硝基芳烃和硝基氧代有机硫酸盐化合物仅占WSON的一小部分，从春季的约1%到秋季的约4%不等，不过在受当地生物质燃烧影响的个别样品中，这些化合物对WSON的贡献高达28%。夏季氧化态有机氮物种的浓度最高（平均为0.65纳克氮/立方米），同时更高代含氧萜烯酸的相对丰度也更高，这表明与更老化的气溶胶有关。秋季观察到硝基芳烃（如硝基邻苯二酚和甲基硝基邻苯二酚）、左旋葡聚糖和老化的二次有机气溶胶（SOA）示踪剂的浓度最高，与老化的生物质燃烧羽流有关。在这个低NO（通常<1 ppb）的地点，夜间硝酸根自由基化学是观察到的硝基氧代有机硫酸盐最可能的形成途径。由异戊二烯在低NO条件下形成的异戊二烯衍生的有机硫酸盐（分子量216，由2-甲基四醇衍生）是最丰富的单一有机硫酸盐。硝基芳烃+有机硫酸盐+萜烯酸的浓度加权平均WSON/WSOC比值比整体气溶胶的WSON/WSOC比值低1个数量级，这表明存在其他未表征的高氮含量物种。尽管硝基氧代有机硫酸盐和硝基芳烃在WSON中所占比例较小，但我们的研究结果为这些大气有机氮中很大程度上未表征的成分的大气形成过程和来源提供了新的见解，这也有助于改进大气模型，以更好地理解活性氮的化学性质和沉降。
