Measurement report: Characteristics of nitrogen-containing organics in PM_2.5 in Urumqi, northwest China: differential impacts of combustion of fresh and old-age biomass materials

Abstract. Nitrogen-containing organic compounds (NOCs) are abundant and important aerosol components, deeply involving in global nitrogen cycle. However, the sources and formation processes of NOCs remain largely unknown, particularly in the city (Urumqi, China) farthest from the ocean worldwide. Here, NOCs in PM_2.5 collected in Urumqi over a one-year period were characterized by ultrahigh-resolution mass spectrometry. The abundance of CHON compounds (mainly poor-O unsaturated aliphatic-like species) in the positive ion mode was higher in the warm period than in the cold period, which was largely attributed to the contribution of fresh biomass material combustion (e.g., forest fires) associated with amidation of unsaturated fatty acids in the warm period, rather than the oxidation processes. However, CHON compounds (mainly nitro-aromatic species) in the negative ion mode increased significantly in the cold period, which was tightly related to old-age biomass combustion (e.g., dry straws) in wintertime Urumqi. For CHN compounds, we found that alkyl nitriles and aromatic CNH compounds showed higher abundance in the warm and cold periods, respectively. It further confirmed different impacts of the combustion of fresh- and old-age biomass materials on NOC compositions. Our results clarify the mechanisms by which fresh and old-age biomass materials emitted different NOCs.