Measurement report: Characteristics of nitrogen-containing organics in PM_2.5 in Ürümqi, northwestern China – differential impacts of combustion of fresh and aged biomass materials

Abstract

Nitrogen-containing organic compounds (NOCs) are abundant and important aerosol components deeply involved in the global nitrogen cycle. However, the sources and formation processes of NOCs remain largely unknown, particularly in the city (Ürümqi, China) farthest from the ocean worldwide. Here, NOCs in PM2.5 collected in Ürümqi over a 1-year period were characterized by ultra-high-resolution mass spectrometry. The abundance of CHON compounds (mainly oxygen-poor 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 aged biomass combustion (e.g., dry straws) in wintertime Ürümqi. For CHN compounds, alkyl nitriles and aromatic species showed higher abundance in the warm and cold periods, respectively. Alkyl nitriles can be derived from fresh biomass material combustion associated with the dehydration of amides (the main CHON compounds in the warm period). In contrast, aromatic species were tightly related to aged biomass burning. These findings further suggested different impacts of the combustion of fresh and aged biomass materials on NOC compositions in different seasons. The overall results shed light on the mechanisms by which fresh and aged biomass materials release different NOCs during combustion.