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

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.

Field Evidence for Constraints of Nearly Dry and Weakly Acidic Aerosol Conditions on the Formation of Organosulfates

“…However, the optimal temperature or RH range to which these models can be applied may not be as broad as that of ISORROPIA-II (as the wide temperature and RH range in this study) (Supporting Information). 16,38,41−46 Furthermore, although previous studies have applied the ISORROPIA-II to predict aerosol pH and ALW in arid environments, 16,21,47,48 we also acknowledge that our prediction results cannot be used as precise values but as the optimal solution among numerous prediction methods (Supporting Information).…”

“…Urumqi (northwest China) experiences dry and dusty summers, 21 potentially resulting in nearly dry and nonacidic particle properties. Increased air relative humidity (RH) and aerosol sulfate and nitrate levels (derived from burning fossil fuels for heating) 21 in winter can increase particle acidity and ALW concentrations. 22,23 Thus, Urumqi is an ideal region to study the formation of aerosol OSs under different levels of particle acidity and ALW concentration.…”