Contributions of primary emissions and secondary formation to nitrated aromatic compounds in themountain background region of Southeast China

As a major component of brown carbon (BrC), nitrated aromatic compounds (NACs) have a significant role in the atmosphere's ability to absorb light. However, the sources and major influencing factors of NACs in the mountain background atmosphere are mostly lacking. Based on a thorough field investigation of NACs from fine particle samples taken in 2014 and 2015 at the peak of Mt. Wuyi (1139 m.a.s.l.), the current work discussed the seasonal fluctuations in their composition, their sources, and the important influencing factors. The total abundance of nine quantifiable NACs increased significantly in the winter (3.9 ± 1.5 ng m-3) and autumn (2.1 ± 0.94 ng m-3), whereas it decreased in the spring (1.3 ± 0.75 ng m-3) and summer (0.97 ± 0.36 ng m-3). According to the results of structural equation modeling, the majority of NACs (93 %) were influenced by biomass, coal, and petroleum combustion over the entire year. This work identified the origins of NACs with applying the positive matrix factorization receptor model. The five major source factors were biomass burning, coal combustion, secondary formation by nitration reaction, secondary formation by photochemical reaction, and other sources. Among them, biomass burning and coal combustion played an important role, especially in the wintertime, with a contribution of more than 50 %. Meanwhile, contributions from secondary formation were significant in these remote areas, which were mainly by photochemical reaction in the summertime and nitration reaction in the wintertime. Further analysis indicated that the formation of NACs was comparatively sensitive to NO2 under low-NOx conditions, suggesting that NACs would become significant in the aerosol characteristics when nitrate concentrations decreased as a result of emission reduction measures.