Ammonia emissions from fertilizer application as a dominant controller of nitrogenous aerosols in the urban atmosphere of Kunming, Southwest China

Abstract

The complex nature of the sources and formation processes of nitrogenous aerosols are a significant problem in cities. Despite being recognized as a less polluted city, Kunming has encountered unexpected air pollution events in the past few years. In this study, PM_2.5 samples collected from Kunming were analyzed to ascertain the source apportionment of nitrogenous aerosols. It was observed the δ¹⁵N values of the total nitrogen (TN) in PM_2.5 over the sampling period varied from 2.4‰ to 18.4‰ (12.0 ± 2.6‰). An isotope-based source apportionment method was employed to quantify the proportional contribution from six potential sources: NH₃ from fertilizer application (63.1%), livestock excreta (9.5%), the mixing source (5.5%); NO_X from vehicle emissions (8.5%), biomass burning (7.1%), and coal combustion (6.4%). Notably, the content of the nitrogen species demonstrated a consistent increase from late October. The total nitrogen levels and nitrogen oxidation ratios (NOR) were found to be sensitive to the changes in temperature and aerosol liquid water content (ALWC). It can be inferred that the higher ALWC levels enhanced the hydrolysis of N₂O₅, thereby promoting the conversion of NO₂, boosting the growth of nitrogenous aerosols, and leading to elevated NOR values. The PSCF analysis indicated that nitrogenous aerosols in urban Kunming were mainly affected by local and regional “hot spots” of NH₃ emissions from fertilizer application. This research not only presents a robust method for quantifying the primary sources of TN in PM_2.5, but also offers insights into air quality assessment and the management of atmospheric nitrogen pollution in Kunming.