Observation of Water-Soluble Ions in Southwest China: Secondary Ion Formation and Biomass Burning Impact

Objective To investigate the seasonal characteristics, sources, secondary generation of water-soluble ions (WSIs) in PM 2.5 and the impacts of biomass burning (BB) on WSIs in Southwest China. Methods A total of 560 PM 2.5 samples were collected in three cities, with nine WSIs analyzed for concentrations. Source contributions were identified by combining positive matrix factorization (PMF) model and backward trajectory analysis. The changes of sulfur oxidation ratio (SOR), nitrogen oxidation ratio (NOR), meteorological factors and O 3 were analyzed, and the formation of secondary ions was discussed. Results The daily concentration of total WSIs ranged from 5.08 to 27.18 μg m −3 , averaging 10.90 ± 3.91 μg m −3 and representing 29.52% of the PM 2.5 mass. The seasonal variation characteristics of PM 2.5 , secondary inorganic ions (SNA) and K + were consistent, and the peak concentrations appeared in spring. BB and secondary inorganic aerosols dominated WSIs sources, with Southeast Asia (SEA) identified as the primary origin area of springtime air masses. In spring, elevated O 3 levels and high ammonia emissions jointly promoted the photochemical reaction of SO 2 , NO 2 and NH 3 to generate (NH 4 ) 2 SO 4 and NH 4 NO 3 . The rising concentrations of K and NH resulted in the alkalinity of aerosols. Conclusions BB had a significant effect on WSIs in PM 2.5 in Southwest China. The focus of spring particulate matter pollution control in Southwest China is to establish an effective cross-border air pollution prevention and control system, and attention should be paid to the management of O 3 and NH 3 emissions.