Spatial Distribution and Formation Mechanism of Water-soluble Inorganic Ions in PM2.5 During a Typical Winter Haze Episode in Guilin, China.

Abstract

A 5-day PM2.5 sampling campaign was conducted during a typical haze episode from December 16 to 20, 2016, at five urban sites and one background site in Guilin, a famous tourist city in Southern China. A total of 30 PM2.5 samples were collected, and water-soluble inorganic ions (WSII) (SO42-, NO3-, NH4+, Ca2+, K+, Cl-, Na+, and Mg2+) were determined using ion chromatography. Correlation analysis, principal component analysis, and coefficient of divergence were applied to identify the formation mechanisms of secondary inorganic ions, potential sources, and spatial distribution of WSII. The average mass concentrations of PM2.5 at each sampling site were 71.6-127.85μgm-3, which were more than the National Ambient Air Quality Standard (GB3095-2012, GradeII (35μgm-3)) in China. SO42- NO3-, and NH4+ were the major WSII, accounting for 34.43-40.59% of PM2.5 mass. NO3-/SO42- ratio revealed that stationary sources-induced PM2.5 was still remarkable. Cl-/Na+ ratio and their strong correlation (r = 0.824) indicated that atmospheric transport from outside urban region played an effective role during the haze episode. Spatial variations of WSII are not pronounced at five urban sites except the background site. High relative humidity and O3 contributed to evidently influence the transformation of SO2 to SO42- but not obvious to NOx oxidation. Finally, the major sources of WSII are identified as the mixture of sea salt, coal combustion, biomass burning, vehicle exhaust and agricultural emissions (66.892%), and fugitive sources (19.7%).