Inorganic ion chemistry of local particulate matter in a populated city of North China at light, medium, and severe pollution levels.

Abstract

Twenty-six pairs of PM2.5 and PM10 samples were collected during haze episodes in Zhengzhou (113°28 E, 34°37 N), a highly populated city in North China. The samples were used to examine the inorganic ion chemistry of particulate matter (PM) of local origin at light (PM2.5\u202f<\u202f60\u202fμg\u202fm-3 and PM10\u202f<\u202f135\u202fμg\u202fm-3), medium (PM2.5: 60-170\u202fμg\u202fm-3 and PM10: 135-325\u202fμg\u202fm-3), and severe (PM2.5\u202f>\u202f170\u202fμg\u202fm-3 and PM10\u202f>\u202f325\u202fμg\u202fm-3) pollution levels. At the light and severe pollution levels, the increase of PM10 was accounted for by the increase of PM2.5, and the variation of PM10-2.5 was small. In contrast, the increase of PM10 at the medium pollution level was caused by the increase in both PM2.5 and PM10-2.5. Sulfate (SO42-), nitrate (NO3-), ammonium (NH4+), and chloride in the form of ammonium chloride (Cl-S) accounted for 47.8% and 60.3% of the PM2.5 mass at the light and severe levels, respectively. These values indicate a large contribution of secondary inorganic species to the PM2.5 growth. As the pollution level changed from light to medium, the contribution of SO42- to the growth of PM2.5 decreased from 49.0% to 15.1%, while those of NO3- and Cl-S increased from 25.1% and 0.6% to 32.5% and 2.8%, respectively, indicating the substantial production of nitrate and chloride. At the severe level, the contribution of SO42- was 30.1%, while those of NO3- and Cl-S were 5.9% and 0.5%, respectively, suggesting a hindering effect of sulfate on the production of nitrate and chloride. These results indicate that the production of secondary species with the increase of PM2.5 was dominated by sulfate-associated conversions at the light and severe pollution levels and was substantially influenced by nitrate- and chloride-associated conversions at the medium pollution level. The estimation of carbonate presence in the PM indicates that part of the carbonate in coarse particles (PM10-2.5) of crustal origin enhanced sulfate production via heterogeneous surface reactions. Quantification of the contribution of primary and secondary species to PM2.5 showed that it was dominated by both primary and secondary particles at the light pollution level, and it was mainly composed of secondary species at the severe pollution level.