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

Twenty-six pairs of PM and PM 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 (PM < 60 μg m and PM < 135 μg m), medium (PM: 60-170 μg m and PM: 135-325 μg m), and severe (PM > 170 μg m and PM > 325 μg m) pollution levels. At the light and severe pollution levels, the increase of PM was accounted for by the increase of PM, and the variation of PM was small. In contrast, the increase of PM at the medium pollution level was caused by the increase in both PM and PM. Sulfate (SO), nitrate (NO), ammonium (NH), and chloride in the form of ammonium chloride (Cl) accounted for 47.8% and 60.3% of the PM mass at the light and severe levels, respectively. These values indicate a large contribution of secondary inorganic species to the PM growth. As the pollution level changed from light to medium, the contribution of SO to the growth of PM decreased from 49.0% to 15.1%, while those of NO and Cl 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 SO was 30.1%, while those of NO and Cl 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 PM 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 (PM) of crustal origin enhanced sulfate production via heterogeneous surface reactions. Quantification of the contribution of primary and secondary species to PM 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.