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 PM2.5 and PM to samples were collected during haze episodes in Zhengzhou (113 degrees 28' E, 34 degrees 37'N), a highly populated city in North China.The samples were used to examine the inorganic ion chemistry of particulate maker (PM) of local origin al. light (PM2.5 < 60 mu g m(-3) and PM10 < 135 mu g m(-3)), medium (PM2.5: 60-170 mu g m(-3) and PM10: 135-325 mu g m(-3)), and severe (PM2.5 > 170 mu g m(-3) and PM10 > 325 mu g m(-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.