Comparison of Characteristics and Reactive Oxidative Species of PM2.5 in Xi'an, China During Haze and Clean Days

Abstract

PM2.5 samples were collected in Xi'an, China during haze(2015-11-30-2015-12-9) and clean days(2016-01-13-2016-01-22). PM2.5 mass, organic carbon(OC), elemental carbon(EC), and water-soluble ions(i.e., NH4+, NO3-, and SO42-) were measured to investigate the characteristics of the two typical pollution processes. The result showed that PM2.5 masses were(170±47.5) μg·m-3 and(48.6±17.9) μg·m-3 for the haze and clean days, respectively, with the haze accompanied by low visibility, high humidity, calm wind, and other adverse weather conditions. The secondary inorganic aerosol(SIA) including NH4+, NO3-, and SO42- accounted for 49.8%±13.1% and 19.4%±5.95% of the PM2.5 mass for the haze and clean days, respectively. sulfur and nitrogen oxidation ratios(SOR and NOR) on haze days were 0.282±0.157 and 0.269±0.124, respectively, which were much higher than those measured on the clean days(0.189±0.057 and 0.077±0.046). The average concentration of secondary organic carbon(SOC) for the haze days(6.22±3.87) μg·m-3 was 5 times of that measured in the clean days(1.44±1.63) μg·m-3. Secondary pollution and adverse weather conditions were the main reasons for the heavy haze. Finally, the level of particulate-Reactive Oxygen Species(ROS)was measured by the 2',7'-dichlorofluorescin diacetate(2',7'-DCFH) fluorescence method, and the average ROS concentration (as H2O2) was(4.99±1.54) nmol·m-3 during haze days and(0.492±0.356) nmol·m-3 during clean days. Accumulation effect and secondary oxidation process may be the main reasons for the increase of ROS concentration under the heavy haze conditions in Xi'an. Positive correlations(P<0.05) between the concentrations of ROS and the ions of NO3- and SO42- indicated that ROS may participate in secondary oxidation process of SIA.