Comparison of Characteristics and Reactive Oxidative Species of PM_2.5 in Xi'an, China During Haze and Clean Days

PM_2.5 samples were collected in Xi'an, China during haze (2015-11-30-2015-12-9) and clean days (2016-01-13-2016-01-22). PM_2.5 mass, organic carbon (OC), elemental carbon (EC), and water-soluble ions (i.e., NH₄⁺, NO₃^-, and SO₄^2-) were measured to investigate the characteristics of the two typical pollution processes. The result showed that PM_2.5 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 SO₄^2- accounted for 49.8%±13.1% and 19.4%±5.95% of the PM_2.5 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 SO₄^2- indicated that ROS may participate in secondary oxidation process of SIA.