LAUSR

SO42- and NO3- are important chemical components of fine particulate matter (PM2.5), especially during haze periods. This study selected two haze episodes in Beijing, China with similar meteorological conditions. A monitoring-modeling approach was developed to estimate the secondary conversion ratios of sulfur and nitrogen based on monitored and simulated concentrations. Measurements showed that in Episode 1 (24th-25th October, 2014), the concentrations (proportions) of SO42- and NO3- reached 35.1 μg/m3 (14.9%) and 55.0 μg/m3 (22.9%), while they reached 14.4 μg/m3 (9.3%) and 59.1 μg/m3 (38.1%) in Episode 2 (26th-27th October, 2017). A modeling system was applied to apportion Beijing's SO42- and NO3- in primary and secondary SO42-/NO3- emitted from local and regional sources. Thus, secondary conversion contributions considering the local and regional level were defined. The former primarily focused on Beijing atmospheric oxidation ability and the latter mainly considered the existence form of Beijing SO42-/NO3- under the regional transport impacts. Finally, secondary oxidation ratios were estimated through combining secondary conversion contribution coefficients for simulated and monitored concentrations. At regional level, sulfur oxidation ratios in polluted (clean) days during two sampling periods were 0.57-0.72 (0.07-0.52) and 0.74-0.80 (0.08-0.61), nitrogen oxidation ratios were 0.20-0.29 (0.05-0.15) and 0.34-0.38 (0.02-0.29), indicating that atmospheric oxidation was enhanced when considering regional transport through 2014-2017. At the local level, sulfur oxidation ratios were 0.66-0.71 (0.04-0.48) in haze (clean) days, while nitrogen oxidation ratios were 0.16-0.29 (0.02-0.16). The atmospheric oxidation ability markedly increased in PM2.5 pollution days, but changed only slightly between the two periods.