Double High-Level Ozone and PM2.5 Co-Pollution Episodes in Shanghai, China: Pollution Characteristics and Significant Role of Daytime HONO

In recent years, high fine particulate (PM2.5) pollution episodes with ozone (O3) levels have been observed in Shanghai from time to time. However, their occurrence and characteristics remain poorly understood. Meanwhile, as a major precursor of tropospheric hydroxyl radical (OH) that initiates the formation hydroperoxyl organic peroxy radicals, HONO would inevitably affect O3, but its role O3 during double high-level PM2.5 remains unclear. this study, these were investigated based on field observation urban 2014 2016. Results showed could occur simultaneously. The cases data concentrations accounted for about 1.0% whole sampling period. During episodes, there still existed active photochemical processes, while processes at concentration conductive production accumulation under VOC-limited regime calm atmospheric condition including temperature, moderately relative humidity, low wind speed, which turn enhanced conversions SO2 NO2 secondary sulfate nitrate aerosols further promoted increase deterioration air pollution. Further analysis indicated daytime be strongly negatively correlated most revealing dominant episodes. This study provides important measurement-based evidence understanding significant contribution formation, helps clarify coexistence mechanisms