Double High-Level Ozone and PM_2.5 Co-Pollution Episodes in Shanghai, China: Pollution Characteristics and Significant Role of Daytime HONO

In recent years, high fine particulate (PM_2.5) pollution episodes with high 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 of hydroperoxyl and organic peroxy radicals, HONO would inevitably affect the formation of O₃, but its role in the formation of O₃ during the double high-level PM_2.5 and O₃ pollution episodes remains unclear. In this study, the characteristics of the double high pollution episodes and the role of HONO in O₃ formation in these episodes were investigated based on field observation in urban Shanghai from 2014 to 2016. Results showed that high PM_2.5 pollution and high O₃ pollution could occur simultaneously. The cases with data of double high O₃ and PM_2.5 concentrations accounted for about 1.0% of the whole sampling period. During the double high pollution episodes, there still existed active photochemical processes, while the active photochemical processes at high PM_2.5 concentration were conductive to the production and accumulation of O₃ under a VOC-limited regime and a calm atmospheric condition including high temperature, moderately high relative humidity, and low wind speed, which in turn enhanced the conversions of SO₂ and NO₂ and the formation and accumulation of secondary sulfate and nitrate aerosols and further promoted the increase of PM_2.5 concentration and the deterioration of air pollution. Further analysis indicated that the daytime HONO concentration could be strongly negatively correlated with O₃ concentration in most of the double high pollution episodes, revealing the dominant role of HONO in O₃ formation during these pollution episodes. This study provides important field measurement-based evidence for understanding the significant contribution of daytime HONO to O₃ formation, and helps to clarify the formation and coexistence mechanisms of the double high-level O₃ and PM_2.5 pollution episodes.