Abstract
In recent years, high fine particulate (PM2.5) 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 PM2.5 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 PM2.5 pollution and high O3 pollution could occur simultaneously. The cases with data of double high O3 and PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 pollution episodes.
Highlights
Shanghai is located in the eastern coast of the Yangtze River Delta (YRD), adjacent to the East Sea
This result suggested strong atmospheric oxidizing capacity during the double high pollution cases, which would promote the formation of secondary inorganic aerosols (SIA) and further enhance the concentration of PM2.5
This study focused on investigating the pollution characteristics and occurrence mechanisms of double high-level O3 and PM2.5 pollution episodes and the significant role of daytime HONO in O3 formation in these pollution episodes, based on almost-three-year observation measurements at the Pudong New Area environmental monitoring station in Shanghai
Summary
Shanghai is located in the eastern coast of the Yangtze River Delta (YRD), adjacent to the East Sea. A recent modelling study found that HONO photolysis acted as the dominant source for primary OH production with a contribution of more than 92% based on a winter field campaign conducted at a rural site of the North China. Some modeling studies showed that elevated levels of HONO considerably enhanced the ROx. Atmosphere 2021, 12, 557 photolysis acted as the dominant source for primary OH production with a contribution of more than 92% based on a winter field campaign conducted at a rural site of the North. PM2.5 boundary and O3 pollution episodes.up to now, fewstudy, studies haveon been reported on theobservation impact of HONO on O3 formation during In this based a continuous field from 2014–2016 in Shanghai, a the specific double high-level PM2.5 and O3 pollution episodes. The characteristics and occurrence mechanism of the double high pollution episodes and the role of daytime HONO in O3 formation in these pollution episodes
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