Understanding Ozone Pollution in Yrd from the Perspective of Diurnal Cycles in 2013–17

Ozone has become a severe environmental problem in many cities of China. In Shanghai, ozone pollution days came up with different diurnal patterns. It is vital to understand the mechanism behind it. This paper performed cluster method to ozone pollution days (>100ppb) in summer in Shanghai and got 4 clusters of 111 days. It was found that ozone concentration in Shanghai was strongly influenced by air masses due to background southwesterly. Regional transport from southwestern areas carried masses biogenic volatile organic compounds (BVOCs), precursors of ozone. Meanwhile, there were VOC-limited regime in Shanghai and NO_x-limited regime in surrounding areas. The transition of sensitivity regimes was positive for ozone production as BVOCs increased. Both of them intensified ozone pollution in Shanghai, making daily pollution time exceed 6 hours. Sea-land breeze (SLB) was found to influence ozone peak in another way. When there was typhoon activity in West Pacific, pushing subtropical high farther north, Shanghai had 20%~30% less total cloud cover, bringing in 8%~13% more radiation. These changes promoted photoreaction of ozone and raised land temperature which developed SLB circulation. At last, the SLB convection increased cloud cover and suppressed ozone generation. The impacts of BVOCs should arouse attention in future studies.