Thresholds of Meteorological Factors Conductive to Severe Summer Ozone Pollution in China and Relation of Occurrence Frequency with Large Scale Circulations

Daily thresholds of meteorological factors relative to severe summer ozone pollution are determined in the North China Plain (NCP), the Fenhe River and Weihe River Plain (FWP), the Sichuan Basin (SCB), the Changjiang River Plain (CJR) and Pearl River Delta (PRD) by combing ozone concentrations at air quality monitoring stations and meteorological elements at weather stations. These regions share same daily thresholds, namely maximum temperature above 30 °C, relative humidity below 80%, rainfall below 10 mm and radiation in the scope of 17~27 MJ/m2 together with wind speed in the range of 0.5~3.0 m/s. The adverse meteorological frequency combining daily thresholds of wind speed and radiation shows individual trend and periodic characteristics in each region after conducting 10-year moving average, namely rising with 3~6 percentage points/decade in NCP (in June), FWP (in June, July and August) and CJR (in July) while decreasing with 2~3 percentage points in SCB in July and August. However, there are no apparent trends in PRD. Additionally, these frequencies are periodic with 8.3 years to 25 years. The frequencies are positively related to Western Pacific Subtropical High (WPSH) in NCP, FWP, CJR and PRD, while negatively relevant in SCB. The correlate coefficients between Southern Oscillation and the frequencies vary in regions and months. With cyclo-stationary empirical orthogonal function analysis, we also substantiate impacts of global warming, Pacific Decadal Oscillation, El-Nino and La-Nina on WPSH in two typical months. These would give us more insights on meteorological effects on ozone pollution and be helpful for its projection.