Characteristics of Ozone Concentration in Shanghai and Its Associated Atmospheric Circulation Background During Summer Half-years from 2006 to 2021

Abstract

It is of great importance to scientifically evaluate the impact of weather and climate conditions on the occurrence of O3 pollution in order to improve the accuracy of O3 pollution forecasts， as well as to reasonably control and reduce the adverse effects of O3 pollution. The characteristics of O3 concentration and climate background were analyzed based on daily O3 concentration data， meteorological factors， and NCEP/NCER reanalysis data from 2006 to 2021 in Shanghai. In addition， the differences in atmospheric circulation situations during years with anomalous O3 concentrations were compared and diagnosed from the perspective of climatology. Additionally， the monthly O3 concentration prediction model （seasonal autoregressive integrated moving average with exogenous regressors， SARIMAX） was further established by adding the key meteorological factors. The results indicated that both the whole-year average and summer half-year average O3 concentrations in Shanghai were increasing with fluctuation， and the summer half-year average was much higher than the annual average， up to 36.2%. Furthermore， there was a significant negative correlation between O3 concentration and wind speed （correlation coefficient of -0.826） and a significant positive correlation with the frequency of static wind and the number of days in which the low cloud cover was less than 20% （correlation coefficients of 0.836 and 0.724， respectively）. The monthly mean O3 concentration had a clear periodicity， showing a pattern with a high concentration in the middle period （April to September） and a low concentration at the beginning and end of the periods. High O3 concentration years （2013-2021） were accompanied by more polluted days， lower average wind speed， more small wind （≤1.5 m·s-1） days， more days of low cloud cover of less than 20%， more days of high temperature， higher direct solar radiation， and more sunshine hours. When the location of the stronger West Pacific subtropical high was westward and southward in the summer half-year， Shanghai was influenced by an anomalous westerly wind， which was not conducive to the transportation of clean air from the sea to Shanghai and thus led to the high concentration of O3 pollution. When the long wave radiation emitted from the ground was low in the summer half-year， it was favorable for the increase in ground temperature and caused a high concentration of O3 pollution. Adding direct solar radiation， maximum temperature， and wind speed as exogenous variables to the monthly O3 forecast model could significantly improve the effectiveness of the monthly forecast， with the root mean square error decreasing by 47.7% （from 22 to 11.5） and the correlation coefficient increasing by 11.2% （from 0.819 to 0.911）， which could be applied to the practical prediction of monthly O3 concentration.