Increase in Surface Ozone over Beijing-Tianjin-Hebei and the Surrounding Areas of China Inferred from Satellite Retrievals, 2005-2018

Abstract

This study analyzed the increase in the surface ozone over Beijing-Tianjin-Hebei (BTH) and its surrounding areas during the period of 2005–2018 using satellite-retrieved data. First, a geographically weighted regression (GWR) model was developed to estimate the surface ozone concentration (SOC) between 2005 and 2018 based on Ozone Monitoring Instrument (OMI) ozone profiles. The resultant values and their growth rate were then analyzed. The SOC exhibited significant variation in the spatial distribution over the study area, with the maximum and the minimum values occurring in the southeast and the northeast, respectively. The latter region also displayed the highest SOC growth rate, however, whereas the southwest displayed the lowest one. Additionally, prominent seasonality was observed in the SOC: The concentration peaked during the warm season and troughed during the cold season, but the growth rate showed the opposite trend. The values during the cold season greatly affected the annual spatial distribution and the growth rate of the SOC, whereas those during the warm season significantly contributed to the annual concentration. From 2005 till 2018, the SOC showed an upward trend with an average growth rate of 3.4 µg m^–3 y^–1, with a greater increase in the second half (2012–2018) than the first half (2005–2011) of the study period because of the stronger photochemical reactions caused by the continual increase in HCHO during summer and the weaker NO titration effect caused by the rapid decrease in NO₂ during winter. With constantly rising levels of HCHO and a VOCs-limited regime in the study area, we must formulate an effective reduction scheme for VOCs and NO₂ co-emissions in order to mitigate the surface ozone pollution, despite the risk that decreasing the NO₂ will lead to a certain increase in the SOC.