Photolysis rate in the Beijing-Tianjin-Hebei region: Reconstruction and long-term trend

Abstract With the increasingly severe ozone pollution in the Beijing-Tianjin-Hebei region in recent years, NO2 photolysis has attracted our attention as the main source of tropospheric ozone generation. The photolysis rate of NO2 (J(NO2)) is of great importance for the quantification of photochemical ozone generation and photochemical pollution research, but its long-term changes in the Beijing-Tianjin-Hebei region remain unclear. An efficient reconstruction method for J(NO2) was established via simulations with the troposphere ultraviolet and visible radiation (TUV) model and the observations of J(NO2) and total ultraviolet (UV) radiation at a typical suburban site in the Beijing-Tianjin-Hebei region. We introduced the cloud modification factor in this method which improved the R2 value between the calculated and observed J(NO2) values from 0.64 to 0.94 over the TUV clear-sky simulation level. A long-term (2005–2019) J(NO2) dataset in Xianghe was reconstructed by applying this method. The annual mean J(NO2) value reached 3.5*10−3 s−1 from2005 to 2019. The annual mean J(NO2) value fluctuated slightly from 2005 to 2012 but increased evidently in the last 7 years. The increase rates of the annual mean J(NO2) value and O(3P) generation via NO2 photolysis were 1.3*10−4 s−1 per year and 1.1*10−3 ppb•s−1 per year, respectively, from 2013 to 2019. From 2005 to 2019, J(NO2) exhibited the sharpest increasing trend in summer, but increased the least in spring. The monthly mean J(NO2) value was the highest in May and the lowest in November. Our results demonstrated that to control the ozone concentration in the Beijing-Tianjin-Hebei region, we must pay more attention to the reduction in volatile organic compounds (VOCs) under the year-by-year increase in ambient J(NO2).