The impact of aerosols on photolysis frequencies and ozone production in urban Beijing during the four-year period 2012–2015

Abstract. During the period 2012–2015, the photolysis frequencies were measured at the Peking University site (PKUERS), a representative site of urban Beijing. We present a study of the effects of aerosols on two key photolysis frequencies, j(O 1 D) and j(NO 2 ). Both j(O 1 D) and j(NO 2 ) display significant dependence on AOD with a nonlinear negative correlation. With the increase in AOD, the slopes of photolysis frequencies vs AOD decrease, which indicates that the capacity of aerosols to reduce the actinic flux decreases with AOD. In addition, the slopes are equal to 4.21–6.93 × 10 −6  s −1 and 3.20 × 10 −3  s −1 per AOD unit for j(O 1 D) and j(NO 2 ) respectively at SZA of 60°, both of which are larger than those observed in the Mediterranean. This indicates that the aerosols in urban Beijing have a stronger extinction on actinic flux than absorptive dust aerosols in the Mediterranean. Since the photolysis frequencies strongly depended on the AOD and the solar zenith angle (SZA), we established a parametric equation to quantitatively evaluate the effect of aerosols on photolysis frequencies in Beijing. According to the parametric equation, aerosols lead to a decrease in j(NO 2 ) by 24.2 % and 30.4 % for summer and winter, respectively, and the corresponding decrease in j(O 1 D) by 27.3 % and 32.6 % respectively, compared to an aerosol-free atmosphere. Based on an observation campaign in August 2012, we used the photochemical box model to simulate the ozone production rate (P(O 3 )). The simulation results shows that the monthly average net ozone production rate is reduced by up to 25 % due to the light extinction of aerosols. Through further in-depth analysis, it was found that particulate matter concentrations maintain high level under the condition of high concentrations of ozone precursors (VOCs and NO x ), which inhibits the production of ozone to a large extent. This phenomenon implies a negative feedback mechanism in the atmospheric environment of urban Beijing.