The impact of aerosols on photolysis frequencies and ozone production in Beijing during the 4-year period 2012–2015

Abstract

Abstract. During the period 2012–2015, photolysis frequencies were measured at the Peking University site (PKUERS), a site representative of Beijing. We present a study of the effects of aerosols on two key photolysis frequencies, j(O1D) and j(NO2). Both j(O1D) and j(NO2) display significant dependence on aerosol optical depth (AOD; 380 nm) with a non-linear 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. The absolute values of slopes are equal to 4.2–6.9×10-6 and 3.4×10-3 s−1 per AOD unit for j(O1D) and j(NO2) respectively at a solar zenith angle (SZA) of 60∘ and AOD smaller than 0.7, both of which are larger than those observed in a similar, previous study in the Mediterranean. This indicates that the aerosols in Beijing have a stronger extinction effect on actinic flux than absorptive dust aerosols in the Mediterranean. Since the photolysis frequencies strongly depended on the AOD and the 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 seasonal mean j(NO2) by 24 % and 30 % for summer and winter, respectively, and a corresponding decrease in seasonal mean j(O1D) by 27 % and 33 %, respectively, compared to an aerosol-free atmosphere (AOD =0). Based on an observation campaign in August 2012, we used a photochemical box model to simulate the ozone production rate (P(O3)). The simulation results shows that the monthly mean daytime 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 a high level under the condition of high concentrations of ozone precursors (volatile organic compounds, VOCs, and NOx), which inhibits the production of ozone to a large extent. This phenomenon implies a negative feedback mechanism in the atmospheric environment of Beijing.

Reversal of Aerosol Properties in Eastern China with Rapid Decline of Anthropogenic Emissions

“…Previous studies in China have shown intense interactions between aerosols and the boundary layer [13], aerosols' indirect effects on clouds [11][12][13][14][15][16][17][18][19][20][21][22], and photochemistry [14,20]. Within the last five years, the frequent haze pollution (AOD > 1.0) in northern China has decreased from more than 80 days/year to less than 30 days, and aerosol properties, including absorption and size distribution, have undergone rapid reversal in variation trends.…”

“…Generally, the rapid reduction in anthropogenic emissions in China has led to dramatic changes in aerosol loading and optical properties, which can in turn alter photochemical process and regional climate effects [12][13][14]. Figure 10 Despite the rapid decline in PM2.5 concentration, regional O3 pollution in summer has been a major air quality problem in eastern China.…”

“…As shown in Figure 11, the average of daytime maximum 8-h O 3 concentration in northern China increased from~120 µg/m 3 in 2013 to~170 µg/m 3 in 2018, indicating prevalent O 3 pollution exceeding the O 3 standard of 160 µg/m 3 in China. It has been found that high-concentration aerosols can reduce O 3 production rate by~25% in Beijing though intense light extinction [14]. As one of the primary gaseous precursors of both PM 2.5 and O 3 , the emission of NO x decreased obviously during the years 2013-2018 [17].…”

“…These aerosol particles can modify clouds' droplet size and lifetime [11], and thereby affects the frequency and intensity of regional precipitation [12]. In the other hand, an aerosol's radiative effects, depending on its constituents and vertical profiles, can in turn regulate the chemical and physical processes of air pollution [13,14]. Nevertheless, the amount and sources of anthropogenic emissions in eastern China during the past few decades have undergone dramatic changes due to both rapid economic development and changing control measures [5,[15][16][17].…”

“…By contrast, the surface ozone (O 3 ) level exhibits a remarkable increase trend [20]. It has been found that light extinction of aerosols at high concentrations can strongly inhibit O 3 production rate [14,20]. Furthermore, the rapid decline of anthropogenic emissions in China can bring about considerable variability in aerosol optical and microphysical properties [21], which is significant in atmospheric chemical and physical processes.…”