The annual mean PM2.5 mass concentration has decreased because of the stringent emission controls implemented in Beijing, China in recent years, whereas the nitrate NO3- mass fraction in PM2.5 increases gradually. Low-visibility events occur frequently even though PM2.5 pollution has been mitigated significantly, with the daily mean PM2.5 mass concentration mostly less than 75 μg/m3. In this study, the non-linear relationship was analyzed between atmospheric visibility and PM2.5 based on chemical composition from a two-year field observation. Our results showed that NO3- became the main constituent of PM2.5, especially during the haze pollution episodes. A localized parameterization scheme was proposed between the atmospheric extinction coefficient (σext) and major chemical constituents of PM2.5 by multiple linear regression (MLR). The contribution of NO3- to σext increased with increasing air pollution, and NO3- became the most important contributor for PM2.5 above 75 μg/m3. The visibility decreased with increasing NO3- mass fraction for the same PM2.5 mass concentration when PM2.5 was above 20 μg/m3. The hygroscopicity of PM2.5 increased with increasing mass fraction of hygroscopic NO3-. These results stressed the importance of reducing particulate NO3- and its precursors (for instance, NH3) through effective emission control measures as well as the tightening of PM2.5 standards to further improve air quality and visibility in Beijing.
Keywords: Atmospheric extinction coefficient; Atmospheric visibility; Nitrate; PM(2.5) pollution.
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