Current challenges of improving visibility due to increasing nitrate fraction in PM during the haze days in Beijing, China.

The annual mean PM mass concentration has decreased because of the stringent emission controls implemented in Beijing, China in recent years, whereas the nitrate NO mass fraction in PM increases gradually. Low-visibility events occur frequently even though PM pollution has been mitigated significantly, with the daily mean PM mass concentration mostly less than 75 μg/m. In this study, the non-linear relationship was analyzed between atmospheric visibility and PM based on chemical composition from a two-year field observation. Our results showed that NO became the main constituent of PM, especially during the haze pollution episodes. A localized parameterization scheme was proposed between the atmospheric extinction coefficient (σ) and major chemical constituents of PM by multiple linear regression (MLR). The contribution of NO to σ increased with increasing air pollution, and NO became the most important contributor for PM above 75 μg/m. The visibility decreased with increasing NO mass fraction for the same PM mass concentration when PM was above 20 μg/m. The hygroscopicity of PM increased with increasing mass fraction of hygroscopic NO. These results stressed the importance of reducing particulate NO and its precursors (for instance, NH) through effective emission control measures as well as the tightening of PM standards to further improve air quality and visibility in Beijing.