Current Challenges in Visibility Improvement in Sichuan Basin.

The visibility in the Sichuan Basin (SB) was worse than that in the North China Plain (NCP) for the same level of PM2.5. One well‐known reason is that the aerosol extinction enhancement due to hygroscopic growth in the SB was stronger than that in the NCP because the relative humidity (RH) in the SB was higher than that in the NCP. However, the visibility was worse in the SB under the same PM2.5 and RH levels. The volume extinction efficiency (VEE) is kept between 7 and 8 for different PM2.5 in the SB, while the VEE increased from 4.6 to 7.2 when the PM2.5 level increased from 20 to 180 μg/m3 in the NCP. The difference in VEE resulted from the different particle number size distributions. Our research demonstrates that the VEE is an important factor that influences visibility and provides new insight into visibility impairment for the SB. Plain Language Summary: The emission control strategies implemented by the Chinese government successfully reduced the PM2.5 level to the same level of about 40 μg/m3 in the Sichuan Basin (SB) and North China Plain (NCP) by 2020. However, the low‐visibility (<10 km) event frequency in the SB (44.6%) was significantly higher than that in the NCP (19.7%). One well‐known reason is that the aerosol extinction enhancement due to aerosol hygroscopic growth in the SB was stronger than that in the NCP, as the relative humidity (RH) in the SB (annual mean 75.5%) was higher than that in the NCP (annual mean 53.6%). In this study, we found that the visibility was worse in the SB under the same PM2.5 and RH levels because the aerosol volume extinction efficiency (VEE) in the SB was much higher than that in the NCP. The VEE is kept in the range between 7 and 8 for different PM2.5 levels in the SB, while the VEE increased from a small value of 4.6–7.2 when the PM2.5 level increased from 20 to 180 μg/m3 in the NCP. Our research demonstrates that the VEE is an important factor that influences visibility and provides new insight into visibility impairment for the SB. Key Points: The visibility in the Sichuan Basin (SB) is worse than that in the North China Plain (NCP) for the same PM2.5 and relative humidityThe volume extinction efficiency (VEE) in the SB is kept between 7 and 8, while it increased with PM2.5 from 4.6 to 7.2 in the NCPThe difference in VEE resulted from the different particle number size distributions at the two areas [ABSTRACT FROM AUTHOR]