Current Challenges in Visibility Improvement in Southern China

Stringent emission controls effectively brought down particulate mass concentrations by >30% in the North China Plain (NCP) and Pearl River Delta (PRD) regions. However, the low-visibility (<10 km) frequency in the PRD region barely changed (similar to 5% decrease in Guangzhou), while a decrease of similar to 25% was observed in the NCP. A higher aerosol hygroscopicity and a higher scattering efficiency, induced by differences in aerosol chemical composition and size distribution, were found in the PRD compared to the NCP, explaining why visibility was much worse in the PRD even under the same relative humidity and particulate mass conditions. The unsatisfactory visibility improvement in Guangzhou was explained by a substantial increase (47%) in aerosol optical hygroscopicity since 2013. To further improve visibility in southern China, aside from further secondary inorganic aerosol reduction, the current challenge mainly lies in identifying and reducing the precursors to organic aerosol components, whose hygroscopicity are highly susceptible to changes in emission patterns, for which we urgently need more in-depth studies of the formation mechanism of oxygenated organic aerosol.