No abstract

Atmospheric visibility (VIS), reflecting the atmosphere transmittance, is an important indicator of air quality (Bäumer et al., 2008). Low visibilities exert significant influences on transportation, especially air traffic (Sun et al., 2021), and thus decrease society efficiencies. China has suffered from air pollution with the rapid economic growth over the past years and the visibility deterioration in China has raised public awareness (Guo et al., 2014). The Chinese government has made many efforts to improve haziness including various strategies of emission control since the issuing of the Air Pollution Prevention Control and Action Plan in 2013. PM 2.5 , NO x , and SO 2 were observed to reduce by 33%, 21%, and 59%, respectively (Zheng et al., 2018). However, the low visibility events occur frequently despite the PM 2.5 mitigation (Xu et al., 2020). The deep blue day, which requires the visibility to be larger than 21.52 km, even decreased in China from 1980 to 2018 (S Wang et al., 2022), which indicates that the visibility repairment lags behind the reduction of PM 2.5 . PM 2.5 was brought down by 30% while the low visibility frequency in the Pearl River Delta regions decreased by only 5% (Xu et al., 2020). The increased aerosol extinction efficiency hinders visibility improvement in Eastern China despite that the PM 2.5 level was reduced by 50.8 μg/m 3 (Yao et al., 2021). More efforts should be devoted to visibility repairment along with the reduction of PM 2.5 .By definition, visibility is the distance at which an observer can barely recognize a black object, and the basic formula of atmospheric visibility is VIR = Inε/σ where ε is a contrast threshold of the observer, and σ is the

Nowadays, many cities in China are suffering from both fine particulate matter (PM2.5, particulate matter with an aerodynamic diameter smaller than 2.5 µm) and ozone (O3) pollution. Secondary organic aerosol (SOA) is an important component of PM2.5 and is closely related to the oxidation processes. To investigate the characteristics and formation pathways of SOA during different types of haze pollution episodes, carbonaceous components of PM2.5 and volatile organic compounds (VOCs) were monitored continuously in Chengdu in April 2019, when Chengdu experienced not only PM2.5 pollution (SPP) but also a complex pollution of PM2.5 and O3 (CoP). In the CoP episode, the concentrations of SOA increased by 51.2% as compared to SPP, and the SOA concentrations were positively correlated with PM2.5 mass concentrations. These suggest that SOA drove the increase in PM2.5 levels during the haze event to some extent. The preliminary VOC source analysis based on the feature ratio showed that vehicle emission and fuel volatilization sources were the main sources of VOCs at this urban site. In addition, coal emissions and biomass burning were also important contributors. High-carbon alkanes and aromatic hydrocarbons significantly contributed to the SOA formation. These results provide a preliminary understanding of SOA formation during different types of pollution episodes in Chengdu, which can help us to further understand air pollution in this typical region.