Current challenges in the visibility improvement of urban Chongqing in Southwest China: From the perspective of PM2.5-bound water uptake property over 2015–2021

Abstract

The visibility (VIS) improvement remains challenging such as in Southwest China, although the aerosol loading was reduced markedly. More in-depth understanding of interactions between visibility and aerosol physicochemical properties is required. This study systematically investigated mechanisms behind the interannual variation of low visibility at urban Chongqing from the aspect of PM2.5-bound optical enhancements over a relatively large timescale of 2015–2021. Results show that VIS <10 km frequently exceeded 50% in 2021 despite remarkable emission reductions since 2015, with the most severe challenge in visibility improvement faced in winter. Generally, the annual mean aerosol hygroscopicity and light extinction capacity were promoted during 2015–2021, as demonstrated by a higher optical enhancement factor (f(RH)), aerosol liquid water content per unit PM2.5 mass (i.e., ALWC/PM2.5), and dry mass extinction efficiency (MEEdry) observed for 2021 in comparison to previous years. A pronounced ascending trend existed in the annual mean fraction of nitrate (fNO3−), which even surpassed organics and sulfate to be the predominant composition in PM2.5. A similar evolution of fNO3− was observed during the visibility degradation specifically when VIS <10 km, highlighting the importance of highly hydrophilic inorganics in the haze formation. Given that the higher level of fNO3− normally introduced an elevated ALWC/PM2.5 which likely enhanced secondary aerosol formation (e.g., sulfate, nitrate, and secondary organics) via multiphase reactions, the combined effects could further exacerbate the visibility degradation particularly under highly humid and polluted conditions. Stricter controls such as on NOx and volatile organic precursors would be beneficial for the visibility improvement in China.