Increased Aerosol Extinction Efficiency Hinders Visibility Improvement in Eastern China

Abstract

Though China has witnessed substantial particular matter pollution mitigation owing to the strict emission control in recent years, the frequency of haze events with low visibility is not improved as much as expected, especially in cold seasons. Here, 6-year wintertime observations, satellite retrievals, a thermodynamic model, and theoretical calculation were integrated to better understand the complex influence of aerosol chemical composition and hygroscopic behaviors on visibility impairment. We found that the proportion of nitrate in aerosol mass concentration increased by approximately 10% from 2013 to 2018. Such a transition in aerosol chemical compositions together with increasing ambient humidity in past years jointly enhanced aerosol extinction efficiency, and the elevated proportion of nitrate played an increasingly critical role after 2017. The increased aerosol extinction efficiency is responsible for the less improved visibility despite large decrease in aerosol mass concentrations in eastern China. Plain Language Summary Atmospheric aerosol degrades regional visibility as a result of light extinction, which is highly linked with aerosol chemical composition and particle size growth in humid environment. Due to a much sharper drop of sulfur emission than nitrogen emission in China, aerosol has become increasingly nitrate-dominant. The transition in aerosol chemical compositions together with increasing ambient humidity in past years may jointly alter aerosol hygroscopic behaviors and enhance aerosol extinction efficiency. In consequence, although substantial mitigation of aerosol pollution has been achieved in recent years, hazy days with low visibility still frequently occur in eastern China.

Elevated Formation of Particulate Nitrate From N2O5 Hydrolysis in the Yangtze River Delta Region From 2011 to 2019

“…In addition, the hydrolysis of N 2 O 5 also plays a crucial role in nitrate formation, which influences the conversion of N 2 O 5 to nitrate (Wang, Chen, et al., 2020; Wen et al., 2018; Sun et al., 2018). Elevated nitrate proportion in PM 2.5 has been demonstrated to promote N 2 O 5 hydrolysis by combining the slightly increased RH (Liu et al., 2020; Y. Wang et al., 2020). Figure S9 in Supporting Information shows the variations of aerosol liquid water content (ALWC) as a function of nitrate proportion in PM 2.5 .…”

Estimation of Aerosol Extinction Coefficient Using Camera Images and Application in Mass Extinction Efficiency Retrieval

“…We inferred that the high values of α e f f 2.5 were caused by the changes in the size and composition of aerosols. Liu et al [45] confirmed that the MEE in Nanjing, China, increased from 7.1 m 2 g −1 in 2013 to 9.3 m 2 g −1 in 2018. Joo et al [46] also stated that the MEE of PM 2.5 increased by 0.72 m 2 g −1 annually during 2015-2019.…”

“…Second, we did not consider the effects of relative humidity. Many studies confirmed that relative humidity has a high impact on visibility [30,35,36,[42][43][44][45][46]. The correlation coefficient of the mass concentration of PM 2.5 and visibility data increased from 0.81 to 0.89 when correcting for the relative humidity [47].…”

The deep blue day is decreasing in China

“…On the other hand, fewer air pollutions do not directly bring in more blue skies, considering days with higher cloud cover or precipitation. The inconsistency between dropping PM and unchanged low visibility events (Ding et al 2016 ) can be seen even during the COVID-19 lockdown period (Huang et al 2020b ; Huang et al 2020c ; Liu et al 2020 ). All mentioned above try to convey the non-synergy correlation between air pollution and blue days when considering the unfavorable meteorological conditions dominated by large-scale synoptic circulation (Bei et al 2020a ).…”