A Review on the Extinction Contribution of Aerosol Components in the United States and China

Aerosol extinction is the main factor affecting atmospheric visibility. The nonlinear relationship between aerosol concentration and its scattering and absorption characteristics leads to considerable uncertainty regarding the influence of aerosol on visibility. In the United States, the concentrations of RCFM (reconstructed fine mass) ranged from 1.4 µg m⁻³ to 19.4 μg m⁻³, and reconstructed aerosol extinction coefficients ranged from 10.0 Mm⁻¹ (1 Mm^–1=10^–6 m^–1) to 172.5 Mm⁻¹ during 1988–2008; these data were obtained from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. The average fine particulate matter (PM_2.5) concentrations in various regions of China ranged from 14.3 µg m⁻³ to 188.3 μg m⁻³ from 2006 to 2018, corresponding to the reconstructed aerosol extinction coefficients of 52.6–1044.0 Mm⁻¹. The PM_2.5 concentration in the United States was comparable to that in Sanya, China. In most cases, sulfate was the largest contributor to aerosol extinction, accounting for up to 77%, followed by organic matter with a maximum of 50%. By contrast, nitrate contributed more to aerosol extinction only in southern California, exceeding 30%. The difference in aerosol extinction between the east and west was more significant than the difference in aerosol concentration because of the higher relative humidity in the east than in the west. In China, sulfate and organic matter were the main components responsible for aerosol extinction, accounting for 21%–57% and 21%–39%, respectively. Meanwhile, nitrate was the main extinction-contributing component only in central China, exceeding 30%. Aerosol concentration and extinction levels were considerably higher in China than in the United States. The IMPROVE equation exhibited obvious uncertainties that underestimated the high extinction coefficient and overestimated the low extinction coefficient in China (i.e., −60%–35%). The more severe the pollution, the more obvious will be the aerosol extinction coefficient underestimation. This uncertainty will negatively affect the accuracy of the implementation of air pollution prevention and Blue Sky programs. An accurate accounting of the aerosol extinction by various components and tracing of aerosol sources need to be conducted urgently to provide a basis for developing a scientific program to control the aerosol sources for improving atmospheric visibility in various environmental climate regions.