Study On Optical Properties And Related Parameters Of Aerosols Near The Ground In Beijing Urban Area

Aerosols have important effects on global climate change,air quality and human health,and continue to contribute the largest uncertainty to the total radiative forcing estimate.The accurate description of aerosol optical properties and related parameters can provide a scientific basis and effective constraints for the parameterization of aerosol optical properties in regional climate models,which is of great significance for reducing the uncertainty in the assessment of radiative forcing.The measurement of aerosol optical properties of submicron（PM₁）and sub-10μm particles(PM₁₀)under dry conditions（RH<30%）were conducted in Beijing from 2018 to 2020.Changes in key aerosol optical parameters such as scattering coefficient,absorption coefficient,scattering?ngstr?m exponent（SAE）,single scattering albedo（SSA）,backscattering ratio（b）,asymmetry parameter（g）,mass scattering efficiency（MSE）and their relationships between different optical properties were investigated.Notably,it is the first time for long time concurrent measurement of aerosol optical properties for PM₁and PM₁₀ in Beijing.During the entire observation period,the average values of aerosol scattering coefficients at 550 nm for PM₁,PM₁₀,and PM_1-10 in Beijing urban area are134.6±128.8 Mm^-1,197.2±199.7 Mm^-1,and 63.8±77.8 Mm^-1,respectively.The mean values of aerosol absorption coefficients at 550 nm for PM₁,PM₁₀,and PM_1-10 are15.7±12.2 Mm^-1,18.1±14.6 Mm^-1 and 2.9±2.8 Mm^-1,respectively.The annual average values of the aerosol scattering coefficient,absorption coefficient and the PM_2.5mass concentration all decrease.From 2018 to 2020,PM_2.5 mass concentration decreases from 52.7μg m^-3 to 38.9μg m^-3.At the same time,the scattering coefficients decreased by 20.1%for PM₁,24.7%for PM₁₀ and 33.6%for PM_1-10;the absorption coefficients of PM₁,PM₁₀ and PM_1-10 decreased by 40.8%,41.1%and 44.4%,respectively.This shows to a certain extent that the air quality in Beijing urban areas gradually improves from 2018 to 2020 and the control of fine particles should be taken more attachment.Besides,higher SAE(PM₁,1.92和PM₁₀,1.53),submicron scattering ratio and submicron absorption ratio during the study period suggest that size distributions are dominated by fine mode aerosols in Beijing urban area and the submicron aerosols are the main contributor to scattering and absorption.There are significant seasonal changes in aerosol optical properties for PM₁ and PM₁₀.The seasonal variation of the aerosol scattering coefficient are consistent with the seasonal variation of the PM_2.5mass concentration.The scattering coefficient and the PM_2.5 mass concentration are the largest in spring.This is related to the occurrence of frequent heavy pollution events in the spring during the observation period which is suggested that air quality has been improved,but more attention needs to be paid to the air quality in spring.The seasonal variation of absorption coefficient is higher in autumn and winter and lower in spring and summer,which is related to the higher light absorption aerosol concentration in autumn and winter than that in spring and summer.The single scattering albedo at 550nm for PM₁ and PM₁₀ are 0.87±0.05 and 0.89±0.05,respectively,which indicates that the scattering aerosols in Beijing urban area are dominant and absorbing aerosols mainly appear in fine particles.Based on the observation results,this paper further calculated the aerosol direct radiative forcing efficiency at the top of the atmosphere,and found that the aerosol direct radiative forcing efficiency at PM₁ and PM₁₀ were-23.9 W m^-2 and-24.7 W m^-2,respectively.According to the relationship between the aerosol optical properties,it can be seen that as the scattering coefficient increases,the SAE and sub-micron scattering ratios gradually decrease,which indicates that in a turbid atmosphere,the particle size distribution of fine particles has shifted to larger sizes and the contribution of submicron aerosols to scattering decreases at high aerosol loading.MSE is higher during heavy pollution periods,suggesting that not only high aerosol loading but also high MSE contributes to the degradation of visibility during heavy pollution episodes.