Determination Of Mineral Dust Light Absorption Contribution And Effects On Black Carbon Light Absorption Enhancement Estimation

The optical properties of aerosols are an important factor affecting the earthatmosphere energy balance.Black carbon(BC)is the primary light absorbing component in atmospheric aerosols,Brown carbon(BrC)and mineral dust(MD)also contribute to light absorption but mainly in the ultraviolet.Black carbon and brown carbon had been studied extensively,but there is limited attention on the light absorption contribution of mineral dust.The light absorption capacity of BC is enhanced after atmospheric aging process due to the coated.The key parameter to characterize the enhancement of black carbon absorption is the black carbon absorption enhancement coefficient(Eabs),which has an important impact on the climate effect of BC and become a hot topic currently.In recent years,many observational studies on the enhancement of light absorption of black carbon have been carried out in China and the most focusing on the North China Plain,the Yangtze River Delta and the Pearl River Delta.However,there are limited studies of fields observational on the enhancement of light absorption of black carbon in central China.Therefore,this study chooses Wuhan,the political and economic center of Central China,to carry out the study on the Eabs.The traditional method uses empirical absorption Angstrom exponent(AAE)to estimate the absorption of mineral dust or brown carbon,and this method can only handle the two components scenario(BC/BrC or BC/MD),the absorption contribution of the three components cannot be studied simultaneously,leading to substantial uncertainties.In this study,a new method was introduced to calculate the light absorption contributions of mineral dust(babs_MD)by the minimum R-squared method(MRS)using one year of online observation data in Wuhan(November 2018 to October 2019),then the absorption of secondary brown carbon(babs_BrCsec)and the absorption enhancement coefficient of black carbon is obtained after deducting the absorption of mineral dust.The purpose of this study was to investigate the mineral dust light absorption contribution and effects on Eabs,and to analyze the characteristics of Eabs variation in Wuhan and the factors that affect Eabs.The annual mean value of PM2.5 was 41.32±26.17 μg m-3 during the observation period,with significant seasonal variation,the highest concentration in winter,followed by spring,and lower concentrations in summer and autumn.Inorganics(NO3-,SO42and NH4+)were found to be the most abundant components in PM2.5 in this study,of which nitrate contributed the most,especially in winter.The average mass concentration of MD was 4.21 ±5.37 μg m-3,accounting for 9%of the PM2.5 mass concentration,and the highest concentration in spring was 11.21±7.28 μg m-3;The annual mean value of MD light absorption(babs370_MD)was 2.74±3.50 Mm-1,with obvious seasonal changes,among which spring was the highest at 7.62±4.95 Mm-1,and the light absorption accounted for 20%,indicating that the mineral dust absorption contribution cannot be ignored.By comparing the dependence of AAE370/880_total and AAE370/880_NMD with the proportion of MD in PM2.5,the rationality of the MRS method to identify MD light absorption was confirmed.In this study,the annual average value of secondary brown carbon absorption(babs370_BrCsec)was 2.00±3.13 Mm-1,and showed an obvious monthly variation trend,with high values was found in November and December.AAE370/880_NMD has a positive dependence on babs370_BrCsec,indicating that the increase of secondary brown carbon will lead to the increase of AAE,thus increasing the wavelength dependence of aerosol.Furthermore,research found that babs370_BrCsec has a positive dependence on FeT,suggesting that the iron-containing component has a promoting effect on the light absorption of secondary brown carbon.The Eabs calculated before and after separation of babs_MD were explored,and the results shows that unresolved light absorption of MD could lead to Eabs overestimation,and the difference between Eabs before and after separation of babs_MD was 12.0%.Although the effect of MD on Eabs is not significant from the annual average,the seasonal differences are very high.For example,the MD light absorption is not identified in spring result in a difference of 37.5%in Eabs,which shows that it is necessary to subtracting babs_MD for calculating Eabs.The annual mean value of Eabs520_NMD during the observation period was 1.25±0.47.and Eabs520_NMD was higher in summer(1.41±0.64),followed by autumn(1.17±0.31),winter(1.15±0.22)and spring(1.08±0.18).High Eabs520_NMD in summer was associated with the generation of more active secondary pollutants.In addition,the analysis found that Eabs520_NMD has a positive dependence on the amount of coating(including inorganic salts,secondary organic carbon,and sum of them),which proves the influence of these coating materials on Eabs,it also proves the rationality of obtaining Eabs for this research from the side.This study uses KBB+/EC as an indicator of biomass burning,combined with backward trajectories and fire spots to explore the effect of biomass burning on particulate observed at observation sites.The results showed that biomass burning contribution to EC,but the absorption cross-section coefficient(MAE520_NMD)and AAE370/880_NMD had no dependence on KBB+/EC.Further analysis showed that the increase of AAE370/880_NMD was affected by Eabs,suggesting that the wavelength dependence of light-absorbing aerosols in Wuhan is mainly affected by the enhancement of light absorption,while the effect from biomass burning is limited.