Research On Improving Source Apportionment Results Of Light-absorbing Carbonaceous Matters With The Real Measured (?)ngstr(?)m Absorption Exponent

As two types of light-absorbing carbonaceous matter,black carbon（BC）and brown carbon（Br C）are important to atmospheric environment,human health and climate changes.BC and Br C are mainly from biomass burning and fossil fuel combustion.Accurate source apportionment results of BC and light absorption of Br C are significant to alleviate its adverse effects on human health,atmospheric environment and climate changes.Using the optical methods to apportion the sources of BC and light absorption of Br C has advantages of convenience and high-temporal resolution,and it is suitable for long-term observation datasets processing and is widely used.Apportioning BC sources and light absorption of Br C with optical methods are highly reliable to the choice of absorption（?）ngstr（?）m exponent（AAE）.The AAE values for biomass burning(AAE_bb)and fossil fuel combustion(AAE_ff)show the obvious spatial-temporal variations,suggesting that it is unreasonable to apportion BC into biomass burning(e BC_bb)and fossil fuel combustion(e BC_ff)with fixed AAE values using the Aethalometer model.The attribution of Br C light absorption using the optical method always adopted the fixed AAE value（i.e.,AAE=1.0）.The AAE value of BC shows variations and it is determined by several factors.This paper aimed at improving the source apportionment result of the Aethalometer model with the real measured AAE values from different emission sources.The secondary Br C light absorption was calculated using the corrected ambient aerosol AAE by introducing the light absorption enhancement coefficient.The spatial-temporal variations of BC sources and secondary Br C light absorption coefficient calculated from the improved methods were finally discussed in this paper.Measurements of aerosol from different emission sources showed that AAE for biomass burning and coal combustion varied in the wide ranges of 1.42±0.21～2.90±0.65 and 1.27±0.06～1.67±0.62 respectively,and they were positively correlated to the ratio of organic carbon to elemental carbon.AAE for industrial emissions and vehicle emissions varied in narrow ranges of 1.07±0.31～1.38±0.28 and 1.01±0.10,respectively,and they were negatively correlated to the ratio of organic carbon to elemental carbon.Based on the real measured AAE and the assumption that if the tracer and black carbon from biomass burning experienced the same atmospheric removal rates,the intercept of the linear regression between them should approach 0.A method using the potassium from biomass burning(K_bb⁺)to constrain the optimal AAE_bb and AAE_ff was developed in this paper.The optimal AAE_bb and AAE_ff within the interannual scale were1.76 and 1.12,respectively,which were within the ranges of measured AAE from different emission sources.The method was further applied to seasonal,monthly and diurnal datasets to find out the optimal AAE combination by comparing the BC source apportionment results using the optimal AAE and fixed AAE values(AAE_bb=2.0,AAE_ff=1.0).The optimal combination was obtained using the constraint at a diurnal resolution and the optimal AAE_bb and AAE_ff varied in the ranges of 1.68～1.85 and 1.05～1.19,respectively.Compared to the BC source apportionment results using the fixed AAE values,the Pearson correlation coefficients between e BC_bb and K_bb⁺improved by43.8%and it also improved by 10.0%between e BC_ff and NO₂using the optimal AAE values constrained at the diurnal resolution.The optimal AAE values constrained at a diurnal resolution were further used to apportion the BC sources at an urban station in Wuhan from March 2018 to February 2019.The variation characteristics of source apportionment results were further discussed under different atmospheric pollution situations,temporal resolutions and potential geographical regions of backward air masses.Results showed that the increasing of e BC_bb contribution to BC was one of the reasons for air quality deterioration from clean to pollution.The seasonal patterns of BC with high levels in winter and low levels in summer were mainly due to the emission strength variations.BC for air masses from different potential regions showed obvious characteristics with the highest BC levels for air masses from local and the highest e BC_ffand e BC_bb percentage contributions from southern China and southwestern China,respectively.The AAE of BC aerosol was set as 1.0 to apportion the light absorption of BC at other wavelengths using the optical method.The Mie theoreical calculation showed that the AAE of BC aerosol was not identical to 1.0 and it was influenced by the diameter of BC particle,coating materials and mixing state of BC.Results from the real measured AAE values also showed that the AAE of different types of aerosol was not identical to1.0,suggesting that there were other sources contributed to the light-absorbing of primarily emitted aerosol.In this paper,the light absorption enhancement coefficient was introduced to the correction of ambient aerosol AAE to get the AAE of primarily emitted aerosol（AAE_p）.AAE_p showed obvious seasonal variations with the highest value in winter（1.36）,followed by autumn（1.25）and the lowest value in summer（1.12）.The light absorption coefficient of primarily emitted aerosol was calculated using the AAE_pand the secondary Br C absorption coefficient was further calculated by subtracting the primary absorption from the total absorption.The variation characteristics of secondary Br C absorption were further discussed at different temporal resolutions,atmospheric pollution situations and potential geographical regions of backward air masses.Results showed that secondary Br C light absorption contributed 7.15%to the total light absorption at most.Secondary Br C light absorption increased from clean to pollution due to the reduced O₃ levels and enhanced formation of secondary organic carbon.The meteorological parameters and O₃ levels showed the highest correlations with the secondary Br C light absorption within monthly and diurnal scales,respectively.Secondary Br C light absorption for air masses from different potential regions showed obvious characteristics with the highest value for air masses from the local region.Secondary Br C light absorption for air masses from southern China was higher than that from northern China in spring and autnmn,while it was higher for air masses from northern China than southern China in summer and winter.The optimization methods to apportion BC sources and light absorption of Br C in this paper contribute to more accurately identify the spatial-temporal variations of BC and Br C,which are significant to evaluate their adverse effects on global climate change,regional atmospheric environment and human health.