观测约束下的煤矿区吸光气溶胶特性模拟研究

Simulation of Absorbing Aerosol Characteristics in Coal Mine Area Under Observation Constraint

利用黑碳仪、光学粒度仪和太阳光度计在山西省长治市煤矿区开展连续实地观测,结合“核-壳”假设下的米散射模型,对煤矿区吸光气溶胶光学特性进行建模,提出一种基于多源地基观测快速反演吸光气溶胶单次散射反照率(SSA)的方法,并通过使用吸收Angstrom指数对模拟结果进行约束,分析了不同混合状态下吸光气溶胶光学特性的变化规律。结果表明:1)煤矿区吸光气溶胶粒径分布与污染条件相关,重度污染条件下粒径较大,中度污染条件下粒径较小,其中不同污染条件均在0.28~0.3μm等较小粒径区间内存在峰值,这表明排放源对粒径分布有重要影响;2)基于“核-壳”假设的米散射模型可以模拟不同混合状态下吸光气溶胶的SSA,以440 nm粒径为例,在均匀假设下SSA为0.43~0.72,在非均匀假设下SSA为0.41~0.65,这表明使用多波段、多粒径信息可以更准确地获取吸光气溶胶光学特性;3)吸收Angstrom指数只能在部分粒径区间与SSA相匹配,且无法应用至较大粒径区间,表明通过吸收Angstrom指数评估吸光气溶胶SSA的方法会产生偏差,应综合考虑粒径分布、混合状态和波长对其产生的影响。

Objective In coal mining regions,extensive coal dust is generated during mining,transportation,and storage,coupled with substantial black carbon produced resulting from incomplete coal combustion in the industry chain.Over time,these materials form absorbable substances,evolving into core-shell aerosols with inorganic salt shells.These aerosols,including sulfate,nitrate,and water,exert significant climate impacts through direct and indirect radiation effects.The environmental and radiative forcing effects are substantial.Absorbing aerosol demonstrates strong solar radiation absorption across the ultraviolet to infrared spectrum.However,past studies primarily focus on their optical properties in visible and infrared bands,overlooking ultraviolet band absorption.Current research often assumes a lognormal particle size distribution for absorbing aerosols,neglecting variations in distribution and optical properties resulting from diverse emission scenarios.Therefore,a thorough analysis of absorbing aerosol optical properties at local scales is crucial.Quantitative assessments of particle size distribution,mixing state,and spatio-temporal variations are vital for elucidating the intricate interactions with boundary layer development,radiative forcing changes,and air pollution.Methods In our study conducted in the coal mining area of Changzhi City,Shanxi Province,various datasets are collected,including surface black carbon concentration,particle size distribution,and columnar aerosol optical depth(AOD).The investigation commenced with the utilization of the variance maximization method to categorize AOD data into distinct pollution events.Subsequent analysis involved evaluating the particle size distribution corresponding to different pollution degrees through probability density functions.The uncertainty of particle size for the desorption aerosol core and shell is then determined by integrating black carbon mass concentration data and particle size distribution information.These uncertainties are then used as input parameters to run the Mie scattering model based on the“core-shell”structure.This process results in the inversion of the multi-band optical characteristic parameters of absorbing aerosol in the coal mining area.The computations are carried out under both the assumption of a uniform distribution and a non-uniform distribution,representing different mixing degrees of aerosols.To complete the picture,the uncertainty interval for the single scattering albedo(SSA)of absorbing aerosol was constrained through the application of absorptionÅngström exponent(AAE)theory.This comprehensive approach provides a nuanced understanding of the complex dynamics of absorbing aerosol in the specific context of coal mining environments.Results and Discussions In the coal mining area,absorbing aerosols are influenced by emission sources,manifesting a particle size distribution divergent from the lognormal model.Under various pollution conditions,robust peaks are discernible in smaller particle size ranges(0.28-0.3μm),with weaker peaks present around 0.58-0.65μm.The relative proportion between the two peaks fluctuates in tandem with the pollution severity(Fig.3).Using the Mie scattering model,the optical characteristics of absorbing aerosol are inverted based on AOD information,black carbon mass concentration,and particle number concentration.Results indicate that under the assumption of a uniform distribution(Fig.4),the average size of the“core”particles at 0.28,0.58,and 0.7μm is relatively low,leading to corresponding patterns in SSA with changes in“core”particle size.Additionally,the average“core”particle size shows no significant variation with changes in wavelength in different size ranges.SSA decreases with increasing wavelength,with greater fluctuations in the smaller particle size range(0.25-0.58μm)and more stable changes in the larger particle size range(0.58-1.6μm).Under this assumption,the AAE theory is found to be inapplicable.In the case of a non-uniform distribution(Fig.5),SSA values exhibit a slow,followed by a gradual and then rapid increase in the shortwave region,while in the longwave region,SSA first rapidly increases and then gradually levels off.For shorter wavelengths(500 nm and above),AAE theory proves effective for absorbing aerosol with smaller particle sizes.For longer wavelengths(675 nm and above),AAE theory is applicable to absorbing aerosol with moderate particle sizes.However,for larger particles such as coal dust,AAE theory is not suitable.It is noteworthy that,under both assumptions,the inversion results of SSA values in the longwave spectrum(such as 870 and 936 nm)are relatively lower compared to the shortwave spectrum(such as 440 and 500 nm).This discrepancy will lead to an underestimation of emission quantities.Conclusions We conduct on-site observations in the coal mining area of Changzhi City,Shanxi Province,aiming to capture the variation characteristics of AOD,particle concentration,and black carbon mass concentration.Utilizing the Mie scattering model based on the“core-shell”hypothesis,we simulate the SSA of absorbing aerosol under two different mixing states.Additionally,we calculate the optical variations of absorbing aerosol constrained by the AAE.The research findings reveal the following:1)The particle size distribution of absorbing aerosol in the coal mining area deviates from the assumptions made in previous studies,which typically assumed single or double-peaked distributions.Influenced by emission sources,the characteristics vary under different pollution conditions.Smaller particles predominantly originate from the incomplete combustion of coal in local power plants and coking factories,producing black carbon.Larger particles stem from the aging processes of black carbon in the atmospheric environment and coal dust generated during coal transportation.2)Comparison of the SSA variations under different mixing states simulated by the two hypotheses indicates that particle size,mixing state,and spectral range significantly impact the SSA of absorbing.In contrast to previous studies using the infrared spectrum,the present investigation reveals higher SSA values in the ultraviolet and visible light spectrum,suggesting a potential underestimation of black carbon emissions.3)The AAE theory is applicable only to certain particle size ranges in different spectral bands.For large-sized absorbing aerosol in the coal mining area,using the AAE theory to estimate SSA introduces uncertainty,and applying the AAE assumption across all particle size ranges leads to an underestimation of emissions.These findings underscore that the distribution characteristics of SSA in absorbing aerosol do not strictly adhere to the power-law relationship of the AAE index but are collectively determined by particle size distribution,mixing state,and spectral range.