Top-down Estimate Of Regional Black Carbon Emissions Using Ground And Satellite Observations For The Yangtze River Delta Region

Black Carbon(BC)is a crucial component of absorbing aerosols and affects the radiation balance of the Earth-atmosphere system directly by absorbing sunlight.The physicochemical and optical properties also have adverse effect on human health and visibility.Research on BC emission inventory helps better understand the intensity,spatial and temporal distribution of BC emissions and its climate effect.It will also provide scientific support for the control of visibility reduction.Due to the complicated BC emission sources,estimate of BC emissions based on bottom-up approach is highly uncertain.It is of great scientific and policy significance to constrain BC emission inventory using different observations and chemical transport model(CTM).Assuming a near-linear response of BC concentrations to emission changes,this study integrated a CTM,a multiple regression model and hourly ground observations to optimize BC emissions at monthly,emission sector and city cluster level.We derived top-down emissions and reduced deviations between simulated and observed BC concentrations for southern Jiangsu city cluster Scaled from a high-resolution invertory for 2012 based on changes in activity levels without including improved emission controls from 2012 to 2015,the BC emissions in southern Jiangsu were calculated at 26.99 Gg/yr for 2015(JS-prior).Constrained with the top-down method,BC emissions were estimated at 13.41 Gg/y(JS-posterior)50.37%smaller than JS-prior,implying the effectiveness of air pollution prevention measures on emission abatement.Applying JS-prior in CTM the a nnual mean concentration of BC at Jiangsu Provincial Academy of Environmental Science(PAES)was simulated at 3.39μg/m3,36.69%higher than the observed 2.48 μg/m3.It was simulated at 3.4μg/m3 at Xianlin Campus of Nanjing University(NJU),10.18%lower than the observed 3.83 μg/m3.Application of JS-posterior in CTM reduced the deviations between simulations and observations at two ground sites effectively.At PAES,in particular,the simulated annual mean declined to 2.57μg/m3 and the annual normalized mean error(NME)decreased from 72.00%to 57.55%.However,application of JS-posterior slightly enhanced NMEs in July and October at NJU where simulated concentrations with JS-prior were lower than observations,implying that reduction in total emissions could not correct modeling underestimation.The effects of observation site,the a priori emission inventory and wet deposition on top-down estimate were further quantified.The best modeling performance was obtained when observations of both sites were used with their difference in spatial functions considered in emission constraining.Including more measurements with better spatiotemporal coverage would improve the understanding of spatial distributions of BC emissions.Similar results after top-down constraining could be obtained in emission levels,spatial distributions and CTM performances,even clear difference existed in the a prior bottom-up inventories,implying that the impact of the a priori inventory was limited on top-down estimate.The impact of wet deposition on the multiple regression model was demonstrated moderate through data screening based on simulated wet deposition and satellite-derived precipitation.Ground observation sites are incomplete in their spatial coverage.Satellite measurements of Absorption Aerosol Optical Depth(AAOD)have much broader temporal and spatial coverage and can be used to better constrain BC emissions.Combining the available ground observations and soot aeroaol optical depth data retrieved from Muiti-angle Imaging SpectroRadiometer,the annual mean AAOD of the Yangtze River Delta(YRD)in 2015 was 0.062 at 550 nm.Based on the quantitative relationship between non-light absorbing components in particles and BC light absorption enhancement,the annual mean Mass Absorption Efficiency(MAE)of BC in YRD was 14.07 m2/g,higher than that of fresh BC(7.5±1.2 m2/g).The obvious seasonal and spatial variation of MAE were found.Winter had lower value than other seasons while northern Jiangsu and northern Anhui had higher values than other regions,resulting from higher concentration level of secondary inorganic aerosol precursors(SO2 and NO2)and stronger photochemical oxidation ability.More secondary inorganic aerosols coatings enhanced BC absorption.Applying Multi-resolution Emission Inventory for China in 2015 in CTM(EMEIC),the annual mean simulated AAOD in YRD was 0.036,42.23%lower than observation,indicating the underestimation of the emission inventory.Based on nonlinearity between changes in AAOD and those in BC emissions,the top-down eatimate of BC emissions in YRD was 336.48 Gg/yr(Esat),97.34%larger than EMEIC.The annual mean simulation increased to 0.052 and annual NME decreased from 55.02%to 38.30%,reducing the deviations between simulated and observed AAOD.BC emissions in Esat and JS-posterior in southern Jiangsu were further compared and significant difference between two constraining method was found.The former was 3.42 the latter,mainly attributing to the difference between two the a prior emission inventories(the spatial correlation was less than 0.05),the bias between ground and satellite observations(51%)and discrepancy of two constraining methods(like near-linearity VS non-linearity,spatial resolutions).