Molecular Composition And Light Absorption Characteristics Of Brown Carbon Emitted From Solid Fuel Combustion In The Qinghai-tibet Plateau

BrC（Brown carbon）is a carbon aerosol with strong absorption in the near ultraviolet and visible regions.The emission from the combustion of residential solid fuels is an important source of BrC in the atmosphere.In the QTP（Qinghai-Tibetan Plateau）,the consumption of solid fuel,including the dung of animals such as yak dung and sheep dung,reached 10,990 Gg during the heating season,resulting in serious indoor air pollution.Moreover,heavy BrC pollution leads to great uncertainty about the influence of molecular absorption characteristics and its radiative forcing on BrC in climate-sensitive QTP.Therefore,three kinds of common residential solid fuels（yak dung,sheep dung,and bitumite）in the QTP were selected in this study,cooking and heating scenarios were differentiated,and the field combustion experiment were carried out to collect BrC samples from source combustion emissions.Secondly,the molecular compositions and optical characteristics of the combustion of BrC were determined by ultra-high performance liquid chromatography quadrupole time-of-flight mass spectrometer,UV-Vis+liquid waveguide capillary flow cell spectrophotometer and Fourier transform infrared spectrometer.The molecular composition and light absorption coefficient were combined by machine learning of the partial least square method to obtain the individual molecular light absorption characteristics.Finally,the emission characteristics and light absorption characteristics of typical BrC chromophore chemical composition were revealed by combining with quantum chemical simulation calculation for chromophore molecules with known structure in BrC.The research results obtained in this paper are as follows:The Absorption（?）ngstr（?）m Exponent（AAE）values of BrC of sheep dung and yak dung at 300-550nm were 4.7-5.5,and the average MAE（mass absorption efficiency）values of BrC of yak dung,sheep dung,and bitumite were 1.31,0.76,and 0.95 m²g^-1,respectively.MAE values in the heating scenario were 1～2 times lower than those in the cooking scenario,indicating that BrC emitted in the cooking scenario had a stronger light absorption capacity,especially for yak dung.The average emission factor of PM_2.5(EF_PM2.5)was 2.77±1.46 g kg^-1for yak dung,5.63±2.14 g kg^-1for sheep dung,and3.31±1.77 g kg^-1for bitumite,respectively.Compared to bitumite burning,animal dung fuels in the QTP can release a large amount of BrC and have relatively high BrC light absorption characteristics.In addition,in terms of radiative forcing,the simple radiative forcing effect（SFEs）of BrC emitted by the combustion of the three solid fuels at the wavelength of 300-400 nm in the cooking scenario is 1.5-2 times that of the heating scenario combustion.The SFEs of incomplete combustion of animal dung fuels are more likely to produce highly aromatic compounds that cause higher BrC SFE during anoxic heating combustion.The BrC molecular composition of three solid fuels showed the characteristics of CHON>CHO>CHONS in terms of quantity,ion intensity,and molecular light absorption coefficient(Mb_abs),indicating that BrC molecules containing N have strong light absorption ability.Among them,CHO and CHON（N₂+N₄）in BrC emissions from source combustion are characterized by long carbon chains and high unsaturated levels,contributing up to 99.7%of the total b_abs.In the BrC emission of bituminous coal,the proportion of Mb_absof CHONS is significantly higher than that of the other two fuel types.The molecules of CHONS are all high oxygen-containing molecules and have high unsaturation,indicating that the light absorption of S-containing molecular formula in bituminous coal is strong.Nitrated aromatic compounds（NACs）are the largest emitting compounds in the known BrC chromophores,accounting for 50.6-73.0%of the identified chromophores.The emission characteristics of NACs in both combustion scenarios show that the combustion of animal dung is greater than that of bitumite,and EF_NACsin heating scenario are 1.8-2.6 times that in cooking scenario,which may be because incomplete combustion in heating scenario may lead to the emission of more unsaturated aromatic compounds.Levoglucan contributed the most to the emission of yak dung and sheep dung under the two combustion scenarios,while arabitol and sucrose contributed the most to the emission of bitumite for heating（65.6%）and cooking（22.7%）,respectively.In addition,quantum chemical simulations reveal that the identified chromophores have obvious absorption in both the UV and near-UV regions,and the theoretical relative contribution of light absorption indicated that NACs was the substance with the highest proportion in chromophore.In this study,the analysis and determination of light absorption capacity and molecular composition of BrC is an important supplement to the BrC emission inventory in the QTP and also provides a scientific basis for the environmental impact assessment of BrC in this region.