Vertical Distribution And Radiative Effects Of Absorbing Aerosols In A Typical City In The Semi-arid Region Of Northwest China

Atmospheric aerosols are provided with the ability to absorb and scatter solar radiation and affect the radiation balance of the Earth-atmosphere system directly or indirectly through aerosol-radiation and aerosol-cloud interactions.Absorbing aerosols can heat the atmosphere by absorbing solar radiation,causing complex and important effects on the boundary layer processes,the weather and climate,and have long been one of the hot issues for research in atmospheric science.However,given shortcomings in both their observations and simulations,there are still large uncertainties in the assessment of aerosol radiation effects,with the simulation capability of numerical atmospheric models limited by difficulties in quantifying its key role.Absorbing aerosols and their vertical distribution play a crucial role in accurately quantifying their direct radiative effects.It is consequently of great scientific value to carry out research on their optical properties and vertical distribution so as to accurately calculate the aerosol radiative forcing.Arid and semi-arid regions in China,mainly in the western part,account for about 40%of the total land area,and are considerably sensitive to the response of global climate change,while climate change in these regions places essential and profound effects on water resources,agriculture,ecological environment and extreme weather.The role played by absorbing aerosols in air pollution and aerosol radiative forcing is particularly highlighted in Lanzhou,a typical city in the semi-arid region of northwest China featured by its strong radiation,heavy pollution and complex sources of absorbing aerosols.First,the contributions of different types of absorbing aerosols to the aerosol absorption coefficients and the changes of aerosol absorption under different levels of fine mode particulate matter(PM_2.5)pollution were analyzed by virtue of comprehensive observations of aerosol physicochemical properties from November 2019 to November 2020 at the Lanzhou Atmospheric Component Super Monitoring Station(LACMS).Then,to obtain the boundary layer and aerosol vertical sounding datasets with high spatial and temporal resolution,and to deeply analyze the characteristics and influencing factors of aerosol absorption properties and vertical distribution,a tethered air-boat integrated sounding experiment was conducted at the Semi-Arid Climate and Environment Observatory of Lanzhou University(SACOL)in the winter of 2018 and 2019.Finally,the vertical distributions of the observed aerosol parameters are input into the SBDART radiative transfer model to calculate the vertical profile of radiative heating rate and radiative forcing,through which,more realistic aerosol radiative heating rate and direct radiative forcing are obtained,the uncertainty in the calculation of aerosol radiative effects is reduced,and the key role of absorptive aerosols is quantified.The main results and conclusions are summarized as follows.(1)The absorption coefficients for all seasons in Lanzhou were reconstructed based on the multiple linear regression method,which further clarified the contributions of three absorbing components,i.e.,black carbon(BC or EC),fine dust(FS)and absorbing organic carbon(OC),to the absorption coefficients in Lanzhou.In the most polluted winter,black carbon,fine dust and absorbing OC accounted for77.4%,16.6%and 6.0%of the total aerosol absorption,respectively,while with the increase of PM_2.5pollution level,the contribution of EC increased from 68.3%to80.5%and that of FS decreased from 25.5%to 13.9%.(2)In the winter of Lanzhou,SSA decreased rapidly with the increase of PM_2.5,which meant that the aerosol absorption enhanced rapidly under heavy pollution,contrary to the study results that SSA increased with the increase of PM_2.5in eastern China.This resulted from the fact that the low relative humidity in the winter of Lanzhou made the scattering enhancement of aerosols caused by hygroscopic growth not obvious enough,and the aerosol hygroscopicity decreased with the increase of PM_2.5,so that the scattering coefficient and PM_2.5were basically linearly related.When PM_2.5exceeded 69?g·m^-3,the percentage of EC in PM_2.5mass concentration increased rapidly,resulting in a faster increase of absorption coefficient and a rapid decrease of SSA when PM_2.5pollution was heavier.Meanwhile,it was pointed out in the study that there was a significant linear relationship between aerosol absorbance(SSA)and aerosol radiative forcing efficiency in the winter of Lanzhou.(3)There were four main types of vertical distribution of eBC(or absorbing aerosol)in the winter of Lanzhou,i.e.,Type A,a bimodal distribution with a peak near or above the ground and the top of the boundary layer;Type B,a decreasing distribution with peak eBC only at the ground;Type C,a uniform distribution with no obvious eBC fluctuation in the boundary layer;and Type D,a uniform distribution in the boundary layer with an eBC peak above the boundary layer,where there was a high value distribution.Difference in the vertical distribution of eBC mainly resulted from reasons including the horizontal and vertical transport of absorbing aerosols,daily changes in the boundary layer,ground pollutant emissions and topography.(4)A method for SSA calculation based on sounding observations was proposed,i.e.,the combination of the absorption coefficient obtained from portable black carbon meter measurements and the scattering coefficient obtained from the aerosol particle size spectra observed by portable particle counters by meter scattering calculations.The results confirmed the vertical distribution of eBC(or absorbing aerosol)as the main contributor to that of SSA,except in some typical dust events.(5)Characteristics and changes of absorption aerosols and vertical distribution of SSA in the winter of Lanzhou were analyzed and summarized by virtue of the boundary layer standardization method.The vertical distribution of eBC in the boundary layer at 02:00,08:00 and 20:00 showed a decreasing trend with the increase of the height,but uniformly distributed at 14:00.Absorbing aerosols mainly originate from anthropogenic emissions at ground level(i.e.,the aerosol absorption coefficient at ground level is the largest),while the scattering aerosols might appear in the atmosphere through new particle generation vertically throughout the boundary layer at night,which contributes to the SSA decrease within the boundary layer with the increase of the height at the same periods.However,the strong turbulent mixing effect at 14:00 mixed all the particles in the boundary layer more uniformly,resulting in the uniform distribution of SSA in the boundary layer.Meanwhile,the absorbing aerosol represented by BC is more easily transported to a high altitude than other scattering particles,so the SSA above the boundary layer decreased with the increase of height at 14:00.(6)Measured absorption coefficients and SSA were adopted as input parameters of the radiative transfer model SBDART,so that more realistic aerosol radiative heating profiles and aerosol direct radiative forcing were obtained,and the key effects of absorbing aerosols and their vertical distributions in the calculation of heating rates and radiative forcing were better quantified.The aerosol radiative heating rate also showed a clear bimodal distribution for the bimodal eBC vertical distribution of class A while for the decreasing eBC vertical distribution of class B,the heating rate displayed a decreasing distribution in the vertical direction.The adoption of the assumed SSA vertical distribution(i.e.,not considering the vertical distribution of absorbing aerosols)would significantly underestimate the aerosol radiative heating rate corresponding to the peak eBC in classes A and B..Besides,given the SSA decrease with the increase of the height,aerosol radiative heating rate corresponding to the uniform eBC vertical distribution of class C increased incrementally as the height increases.In terms of radiative forcing,the adoption of assumed SSA vertical distributions also significantly underestimated the aerosol heating effect on the atmosphere for different absorbing aerosol vertical distributions.(7)Characteristics and variations of aerosol radiative heating rate and radiative forcing during the daytime in the winter of Lanzhou were summarized by virtue of the boundary layer normalization method,with the importance of actual observations of absorbing aerosols and their vertical distributions further emphasized in case of improving the radiative transfer model calculation results.At the same time,the radiative forcing of aerosols was overestimated by 7.2 W·m^-2at the top of the atmosphere,and underestimated by 5.4 W·m^-2at the ground and 12.6 W·m^-2within the atmosphere,with relative errors of 19.3%,7.5%,and 35.6%,respectively.Errors mainly resulted from the underestimation of atmospheric aerosols absorption in the boundary layer.The aerosol vertical heating rate outside the boundary layer at 14:00was more frequently influenced by SSA,while the adoption of assumed SSA vertical distribution significantly underestimated the aerosol absorptivity above the boundary layer,leading to a significantly lower calculated radiative heating rate outside the boundary layer,while the radiative forcing caused by aerosols was overestimated by12.0 W·m^-2at the top of the atmosphere,and underestimated by 9.6 W·m^-2at the ground level and 21.6 W·m^-2inside the atmosphere,with relative errors reaching36.9%,12.4%,and 48.4%,respectively.