Quantitative Classification And Radiation Effects Of Absorbing Aerosols In The Semi-arid Region Of China

Absorbing aerosols in atmospheric aerosols can change the radiation balance of the Earth-atmosphere system through absorption and scattering solar radiation,and have significant effects on weather and climate.It is very important to clarify the optical properties and radiation effects of absorbing aerosols to reduce the uncertainty in the assessment of aerosol radiation effects,improve the parameterization of numerical models,and improve the simulation and prediction capability of weather and climate.The semi-arid region of China is characterized by low cloudiness,strong solar radiation,aridity and low rainfall,dust weather,heavy industrial emissions,complex aerosol sources,and extremely sensitive to the response to global climate change.The absorption aerosols in the semi-arid region of China have both black carbon（BC）and brown carbon（Br C）as well as dust,and there are more complex aerosol processes and radiation forcing.However,the current understanding of the absorption properties of BC,Br C,and dust in the semi-arid region of China and their radiation contributions are still unclear and the radiation parameterization is highly biased,thus it is especially urgent and important to study the optical properties and radiation effects of the absorbing components of aerosols in the semi-arid region.This paper used the aerosol observation data in the semi-arid region of China accumulated by Lanzhou University,improved the absorbing aerosol quantitative classification algorithm combining the organic component source analysis and dust quantification theory,elucidated the spatial and temporal distributions of different absorbing aerosol components in the semi-arid region of China,calculated and analyzed the absorption properties of BC,Br C,and dust and their radiation forcing efficiency,providing the basis for further research on radiation parameterization of absorbing aerosols in the semi-arid region.（1）The spatial and temporal distributions of aerosol physical and optical properties in the semi-arid region of China were analyzed using aerosol observation data at four observation sites（Semi-Arid Climate and Environment Observatory of Lanzhou University（SACOL）,Dunhuang,Zhangye and Zhangbei）,and the aerosols were classified according to the mixing state of BC,Br C,and dust using the optical classification method.The results showed that the semi-arid region of China was controlled by two or three absorbing components with the participation of dust in most cases.The analysis of the multi-year observations at the SACOL site showed that the contribution of aerosols dominated by the mixing of multiple absorbing components was increasing,and their absorption and scattering abilities were increasing.（2）The PM₁ chemical components at the SACOL site in winter 2018 was observed using a high-resolution time-of-flight aerosol mass spectrometer.Three organic components,including biomass burning organic aerosol（BBOA）,coal combustion organic aerosol（CCOA）,and oxygenated organic aerosol（OOA）,were identified using the positive matrix factorization algorithm.Significant aerosol regional transport events were identified during the observation period by analyzing the chemical component data against various data such as ground wind and sounding.The mountains and the planetary boundary layer top limited the aerosol diffusion in the valley,leading to the aerosol accumulation in the valleys,and airflow transported the accumulated aerosols along the valley to the SACOL site.In other words,hilly-gully topography not only promotes the accumulation of aerosol pollution,but also acts as a“transmission tunnel”to transport aerosol pollution to a larger spatial area.（3）Dust mass size distributions were decomposed from the number particle size distributions using the double log-normal distribution fitting method,and the dust mass concentrations were quantified at the SACOL site in winter 2018 and 2019.The results showed that dust aerosols had a high mass contribution in the PM_2.5 size range where anthropogenic aerosols were concentrated.Therefore,it is reasonable to pay attention to the contribution of dust to aerosol absorption in the semi-arid region of China.Combining the potential source contribution function and reanalysis data,it was determined that the dust at the SACOL site mainly came from the Tengger Desert and the Badan Jara Desert in winter 2018 and from the Taklamakan and Tengger Deserts in winter 2019.（4）The traditional absorbing aerosol quantitative classification algorithm was improved using multi-wavelength aerosol absorption coefficients and mass concentrations of dust and organic components.The absorption properties and radiation forcing efficiencies of BC,BBOA,CCOA,and dust at the SACOL site in winter 2018were quantified.The results showed that when the absorption Angstr?m exponent of BC was taken as 1.0,the contribution of BC to the aerosol absorption coefficient at 370nm was 39%,and the light absorption capacities of CCOA,BBOA,and dust decreased sequentially with mass absorption cross sections at 370 nm of 2.67-4.36,1.24-2.03,and 0.21-0.34 m²?g^-1,respectively;the contribution of dust to the direct radiation forcing caused by the non-black carbon absorption could be as high as 19.0%-24.3%at the SACOL site,indicating the complexity and importance of dust in the radiation forcing of aerosols in the semi-arid region of China.