Ground-based Retrieval Of Dust Mass Concentration Over Northwest China

Dust aerosol,a major component of the atmospheric aerosol system,not only plays an important role in the global biogeochemical cycle system,but also has a significant impact on the global radiative energy budget and climate change.The northwestern region of China is a main source of dust in East Asia,quantitative assessment of the dust mass concentration or dust flux is the key to understand the dust transport,sedimentation,and recycling processes in East Asia.However,due to the assumption of dust mass extinction coefficient(k_ext,c)or the influence of anthropogenic aerosol,the dust mass concentration assessment which based on passive or active satellite remote sensing is still with great uncertainty.Therefore,in view of the above-mentioned problems in dust mass concentration assessment,this paper used the combined observation data of Micro Pulse Lidar?MPL?and sun-photometer?CE318?at the SACOL,Minqin,Zhangye and Dunhuang stations,focusing on analysing the identification and classification methods of natural dust and polluted dust in Northwest China.The main conclusions are summarized as follows:?1?Based on the dust particlate depolarization ratio?PDR?and the?ngstr?m exponent of 440-870 nm(AE_440-870),the identification and classification criteria for dust in the Northwest China were determined:Dust,AE_440-870?0.2,PDR>0.30;Polluted dust,0.2<AE_440-870<1.4,0.1<PDR<0.3.The volume concentration spectrum of dust and polluted dust in the Northwest China were significantly different.The dust exhibits an unimodal structure dominated by coarse mode,the Coarse Mode Fraction?CMF?is0.82±0.03,and the polluted dust is bimodal structure,the CMF is 0.39±0.20.?2?The k_ext,c in northwest China is mainly concentrated between 0.30-0.55 m² g^-1,with an average of 0.43±0.05 m² g^-1.The volume extinction ratio?Y?of dust aerosol has a very good linear relationship with the effective radius?X?of coarse mode particles,Y=0.66X-0.33,and the correlation coefficient is as high as 0.97.Based on this,combined with the dust mass density,the effective radius of coarse mode particles can provide a more convenient method for real-time retrieval of k_ext,c.?3?Due to the different distances from dust source and effects of anthropogenic pollution,the vertical variation characteristics of dust and polluted dust mass concentration in northwest China have remarkable differences.Under the dust weather condition,the average dust mass concentration in Dunhuang,Minqin and SACOL stations is about 240?g m^-3,200?g m^-3 and 650?g m^-3,respectively;Under the polluted dust weather condition,the average dust mass concentration profile which excluded anthropogenic pollution showed a decreasing trend with height,its column concentration was much smaller than the dust weather condition and ranged from 60 to70?g m^-3.?4?Sensitivity analysis indicates that the real-time retrieval of k_ext,c and effective removal of anthropogenic pollution are prerequisites for accurate assessment of dust mass concentration.In most studies,it is assumed that the k_ext,c is a constant of 0.37m² g^-1,which will result in an overestimate of the dust mass concentration in dust source area by more than 30%and small impact in the long-distance transmission region.If the effects of anthropogenic pollution are ignored,the mass concentration of polluted dust will be overestimated by approximately one time;If both the k_ext,c is assumed to be a constant and the influence of anthropogenic pollution is not removed,the dust mass concentration will be overestimated by approximately 1.6 times.