Ground-based Remote Sensing Of Dust Mass Extinction Efficiency In Africa And Asia

Dust is one of the most important types of aerosols and plays a key role in the earth-atmosphere system and has a far-reaching impact on the global climate change and biochemical cycle.Africa and Asia are the largest sources of dust in the world,and quantitative assessment of dust flux or atmospheric sediment concentration is a prerequisite for accurate understanding of global dust transport,deposition and circulation However,the greatest uncertainty in the evaluation of atmospheric sediment concentration based on satellite remote sensing comes from the mass extinction efficiency of dust aerosols.Therefore,it is of great scientific significance to carry out remote sensing analysis of dust mass extinction efficiency in Africa and Asia.Based on the analysis of the potential impact of dust aerosols on AERONET observation sites in Africa and Asia,this paper divides the African region into two dust source regions of Sahara desert,Sahel and two transport regions of transatlantic transport path?35 stations in total?Three stations in Asia are selected to represent the typical dust affected areas in East Asia,Central Asia and South Asia.The optical properties of dust aerosols in Africa and Asia are analyzed comprehensively.The mass extinction efficiency of dust aerosols are retrieved.The similarities and differences of the mass extinction efficiency of dust in different dust source areas and transmission areas are compared and analyzed,and the current satellite remote sensing study of atmospheric sediment concentration is evaluated.The significant uncertainty caused by the assumption of dust mass efficiency.In addition,the relationship between the extinction efficiency of dust mass and the effective radius and extinction efficiency of particles is analyzed by Mie scattering theory model,which provides support for the parameterization of the extinction efficiency of dust mass.The main conclusions are as follows:?1?The frequency of dust occurrence in Africa and Asia has significant seasonal variation characteristics.In the Sahel region,the frequency of sand dust is much higher than that in the Sahara desert,and the frequency of sand dust in the transmission region is lower than that in the source region.Due to the influence of different sand sources,the sand dust in East Asia and Central Asia are concentrated in March to April and June to October respectively,and the sand dust in South Asia is concentrated in March to July,with the frequency?36.2%?being about East Asia and East Asia In Central Asia,the frequency of pollution dust is twice as high.Sand dust in Africa is the main contributor to the total optical thickness of the region.The total aerosol optical thickness and sand dust optical thickness in the Sahel region are far higher than those in the Sahara desert and its transmission area,and the source area of sand dust absorption is stronger than that in the transmission area.Asian sand dust and pollution sand dust are the main contributors to the total optical thickness,and the pollution sand dust has a high absorption due to human pollution,especially in South Asia District.In terms of dust particle size spectrum,both African dust and Asian dust as well as pollution dust are dominated by coarse mode with significant difference in peak concentration.?2?There are significant differences in extinction efficiency of dust mass in different dust source areas and transmission areas.The extinction efficiency of dust quality in Africa is quasi normal distribution with narrow frequency distribution,mainly between 0.3-0.55 m²g^-1,increasing from the source area to the transmission area;the extinction efficiency of dust quality in East Asia,Central Asia and South Asia is mainly between 0.3-0.5 m²g^-1,slightly lower than that of dust,and the significant difference is related to the impact of human pollution.Mie scattering model simulation and statistical analysis show that the size of the dust mass extinction efficiency significantly depends on the size of the effective radius and extinction efficiency of the coarse mode particles.Assuming that the extinction efficiency is 2.6,the mass extinction efficiency is inversely proportional to the effective radius of the coarse mode particles,and there is a very good parametric relationship.?3?The assumption of extinction efficiency of dust mass in Africa and Asia will lead to a large error in the assessment of atmospheric sediment concentration.When the extinction efficiency of dust mass is assumed to be 0.37 m²g^-1,the atmospheric sediment concentration in the Sahel,Sahara and trans Atlantic transport areas in Africa will be overestimated to 11%,21%and 26%respectively,resulting in small errors in Asia,only 1%,7%and 4%in East Asia,Central Asia and South Asia.When the extinction efficiency of dust mass is assumed to be 0.64 m²g^-1,the atmospheric sediment concentration in Africa and Asia will be seriously underestimated,among which 36%,30%,27%,42%,46%and 45%will be underestimated in Sahel,Sahara,transatlantic transport area,East Asia,Central Asia and South Asia,respectively.The results show that when the mass extinction efficiency of sand dust in the Sahel,Sahara and trans Atlantic transport areas in Africa are assumed to be 0.41,0.45,0.46 and 0.47 m²g^-1,respectively,and when the mass extinction efficiency in East Asia,Central Asia and South Asia are assumed to be 0.37,0.34 and 0.35 m²g^-1,respectively,the climate average value of atmospheric sediment concentration in the corresponding area can keep within 1%of the real-time inversion results.?4?The annual variation characteristics of atmospheric sediment concentration in Africa and Asia are consistent with that of optical thickness,and the atmospheric sediment concentration corresponding to the period with high optical thickness is also higher.The atmospheric sediment concentration in the Sahel region of Africa is much higher than that in the Sahara desert and transatlantic transport area.The monthly average maximum value for many years is 0.87 gm^-2in June.With the increase of dust transport distance,the atmospheric sediment concentration is significantly reduced.The monthly average atmospheric sediment concentration for many years is below 0.2 gm^-2each month.The monthly average maximum atmospheric sediment concentration in East Asia was 0.33 gm^-2in March,and that in Central Asia and South Asia was 0.24 gm^-2and 0.42 gm^-2respectively in July.The regional difference of atmospheric sediment concentration is mainly affected by the frequency and intensity of sand dust occurrence.Sahel and South Asia have higher frequency and wider month of sand dust occurrence,which leads to greater atmospheric sediment concentration.