Spatiotemporal Patterns Of Aerosol Optical Depth And Its Influencing Factors Throughout Guanzhong Region

Guanzhong region(GZR)is the seat of Xi’an,the political,economic and cultural center of western China.Enhancements of haze weather with accelerating development of economy have attracted extensive attention.Monitoring and quantitative analysis on atmospheric pollution and its causes are crucial significant to the construction of atomospheric and ecological environment in GZR.Aerosol Optical Depth(AOD)indicates the attenuation of light by aerosols,it is one of the most essential optical properties of aerosols and also is one of key indicator affects the balance of surface energy budge,which can serve as indication or early warning of regional atmospheric pollution.The spatiotemporal patterns of AOD and its variation characteristics,and its spatial response to several controls in GZR are investigated.Multi-year average AOD and annual mean AOD in typical years from 2000 to 2020 are estimated by using MCD19A2 datasets and the spatial heterogeneity of AOD are analyzed based on MCD19A2 AOD.Then,the spatial variation trend of AOD and its sustainability are explored at the pixel scale applying Theil-Sen medain trend analysis,Mann Kendall test(MK)and Hurst index;the periodic fluctuation characteristics of AOD series are discussed with the application of extreme-point symmetric mode decomposition(ESMD).Furthermore,topographical,socioeconomic,meteorological and surface characteristic factors closed related to AOD are obtained to explore spatial response of AOD to various factors and the spatial response of AOD to the above four characteristic parameters(a total of 12 influencing factors)are simulated quantitatively based on spatial principal component analysis(PCA).The key results are as follows:(1)Spatial differentiation of AOD throughout GZR.The spatial distribution of AOD in GZR are significantly affected by altitude and the distribution of local pollution sources,showing irregular circular distribution characteristics: AOD is highly concentrated and distributed in the core area of urban agglomeration with lower altitude,flat terrain and intensive population activities,and it gradually decreased taking the high-value center area as the target area,and reaches its minimum in northern mountains and southern Qinling Mountains.The results of EOF showed that there were 6 major spatial patterns of AOD in GZR whose cumulative contribution rate is greater than 85%: The first mode,the most typical distribution state of AOD in GZR,showed regional consistency.The remaining modes are all spatially opposite,and the time coefficients of all modes oscillate repeatedly,However,the time coefficients of the second to sixth modes are more random than the first mode.Further time classification are carried out for each mode,by which we can find the main spatial distribution type of AOD in GZR can be divided into two types:(1)AOD decreases synchronously as a whole,AOD decreased synchronously at about 2/5 of the observation time,showing strong time persistence;(2)AOD increases synchronously,AOD increased synchronously in about 3/10 of the time and showed obvious seasonal recurrent characteristics..(2)Spatiotemporal variation of AOD.In terms of spatial trend,areas where AOD decrease(91.60%)are much larger than the areas where AOD increase(1.54%)based on Theil-Sen medain trend analysis and MK test.It is find AOD continued to decrease on the whole by further combined with the Hurst index analysis results,with 91.28% areas showing positive persistence,namely,91.28% of the regional AOD continued to decrease.For temporal trend,the decomposition results of ESMD show that AOD exhibited a―decreasing-increasing-decreasing‖ trend throughout GZR,which came from the superposition of several different seasonal or inter-annual periodic components,among them seasonal fluctuation played a dominant role in AOD change.There were differences in the fluctuation frequency and fluctuation time of each component and the mutation of AOD mainly occur in Imf1 component,and the mutation occurs mostly in May to July every year by mutation analysis.(3)Factors of spatiotemporal distribution on AOD.Topography,meteorological conditions and socio-economic factors are the three main factors affecting the spatial distribution of AOD in GZR based on PCA.Generally speaking,On the whole,socio-economic and meteorological factors have a positive impact on the variations of AOD in GZR,while topographic and surface feature indicators have a negative effect on the change of AOD,among which the negative effect of altitude and slope is greater,which is the main factor leading to the AOD of urban agglomeration being greater than that of North-South mountain system.Under the combined interaction of various factors,the AOD in GZR will decrease by0.241,that is,the contribution of topographic and surface feature indexes to the current AOD change is greater than that of socio-economic factors and meteorological factors.