Spatio-Temporal Variations Of Aerosol Optical And Micro-Physical Properties With Implications To Radiative Forcing Over Pakistan And Surrounding Asian Regions Over Recent Decades

Aerosol's optical,microphysical and radiative properties are crucial for understanding their impact on regional climate and environment.In the present study,remotely sensed ground-and satellite-based observations by utilizing various statistical approaches,coupled with numerical simulations,were used to identify multiscale changes of the aerosol properties over Pakistan and its environs in South and East Asia(SEA)(1980-2018).The spatiotemporal evolution in aerosol optical thickness(AOT)over environmentally distinct regions in Pakistan were investigated.Results elucidated that an annual increase in AOT_MERRA-2 between 0.002 and 0.012 year^-1,at a 95%confidence level(p<0.05)across the study region.Significant trends were ascertained in the Katawaz Basin(2.29%year^-1),followed by the Balochistan Plateau(2.12%year^-1),Indus Basin(1.87%year^-1),and KDR(SA)(1.56%year^-1).These can be attributed to change in human-induced factors,local and regional emission sources,and prevailing meteorological conditions.Furthermore,a comparison of both retrievals(MERRA-2 and MODIS)in climatology revealed significant differences between the AOT datasets as depicted using different versions,algorithms,and collection methods.Despite the differences in magnitudes,the MODIS AOT product retrieved through the deep blue(DB)algorithm was considered better in terms of different statistical metrics and comparison methods used for evaluation.We compared spatial variations and trends in AOT over SEA on seasonal,annual,and monthly scales.In terms of area coverage and strength,the mean AOT(±SD)were observed,amplified remarkably with an increasing population and change in elevation across Pakistan.The lowest(0.05±0.04)and highest(0.40±0.06)values were observed in autumn and summer seasons,over the desert and coastal regions,respectively.A general increase in the annual AOT over the central to lower Indus Basin is associated with the considerable contribution of dust particles from the desert.The substantial annual heterogeneity in AOT peak values originated from an increasing population and emissions from biogenic and anthropogenic sources in the study domain.Ground-based retrieval analysis(Aerosol Robotic NETwork,AERONET)effectively captured an increasing(decreasing)trend over the Indo-Gangetic Plain(IGP)(North China Plain,NCP),which presented the highest(lowest)percentage of departures in AOT₄₄₀values.The characterization of major aerosol types in SEA presented the dominance of pure dust(PUD,31.90%)followed by polluted continental(POC,24.77%)types of aerosols,with moderate contribution from polluted dust(POD,20.92%),organic carbon dominating(OCD,11.85%),black carbon dominating(BCD,8.77%)and the lowest for the non-absorbing aerosol(NOA,1.79%).These are associated with agriculture,biomass burning,and long-range transport.The aerosol extinction coefficient(EC)vertical profile is strongly dependent on the vertical distribution of columnar aerosol,composition,and relative humidity.It revealed a higher range of>0.31 km^-1 within 2 km above the ground over most areas in East India and China.The volume particle concentration(VPC)of fine-(coarse-)mode was optimum with the highest values observed at Taihu(Karachi)0.139?m³?m^-2(0.061?m³?m^-2)attributed to the geographical location,meteorology,and hygroscopic nature of aerosols.Later,the annual mean(±SD)VPC were noticed high for PUD 0.37±0.01?m³?m^-2,followed by NOA and OCD with0.15±0.07?m³?m^-2 and 0.12±0.07?m³?m^-2for Karachi.This affirms the multiplicity of aerosol sources.The aerosol's direct radiative effects in the short-wave region(0.3?5.0?m)were quantified in the change of ARF in distinct environments of SA by using ground base(AERONET)observations and model(SBDART)simulation.The ARF exhibited significant temporal heterogeneity with a high positive ATM being observed across the regions,experiencing high aerosol burden.High surface ARF(70.12Wm^-2)and top layer ARF(99.78Wm^-2)were noticed for BCD aerosol types.They had corresponding heating values of(2.10)Kday^-1 and(2.54)Kday^-1 being generally high(low)during DJF(SON)in the urban and industrialized regions of SA.Meanwhile,the averaged(±SD)values of ARF at the TOA,BOA,and ATM ranged from-17.36±3.75 to-45.17±4.87 Wm^-2,-64.6±4.86 to-93.7±10.27 Wm^-2,and 40.5±6.43 to 68.25±7.26 Wm^-2,respectively.These are linked to combined influences from natural and anthropogenic absorbing aerosols,with an averaged atmospheric heating rate of 0.9-2.3 Kday^-1 over the different environments in SA.The positive(negative)values of ARF and HR,resulted in net warming associated with the absorption of solar radiation due to the presence of anthropogenic aerosols in the atmosphere.The present study has demonstrated a comprehensive understanding of the physical properties of aerosols over Asian regions.It propels a scientific basis for aerosol science over the continent.This study facilitates the accuracy of climate simulations in prevailing climatic conditions over Pakistan and surrounding Asian regions.The modelling section shows a general diagnostic of the mechanism,which requires further validation and improvement through simulation with AOT data assimilation in future studies.