Diurnal variations in the aerosol physical, optical and altitude distributional characteristics over three locations in eastern India: Implications to black carbon radiative forcing

Abstract

A qualitative assessment on the effect of boundary layer dynamics in the estimation of clear sky black carbon (BC) radiative forcing has been made considering the debate on the lower bound and possible uncertainties in the aerosol radiative forcing estimation. Comprehensive measurements made on aerosol optical and physical properties, near surface BC mass concentrations and Lidar derived aerosol back scatter intensity profiles at three selected locations in India (Visakhapatnam, Kharagpur, and Kolkata) are utilized for this purpose. Sensitivity analysis carried out to estimate the errors in short wave (SW) BC forcing computation indicated that non-incorporation of diurnal changes in the boundary layer depth into the models may lead to over estimation of diurnally averaged (as the study is limited to short wave radiative forcing, diurnally averaged forcing refers to day-time averaged) BC forcing. The relative errors may vary between 7 to 70% depending on the season and the location changes in boundary layer depth day-time behaviour. The results reported in the present study, though specific to the study locations, clearly indicate that a more systematic approach is needed to investigate the sensitivity of aerosol radiative forcing to various atmospheric parameters and processes within the boundary layer, particularly at stations characterised by strong anthropogenic influence and large diurnal temperature variability that affect the boundary layer depth.