The sub-daily variability of aerosols affects the estimates of daily mean aerosol loading. However, large spatial scale estimates of their climate effects are mostly based on snapshots from low orbit satellites that may bias the mean estimate for daily, monthly, or annual timescales. In this study, an attempt is made to estimate the magnitude of such bias based on ground and satellite-based datasets. Using ground-based measurements, we show an apparent asymmetry (of the order of 10-50%) in the sub-daily variability of aerosol loading over the Indian region. For the first time, it is reported that this sub-daily variability has a spatial pattern with an increasing amplitude toward the east of the subcontinent. We also find this variability in aerosol loading is well-captured by the satellites but with a lower amplitude. Our study shows that such differences could alter the annual surface radiative forcing estimates by more than similar to 15 W m(-2) over this region. We find that NASA's Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2), a state-of-the-art model-based chemical reanalysis, is unable to capture these sub-daily variabilities. This implies that both model and satellite-based radiative forcing estimates for large spatial scales should improve aerosol sub-daily information/variabilities for obtaining reliable radiative forcing estimates.

The Indo Gangetic Plain (IGP), one of the most densely populated regions of the world, is a global hotspot of anthropogenic aerosol emissions. In the pre-monsoon season (March-May), the strong westerlies carry transported dust aerosols along with anthropogenic aerosols onto the Bay of Bengal (BoB). The outflow from IGP modulates the aerosol loading and the aerosol direct radiative forcing (ADRF) over the BoB. The quantification of the anthropogenic aerosol impact on the radiative forcing over the outflow region remains inadequate. The enforced shutdown amid the COVID-19 pandemic eased the anthropogenic activities across the country, which helped to examine the magnitude and variability of aerosol loading and subsequent changes in ADRF over IGP and the outflow region of the BoB. Wind trajectory analysis illustrates that the ADRF over the BoB is greater during the days when the winds originated from the IGP region (at the surface-54.2 +/- 6.4 Wm(-2), at the top of the atmosphere,-26.9 +/- 3.4 Wm(-2) and on the atmosphere, 27.0 +/- 3.1 Wm(-2)) compared to the seasonal average (-46.3 +/- 7.1 Wm(-2),-24.9 +/- 4.0 W m(-2) and 20.6 +/- 3.2 Wm(-2), respectively). This indicates that anthropogenic aerosols emission from IGP can contribute an additional 31% of the atmospheric ADRF over the IGP outflow region of the BoB. The reduced aerosol loading during the shutdown period resulted in a reduction of ADRF at the surface, at the top of the atmosphere, and on the atmosphere over the IGP outflow region of the BoB by 22.0 +/- 3.1%, 20.9 +/- 3.4% and 23.2 +/- 3.3%, respectively. This resultant 20-25% reduction in ADRF over the IGP outflow region of BOB matches well with 10-25% reduction in aerosol optical depth (AOD) over the IGP during the shutdown period showing a robust coupling between IGP aerosol emissions and ADRF over the BoB. (C) 2021 Elsevier B.V. All rights reserved.