Aerosol-climate interactions at the local scale are strongly modulated by the biophysical properties of the underlying land surface

The aerosol radiative effect (RE) is strongly heterogeneous due to the horizontal and vertical variability of both aerosol concentration and composition. Of particular note is the disproportionately larger aerosol direct RE at the Earth's surface, which can be strongly negative or positive depending on aerosol diameter and the wavelength of incoming radiation. Moreover, the local surface temperature response to this forcing depends on both the magnitude of the forcing and how energy is dissipated via the biophysical properties of the underlying land surface. All these complexities lead to large uncertainties in our understanding of the aerosol impact on our climate, particularly its spatial variation. Here, we combine a global reanalysis product with a theoretical decomposition of the surface energy budget to separate the local temperature response to the shortwave and longwave components of the aerosol direct RE across the major terrestrial climate zones. We find that the local climate sensitivity is around five times higher during nighttime, when only the longwave direct RE is active, than that to the shortwave direct RE during daytime. This is due to low turbulent mixing during the night. Even during daytime, the local climate sensitivity is stronger for longwave direct RE than for shortwave direct RE, partly caused by the collocation of higher local climate sensitivity, due to aerodynamically smooth, non-vegetated surfaces, and coarse-mode aerosols, that strongly modulate longwave direct RE, in arid regions. As a result of the opposing shortwave and longwave direct RE at distinct times of the day, the global mean diurnal temperature range is reduced by 0.47, with almost half the reduction due to anthropogenic aerosols. We examine the temporal trend of the local temperature response to the aerosol direct RE and find that local climate sensitivity to surface radiative forcing has increased in the tropical zone, possibly due to deforestation; and that this increased sensitivity accounts for a third of the change in temperature response due to aerosol shortwave direct RE in this region from 1980 to 2017. Finally, we discuss how the modulation of local temperature response to aerosols by the surface could conceptually explain the higher climate forcing efficacy of aerosols.