Observed impact of atmospheric aerosols on the surface energy budget

Atmospheric aerosols scatter and potentially absorb incoming solar radiation, thereby reducing the total amount of radiation reaching the surface and increasing the fraction that is diffuse. The surface energy balance is postulated to change with increasing aerosol concentrations through an increase in diffuse light reaching greater portions of the canopy, increasing photosynthetic and transpiration rates, and reducing the Bowen ratio (B). Here we evaluate half-hourly and hourly surface fluxes from six FLUXNET sites in the continental United States over the past decade (2000-2010) in conjunction with satellite-derived aerosol optical depth (AOD) to determine if surface aerosols systematically influence surface energy fluxes. Surface fluxes are binned by satellite-derived AOD value (e.g., high AOD and low AOD) to establish the relationship between aerosol concentrations and sensible heat flux (H) and latent heat flux (λE) partitioning. High AOD reduces net radiation by 7-20% at midday coupled with a 6-30% decrease in H and a 2-13% decrease in λE, although not all sites exhibit statistically significant changes. At one of the four forest sites, the decrease in H is nearly three times that of λE, suggesting that increased photosynthesis leads to an increase in transpiration and a reduction in the Bowen ratio at high AOD. This response is not consistent for all sites, and overall the observed effect of AOD on surface fluxes is generally small (about 10%). These results suggest that aerosols can trigger a variable effect on surface flux partitioning by aerosols at different sites that may be ecosystem dependent, yet the drivers for this ecosystem-dependent response require further exploration.