Effects of aerosol on the local heat budget of the lower atmosphere

The convergence of heat below subsidence inversions was estimated from radiosonde ascents on fine summer days. Sensible heat input from the ground was estimated from measurements of heat fluxes and net radiation over a wheat field. Absorption and back scattering of solar radiation by aerosol throughout the atmosphere were calculated from measurements of solar radiation at the ground. Radiation absorbed by aerosol in the thermal boundary layer heated the lower atmosphere at an average rate of 3.3degC day-’ (60Wrn-’), about twice as fast as estimated for gaseous constituents. An equation is derived to show how the net effect of aerosol on the lower atmosphere depends on the reflection coefficient of the surface and on the aerodynamic and surface resistances to vapour transfer. Over the wheat, the net effect was almost zero when the crop was transpiring fast, but when the crop was mature, aerosol caused net cooling.