This paper provides very good insights into this topic by using ground and aircraft based lidar observations during DABEX field campaign. 'In general, mineral dust was observed at low altitudes (up to 2 km), and a mixture of biomass burning aerosol and dust was observed at altitudes of 2–5 km.'
For clear sky conditions, when the observed low-level dust layer was included in a radiative transfer model, the absorption of solar radiation by the biomass burning aerosols increased by 10%.' This enhancement in absorption is due to reflection of solar radiation by dust aerosols in background up into the biomass burning aerosol layer above. This situation is analogous to presence absorbing aerosols over a bright background. Thus, depending on the distribution of aerosols and the type of aerosols present at different heights in the atmosphere, their radiative effects can be altered. This in-turn changes the differential heating of the atmosphere and hence the atmospheric stability that influences convective and turbulent motions and clouds [Ackerman et al., 2000]. This can be important for both TOA and surface radiation budget. The scenario is a little more complex when aerosols are above clouds. 'For example, the elevation of biomass burning aerosols above marine stratocumulus clouds during the Southern African Regional Science Initiative (SAFARI-2000) greatly enhanced their absorption of shortwave radiation. This led to a positive direct aerosol shortwave radiative effect over the Southern Atlantic, whereas the effect was negative in clear sky conditions [Keil and Haywood, 2003; Abel et al., 2005; Myhre et al., 2003a]'. Thus, the need to consider the treatment and appropriate representation of vertical distribution of aerosol species in the global models when estimating the impact of anthropogenic absorbing aerosols is crucial.