Saturday, September 20, 2008

Impact of Aerosol Vertical Distribution on their Radiative Effects

From over a decade of aerosol research, it is now a well established fact that atmospheric aerosols play a crucial role in altering the earth's radiation budget [IPCC 2007]. Toa large extent, the effect on aerosols on the top-of-atmosphere radiation balance has been addressed using both observations and global models. A close review of literature suggests the imperative need to address the vertical distribution of aerosols in the atmosphere [Gadhavi et al., 2006] and that is exactly what aerosol science is gearing towards. Recently, there have been many research studies that have begun to address this question. One interesting paper by Johnson et al., appeared in JGR last week that talks about this very issue.

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.

Please refer to the paper by Johnson et al (below) for all references in this blog-post:

Johnson, B. T., B. Heese, S. A. McFarlane, P. Chazette, A. Jones, and N. Bellouin (2008), Vertical distribution and radiative effects of mineral dust and biomass burning aerosol over West Africa during DABEX, J. Geophys. Res., 113, D00C12, doi:10.1029/2008JD009848