Modelling the radiative impact of aerosols from biomass burning
during SAFARI-2000
Gunnar Myhre1,2 Terje K. Berntsen3,1 James M. Haywood4 Jostein K. Sundet1 Brent N. Holben5 Mona Johnsrud2 Frode Stordal2,1
1Department of Geophysics, University of Oslo, Oslo, Norway
2Norwegian Institute for Air Research (NILU), Kjeller, Norway
for International Climate and Environmental Research - Oslo (CICERO), Oslo, Norway
4Met Office, Bracknell, UK
5Biospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland
3Center
Introdution
Method
Based on modelling
• A 3-dimentional off-line CTM with pre-calculated meteorological fields from
ECMWF is adopted to calculate the distribution of aerosol from biomass burning.
The horizontal resolution used in the simulations is T63 (1.87°x1.87°).
linked to measurements we estimate the radiative impact
of aerosols from biomass burning during the SAFARI-2000 campaign,
A chemistry-transport model (the Oslo CTM) with meteorological data for the
actual period is adopted to simulate the distribution of the biomass aerosols.
 A radiative transfer scheme is adopted in the calculations of the radiative
impact of the biomass aerosols.
A thorough comparison between our model results and available observations
are made with regard to aerosol optical depth (AOD), the vertical profile, and the
radiative impact of the biomass aerosols. Observations include in situ data from
the Met Office C-130 aircraft, ground based data, and satellite data
Aerosol optical depth (AOD)
The modelled September 2000 monthly mean AOD is shown in Figure 1. A
maximum AOD of nearly 1.0 is estimated with transport pattern to the north
west and south east.
• The treatment of black carbon (BC) and organic carbon (OC) for biomass burning
is adopted from Cooke et al. [1999]. Both BC and OC are separated in a
hydrophobic fraction and a hydrophilic fraction (see further details [Myhre et al.,
2002)].
• The size distribution and refractive index of the particles in the biomass burning
plume are adopted from the Met Office C-130 aircrfat [Haywood et al., 2002] to
model the optical properties (specific extinction coefficient, single scattering
albedo, and asymmetry factor) using Mie theory.
• A BC/OC ratio of 0.12 from [Haywood et al., 2002] and a OM/OC ratio of 2.6 from
Formenti et al. [2002] is used in the calculation of the optical properties.
• We reproduce the single scattering albedo at 0.55 µm of 0.90, which was estimated
by Haywood et al. [2002]. Further, the decrease with wavelength in specific
extinction and single scattering, [Haywood et al., 2002], which is important for the
radiative transfer calculations, is also well reproduced.
Radiative forcing
Clouds strongly influence the radiative forcing due to aerosol from biomass burning
as can be seen from Fig 3. Clouds have a stronger impact on the radiative forcing due
to biomass aerosol than for sulfate aerosols.
Fig 1: Monthly mean modelled AOD for September 2000 in the upper
panel and AOD for September 2000 from MODIS in the lower panel.
Fig 3: Monthly mean radiative forcing due to aerosols from biomass buring during
September 2000. a) Clouds included in the radiative transfer calculations, b) clouds
excluded in the radiative transfer calculations.
Summary
Comparison with AERONET data
 Using the ECMWF meteorological data for the campaign period the model manages
to reproduce some of the main patterns of AOD during period, found both in
satellite retrievals and ground based AERONET measurements.
 The modelled radiative impact of the biomass aerosols compares reasonably well to
measurements (within 20%).
 Local radiative cooling and warming up to 50 Wm-2 magnitude is modelled.
 The clouds strongly influence the radiative impact of the aerosols.
 Globally the aerosols from biomass burning in southern Africa in September 2000
result in a global mean radiative impact of -0.13 Wm-2.
References
Cooke, W.F., C. Liousse, H. Cachier, and J. Feichter, Construction of a 1x1 fossil-fuel emission dataset for carbonaceous
aerosols and implementation and radiative impact in the ECHAM-4 model, J. Geophys. Res., 104, 22,137-22,162,
1999.
Formenti, P., W. Elbert, W. Maenhaut, C. Jost, D. Sprung, M.O. Andreae, H. Barjat, J. Haywood, P. Francis, and S.
Osborne, The C-130 airborne measurements of water soluble and carbonaceous aerosols during the SAFARI 2000 dry
season intensive: chemical characteristics, relevance to the optical properties and emission inventories of African
biomass burning aerosols, J. Geophys. Res., accepted 2002.
Haywood, J., S. Osborne, P. Francis, P. Formenti, and M.O. Andreae, The mean physical and optical properties of
biomass burning aerosol measured by the C-130 aircraft during SAFARI-2000, J. Geophys. Res., accepted 2002.
Myhre, G., T. K. Berntsen, J. M. Haywood, J. K. Sundet, B. N. Holben, M. Johnsrud, and F. Stordal, Modelling the solar
radiative impact of aerosols from biomass burning during SAFARI-2000, accepted J. Geophys. Res., 2002.
Fig 2: Comparison of AOD from the modelled with AERONET for 10 stations
The paper can be found at
http://folk.uio.no/gunnarmy/manuscript/revised/safari/safari_ctm.pdf
A similar paper from the SHADE campaign can be found at
http://folk.uio.no/gunnarmy/manuscript/revised/shade/shade.pdf