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Aerosol and radiation studies with the UK Met Office C-130 aircraft during SAFARI-2000
Jim M. Haywood, Met Office, Farnborough, Hants, United Kingdom; and P. N. Francis, S. R. Osborne, O. Dubovik, B. Holben, P. Formenti, and M. Andreae
The UK Met Office C-130 aircraft measured the physical and optical properties of aerosol in September 2000 during SAFARI-2000. Filter measurements of aged regional haze suggest that the ratio of black carbon (BC) to organic carbon (OC) is approximately 0.12±0.02, and that the contributions to the total aerosol mass are approximately 0.05 BC, 0.25 inorganic compounds, and 0.70 organic matter (OC plus associated elements). The sub-micron size distribution of aged regional haze is similar throughout the period, and may be fitted with three log-normal distributions with geometric mean radii, rn, of 0.12±0.01µm, 0.26±0.01µm, and 0.80±0.01µm and standard deviations, sigma, of 1.3±0.1, 1.5±0.1, and 1.9±0.4. Measurements over 2500km from the emission region show similar rn and sigma for the first two modes while the third mode is absent presumably as a result of sedimentation. At 0.55µm, effective medium approximations suggest a refractive index of 1.54-0.018i for aged aerosol. The single scattering albedos derived using this refractive index and the measured size distributions are consistent with those derived independently from the nephelometer and Particle Soot Absorption Photometer (PSAP). The optical parameters at 0.55µm derived from the first two log-normal distributions suggests a specific extinction coefficient of 5m2g-1, an asymmetry factor of 0.59 and a single scattering albedo of 0.91 at for aged aerosol. Measurements are also performed in a fresh biomass plume less than a few minutes old which contains smaller particles, and a smaller BC/OC mass ratio of 0.08. Vertical profiles of CO, aerosol concentration and scattering show a good correlation, and suggest the aerosols become well-mixed over land regions. Over ocean, the aerosols can be separated from underlying Sc cloud by a clear-gap and a strong inversion which may inhibit the mixing of biomass aerosol and Sc thereby limiting the indirect effect. Radiative closure studies were also performed over a CIMELS site based at the Etosha Pan in Northern Namibia. While the estimates of aerosol optical depth from the radiometric measurements of the C-130 are in reasonable agreement with those from the CIMELS, significantly lower values are obtained by integrating the scattering and absorption coefficients obtained from the nephelometer and PSAP. The most likely explanation for this discrepancy is due to particle losses in the aircraft instrumentation. The aerosol size distribution measured by the Passive Cavity Aerosol Spectrometer Probe (PCASP) and derived from the CIMELS are in excellent agreement over the 0.05-1.0µm radius range, which contains the majority of the optically active particles. C-130 derived refractive indices and single scattering albedos are also shown to be in excellent agreement. These results suggest that data from CIMELS should be used in validating satellite measurements and modelling studies of the radiative effects of aerosols.
Session 1, Radiative Effects of Aerosols
Monday, 3 June 2002, 9:00 AM-11:45 AM
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