1. In-Situ Measurement of Scattering Albedo of Atmospheric Aerosol in the Amazon Region Otmar Schmid 1, Meinrat O. Andreae 1, W. Patrick Arnott 2, Paulo Artaxo 3, Duli Chand 1, Göran Frank 1, Erik Swietlicki 4, and Luciana V. Gatti Results PSAP based ω 0 values should be considered lower limits especially, if the Bond correction is not included. Extensive optical aerosol properties (B abs, B scat ) decreased by more than one order of magnitude from burning to pristine season. For burning, transition and pristine period, the mean ω 0 (550nm) values were 0.93, 0.91, and 0.93, respectively. Although the first two values are higher than typically assumed (ω 0 = 0.87, Penner et al., 2001), they are consistent with the upper ω 0 range reported in the literature for this type of aerosol (see Guyon 2003 and references herein). Fresh aerosol from biomass burning displays significantly lower than average ω 0 values (<0.8). Acknowledgements We gratefully acknowledge support from the European Commission (contract no. EVK2-CT-2001-00110, SMOCC ). Motivation Estimates indicate a significant (~7%) contribution of biomass burning to global aerosol emissions (Andreae, 1991). The impact of atmospheric aerosol on direct radiative forcing is characterized by their optical properties, given by the single scattering albedo ω 0 = B scat /(B scat +B abs ) (= ratio of scattering and extinction coefficient). Photoacoustic Spectrometer (PAS) As seen in the block diagram below, sample air is continuously drawn through an acoustic resonator which is illuminated by a power modulated laser beam (532 nm, 1500 Hz). Light absorption by particles in the resonator generates heat and subsequent pressure pulses which produce an acoustic standing wave in the resonator. The amplitude of the acoustic wave is proportional to the absorption coefficient B abs (Arnott et al., 2000). Experimental Measurement site: Fazenda Nossa Senhor (10°46’S, 62°21’W), Amazon Basin, Brazil; partially deforested region Measurement period: September to November 2002 Dry (RH<35 %), submicron sample aerosol Instrumentation: - Nephelometer (Radiance Research, M903, =545nm) - PSAP (Radiance Research, =565nm) - PAS ( =532nm, Arnott et al., 2000) Literature Andreae, M. O., Biomass burning: Its history, use and distribution and its impact on environment quality and global climate in global biomass burning: Atmospheric, climatic and biospheric implications, Ed. J. Levine, 3-21, 1991. Arnott et al., Rev. Sci. Instrum. 71, 4545-1452, 2000. Bond et al., Aerosol Sci. Technol. 30, 582-600, 1999. Guyon et al., Atmos. Chem. Phys. Discuss., 3, 1367-1414, 2003. Haywood et al., J. Geophys. Res.108,D13, 8473, doi:10.1029/2002JD002226, 2003 Penner et al., 3 rd Report of IPCC, Cambridge University Press, Cambridge, 2001. Objectives Measure ω 0 at a location of active biomass burning over several months covering both dry (burning) and wet (pristine) season Perform simultaneous measurement of particle absorption with a Particle Soot Absorption Photometer (PSAP) and a photoacoustic spectrometer (PAS) to identify potential systematic biases in ω 0. Identify seasonal averages and trends in ω 0. Fresh aerosol appears ‘more black’ (smaller  0 ) than aged aerosol. This is consistent with laboratory experiments on aging aerosol from biomass burning and other in-situ observations (Haywood et al., 2003). The PAS measures 18 % smaller B abs values than the PSAP. Hence, even after applying the Bond correction (Bond et al., 1999), PSAP based ω 0 values should be considered a lower limit. 1 Max Planck Institute for Chemistry, Dept. of Biogeochemistry, PO Box 3060, D-55020 Mainz, Germany 2 Desert Research Institute, Division of Atmospheric Science, Reno, NV 89512, USA 3 Universidade de São Paulo, Instituto de Fisica, CEP 05508-900 São Paulo, SP, Brazil 4 Lund university, Division of Nuclear Physics, PO Box 118, S-22100 Lund, Sweden 5 Instituto de Pesquisas Energéticas e Nucleares, Atmospheric Chemistry Laboratory, CEP 055508-900, São Paulo, SP, Brasil