Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 7 2 2007 10.5194/acp-7-453-2007 http://www.atmos-chem-phys.net/7/453/2007/ http://www.atmos-chem-phys.net/7/453/2007/acp-7-453-2007.html http://www.atmos-chem-phys.net/7/453/2007/acp-7-453-2007.pdf 453 458 2007-01-25 Classification of aerosol properties derived from AERONET direct sun data G. P. Gobbi g.gobbi@isac.cnr.it Y. J. Kaufman I. Koren T. F. Eck Inst. Scienze dell' Atmosfera e del Clima, CNR, Rome, Italy NASA/Goddard Space Flight Center, 613.2, Greenbelt MD 20771, USA Dep. of Environ. Sciences Weizmann Institute, Rehovot 76100, Israel GEST, University of Maryland, Baltimore, MD, USA also at: NASA/GSFC, 614.4, Greenbelt MD 20771, USA Aerosol spectral measurements by sunphotometers can be characterized by three independent pieces of information: 1) the optical thickness (AOT), a measure of the column aerosol concentration, 2) the optical thickness average spectral dependence, given by the Angstrom exponent (α), and 3) the spectral curvature of α (δα). We propose a simple graphical method to visually convert (α, δα) to the contribution of fine aerosol to the AOT and the size of the fine aerosols. This information can be used to track mixtures of pollution aerosol with dust, to distinguish aerosol growth from cloud contamination and to observe aerosol humidification. The graphical method is applied to the analysis of yearly records at 8 sites in 3 continents, characterized by different levels of pollution, biomass burning and mineral dust concentrations. Results depict the dominance of fine mode aerosols in driving the AOT at polluted sites. In stable meteorological conditions, we see an increase in the size of the fine aerosol as the pollution stagnates and increases in optical thickness. Coexistence of coarse and fine particles is evidenced at the polluted sites downwind of arid regions. Angstrom, A.: On the atmospheric transmission of sun radiation and on dust in the air, Geogr. Ann., 11, 156–166, 1929. Barnaba, F. and Gobbi, G. P.: Aerosol seasonal variability over the Mediterranean region and relative impact of maritime, continental and Saharan dust particles over the basin from MODIS data in the year 2001, Atmos. Chem. Phys., 4, 2367–2391, 2004. Chinnam, N., Dey, S., Tripathi, S. N., and Sharma, M.: Dust events in Kanpur, northern India: Chemical evidence for source and implications to radiative forcing, Geophys. Res. Lett., 33, L08803, doi:10.1029/2005GL025278, 2006. Dubovik, O. and King, M. D.: A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements, J. Geophys. Res., 105, 20 673–20 696, 2000. Dubovik, O., Holben, B. N., Eck, T. F., Smirnov, A., Kaufman, Y. J., King, M. D., Tanre, D., and Slutsker, I.: Variability of absorption and optical properties of key aerosol types observed in worldwide locations, J. Atmos. Sci., 59, 590–608, 2002. Eck, T. F., Holben, B. N., Reid, J. S., Dubovik, O., Smirnov, A., O'Neill, N. T., Slutsker, I., and Kinne, S.: Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosols, J. Geophys. Res., 104, 31 333–31 349, 1999. Eck, T. F., Holben, B. N., Ward, D. E., Dubovik, O., Reid, J. S., Smirnov, A., Mukelabai, M. M., Hsu, N. C., O'Neill, N. T., and Slutsker, I.: Characterization of the optical properties of biomass burning aerosols in Zambia during the 1997 ZIBBEE field campaign, J. Geophys. Res., 106, 3425–3448, 2001. Holben, B. N., Eck, T. F., Slutsker, I., Tanre, D., Buis, J. P., Setzer, A., Vermote, E., Reagan, J. A., Kaufman, Y. J., Nakajima, T., Lavenu, F., Jankowiak, I., and Smirnov, A.: AERONET – A federated instrument network and data archive for aerosol characterization, Rem. Sens. Environ., 66, 1–16, 1998. Kaufman, Y. J.: Aerosol optical thickness and atmospheric path radiance, J. Geophys. Res., 98(D2), 2677–2692, 1993. Kaufman, Y. J., Gobbi, G. P., and Koren, I.: Aerosol climatology using a tunable spectral variability cloud screening of AERONET data, Geophys. Res. Lett., 33, L07817, doi:10.1029/2005GL025478, 2006. King, M. D., Byrne, D. M., Herman, B. M.: Aerosol size distribution obtained by inversion of spectral optical depth measurements, J. Atmos. Sci., 35, 2153–2167, 1978. Mélin F. and Zibordi, G.: Aerosol variability in the Po Valley analyzed from automated optical measurements, Geophys. Res. Lett., 32, L03810, doi:10.1029/2004GL021787, 2005. Mishchenko, M. I, Travis, L. D., Kahn, R. A., and West, R. A.: Modeling Phase Functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids, J. Geophys. Res., 102(D14), 16 831–16 847, 1997. Nakajima, T., Takamura, T., Yamano, M.: Consistency of aerosol size distributions inferred from measurements of solar radiation and aerosols, J. Met. Soc. Jap., 64, 765–776, 1986. Nakajima, T. and King, M. D.: Determination of the optical thickness and effective particle radius of clouds from reflected solar radiation measurements. Part I: Theory, J. Atmos. Sci., 47, 1878–1893 1990. O'Neill, N. T., Dubovik, O., and Eck, T. F.: A modified Angstrom coefficient for the characterization of sub-micron aerosols, Appl. Opt., 40, 2368–2375, 2001a. O'Neill, N. T., Eck, T. F., Holben, B. N., Smirnov, A., and Dubovick, O.: Bimodal size distribution influences on the variation of Angstrom derivatives in spectral and optical depth space, J. Geophys. Res., 106(D9), 9787–9806, 2001b. O'Neill, N. T., Eck, T. F., Smirnov, A., Holben, B. N., and Thulasiraman, S.: Spectral discrimination of coarse and fine mode optical depth, J. Geophys. Res., 108(D17), 4559, doi:10.1029/2002JD002975, 2003. O'Neill, N. T., Thulasiraman, S., Eck, T. F., and Reid, J.S.: Robust optical features of fine mode size distributions: Application to the Quebec smoke event of 2002, J. Geophys. Res., 110, D11207, doi:10.1029/2004JD005157, 2005. Schuster, G. L., Dubovick, O., and Holben, B. N.: Angstrom exponent and bimodal aerosol size distributions, J. Geophys. Res., 111, D07207, doi:10.1029/2005/JD006328, 2006. Zheng, M., Salmon, L. G., Schauer, J. J., Zeng, L., Kiang, C. S., Zhang, Y., and Cass, G. R.: Seasonal trends in PM2.5 source contributions in Beijing, China, Atmos. Env., 39, 3967–3976, 2005.