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Title: AEROSOL RADIATIVE PROPERTIES IN THE SEMI-ARID WESTERN UNITED STATES 1493

Author
item PINKER, R. - UNIVERSITY OF MARYLAND
item PANDIETHURAI, G. - INDIAN INST TROP METEOR
item HOLDEN, B. - NASA
item Keefer, Timothy
item Goodrich, David - Dave

Submitted to: Atmospheric Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2004
Publication Date: 10/1/2004
Citation: Pinker, R., Pandiethurai, G., Holden, B., Keefer, T.O., Goodrich, D.C. 2004. Aerosol radiative properties in the semi-arid Western United States.Atmospheric Research 71:243-252.

Interpretive Summary: Extremely small liquid or solid particles in the atmosphere, called aerosols, can have significant impact on Earth¿s climate by absorbing or reflecting incoming radiation. Accurate measurements of the optical properties of these particles are made by earth based instruments and increasingly by satellites. Global climate computer models, which attempt to predict the dynamic processes of heating and precipitation near the earth¿s surface, require realistic representations of these particles for validation of model results. Also, there are natural seasonal and annual fluctuations in the amount of these particles which must be determined. Generally it has been more difficult to measure these over land than over oceans. Building on observations made over the last 25 years at nearby locations, recent observations in southeast Arizona using newer instruments and techniques allow for more types and more frequent measurements of these important variables. Comparisons between similar measurements made during the older and recent observing periods are in good agreement. Significant monthly and seasonal variation was detected, an important consideration for climate models. Observations were also compared to model results with good agreement. New techniques and methods such as these will help improve our ability to detect and respond to changes in climate.

Technical Abstract: A comprehensive characterization of aerosol optical properties, such as aerosol optical depth, Angstrom exponent, volume size distribution, single scattering albedo, and asymmetry parameter at the semi-arid site of Tombstone, Arizona, will be presented. In this region, numerous observations of aerosol optical depth, have been made in the past. Much less is known about other optical characteristics of importance in climate modeling and remote sensing. New observational techniques and inversion methods now allow augmentation of the earlier information. Monthly mean aerosol optical depths at 500 nm were found to be in the range of 0.029 to 0.116, and in good agreement with earlier observations. The monthly mean Angstrom exponent ranged from 0.9 to 1.6, higher in spring and summer and lower in late fall and winter. Volume size distributions exhibit clear dominance of the accumulation mode particles, with a gradual increase from winter to spring and to summer. Annual variation of accumulation and coarse mode radii ranged between 0.05-0.4 mm and 4-8 mm, respectively. Single scattering albedo and asymmetry parameters are also presented and annual mean values at 0.67 mm are found to be 0.81 and 0.59, respectively. The direct radiance measurements at 940 nm allow estimation of precipitable water vapor; the retrieved values compared well with estimates from the National Center for Environmental Prediction (NCEP) regional weather forecast model.