Optical Properties of Aerosol Particles Introduction Atmospheric aerosol particles play a significant role in determining Earth's climate, through their interactions with solar radiation (the direct effect), and through their action as cloud condensation nuclei (the indirect effect). Our current ability to predict the aerosol direct effect on the atmospheric radiation balance on both global and local scales is restricted by a number of factors, among which are the high variability of aerosol properties in space and time, and instrumentation limitations for the production of quantitative size-dependent particle chemical composition and optical properties data with high time resolution. Comprehensive studies of the relationship of aerosol optical properties to particle composition as a function of size are therefore needed to improve our ability to compute radiative scattering by the aerosol on all size scales. Experimental Setup and Instrumentation One goal of this study is the development of a data analysis algorithm to calculate aerosol scattering extinction, and hence overall optical properties, using the size- dependent chemical composition of the particles. The algorithm is being tested with data from comprehensive measurements obtained at the UW Keck Aerosol Laboratory in Laramie, WY during the Elk Mountain/Laramie Aerosol Characterization Experiment (EMLACE) in the summer of A partial data set from August 3, 2005 is shown below. Laramie, WY Surface Aerosol Properties, Laramie, WY Data Analysis Mass Closure Size-resolved chemical composition data (SO4, NO3, NH4, Organics) from AMS Total mass loadings from filter- packs (sulphates, nitrates, organics, black carbon, refractory material) 1.Simple/compli- cated chemical composition model Size-resolved chemical composition (dM/dlogD or dM/dD) Size-resolved Refractive index (n) and density (ρ) 2. Partial Molar Refraction (PMR) method for n calc. (Stelson, 1982) 3.Volume- weighted density in each particle size range Comprehensive size distribution over 15 nm – 20 μm size range Size-distribution measurements from SMPS, PCASP, and APS 4. Diameter conversion and distributions alignment method Size-resolved Refractive index (n) of comprehensive size distribution 2. Partial Molar Refraction (PMR) method for n calc. (Stelson, 1982) Total scattering coefficient measurements from Nephelometer Total scattering coefficient calculated from the sampled aerosol 5. Mie calculations Comparison/ optical closure References Data Processing Algorithm Development Alignment of size distributions for generated aerosols Optical closure (total scattering coefficient) Total extinction coefficient for ammonium sulphate ((NH 4 ) 2 SO 4 ) polydisperse aerosol calculated using Mie theory for TSI Nephelometer wavelengths: red (700 nm), green (550 nm) and blue (450 nm) over degrees (determined by instrument optics), compared with the total scattering coefficient measured by the TSI Nephelometer. Bohren, C. F. and Huffman, D. R.: Absorption and scattering by small particles, John Wiley and Sons, Inc., 1983 DeCarlo, P., Slowik, J. G., Worsnop, D. R., Davidovits, P., and Jimenez, J. L.: Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements. Part I: Theory. Aerosol Science and Technology, 38: 1185–1205, DOI: / Hand, J. L. and Kreidenweis, S. M.: Size corrections based on refractive index for Particle Measuring Systems Active Scattering Aerosol Spectrometer Probe, CIRA report , Colorado State University, Fort Collins, CO, 1996 Stelson, A. W.: Urban aerosol refractive index prediction by partial molar refraction approach, Environ. Sci. Technol., 24, 1676–1679, 1990 Preliminary Results and Conclusions The general data analysis algorithm has been tested using lab-generated aerosols (sodium nitrate, ammonium nitrate, and ammonium sulphate). Size distributions of lab- generated poly- and monodisperse aerosols measured by the APS and SMPS are in excellent agreement. Comparison of PCASP distributions with those from the APS and SMPS show some variability, but are nevertheless reasonable. Comparison of measured scattering extinctions at three wavelengths for dried poly- disperse (NH 4 ) 2 SO 4 aerosol with values calculated by Mie theory using measured size distributions are in very good agreement (≤ 14%). The analytical tools developed in this study are now being applied to the ambient aerosol data acquired during EMLACE. These analyses account for variability in the size-dependent chemical composition of the particles. Data Analysis Algorithm Development Mariya Petrenko, Derek Montague, Peter Liu, and Terry Deshler Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming 82071, USA Based on Size-Resolved Chemical Composition: 0.5 lpm 1 lpm To pump RH Sensor 4 lpm Roof Inlet Heat Coil Ultrafine Condensation Particle Counter (UCPC) Counts number concentration (up to 10 5 cm -3 ) of particles of the size 3 nm and higher Aethalometer Measures mass loading of black carbon (μg/m 3 ) Aerodynamic Particle Sizer (APS) (5.0 lpm) Measures particle size distribution in μm aerodynamic diameter range (resolution 32 chan/decade, averaging time – 1 min) Passive Cavity Aerosol Spectrometer Probe (PCASP) (0.5 lpm) Measures particle size distribution in μm optical diameter range (resolution 30 chan total, averaging time – 1sec) Scanning Mobility Particle Sizer (SMPS) Measures particle size distribution in nm mobility diameter range. (resolution 64 chan/decade, averaging time interval: 303 sec) TSI 3-wavelength Integrating Nephelometer Measures total- and back-scattering coefficients at 3 wavelengths: 700 nm (red), 550 nm (green), and 450 nm (blue) Aerodyne Aerosol Mass Spectrometer (AMS) Measures total mass loading (μg/m 3 ) of volatile aerosol species and their size-resolved mass loading ( nm vacuum aerodynamic diameter range)