Optical Properties of Aerosol Particles

1. Laramie, WY Aerodyne Aerosol Mass Spectrometer (AMS) Measures total mass loading (μg/m3) of volatile aerosol species and their size-resolved mass loading (20-1000 nm vacuum aerodynamic diameter range) Roof Inlet 0.5 lpm Scanning Mobility Particle Sizer (SMPS) Measures particle size distribution in 15-770 nm mobility diameter range. (resolution 64 chan/decade, averaging time interval: 303 sec) Heat Coil 1.Simple/compli-cated chemical composition model 2. Partial Molar Refraction (PMR) method for n calc. (Stelson, 1982) Size-resolved chemical composition data (SO4, NO3, NH4, Organics) from AMS To pump Size-resolved chemical composition (dM/dlogD or dM/dD) RH Sensor Size-resolved Refractive index (n) and density (ρ) TSI 3-wavelength Integrating Nephelometer Measures total- and back-scattering coefficients at 3 wavelengths: 700 nm (red), 550 nm (green), and 450 nm (blue) 3.Volume-weighted density in each particle size range Total mass loadings from filter-packs (sulphates, nitrates, organics, black carbon, refractory material) Aethalometer Measures mass loading of black carbon (μg/m3) Ultrafine Condensation Particle Counter (UCPC) Counts number concentration (up to 105 cm-3) of particles of the size 3 nm and higher Comprehensive size distribution over 15 nm – 20 μm size range 4 lpm 5. Mie calculations Total scattering coefficient calculated from the sampled aerosol Size-resolved Refractive index (n) of comprehensive size distribution 1 lpm Passive Cavity Aerosol Spectrometer Probe (PCASP) (0.5 lpm) Measures particle size distribution in 0.1-3 μm optical diameter range (resolution 30 chan total, averaging time – 1sec) Aerodynamic Particle Sizer (APS) (5.0 lpm)Measures particle size distribution in 0.5-20 μm aerodynamic diameter range (resolution 32 chan/decade, averaging time – 1 min) 2. Partial Molar Refraction (PMR) method for n calc. (Stelson, 1982) 4. Diameter conversion and distributions alignment method Comparison/ optical closure Size-distribution measurements from SMPS, PCASP, and APS Total scattering coefficient measurements from Nephelometer Optical Properties of Aerosol Particles Based on Size-Resolved Chemical Composition: Data Analysis Algorithm Development Mariya Petrenko, Derek Montague, Peter Liu, and Terry Deshler Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming 82071, USA 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. 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 2005. A partial data set from August 3, 2005 is shown below. Experimental Setup and Instrumentation Data Analysis Mass Closure Data Processing Algorithm Development Surface Aerosol Properties, Laramie, WY Preliminary Results and Conclusions Alignment of size distributions for generated aerosols • 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 (NH4)2SO4 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 dataacquired duringEMLACE. These analyses account for variability in the size-dependent chemical composition of the particles. Optical closure (total scattering coefficient) References • 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, 2004. DOI: 10.1080/027868290903907 • Hand, J. L. and Kreidenweis, S. M.: Size corrections based on refractive index for Particle Measuring Systems Active Scattering Aerosol Spectrometer Probe, CIRA report 0373-5352-31, 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 Total extinction coefficient for ammonium sulphate ((NH4)2SO4) polydisperse aerosol calculated using Mie theory for TSI Nephelometer wavelengths: red (700 nm), green (550 nm) and blue (450 nm) over 7-170 degrees (determined by instrument optics), compared with the total scattering coefficient measured by the TSI Nephelometer.