| Summary: | We compare select aerosol Properties derived from the Community Multiscale Air Quality (CMAQ) model-simulated aerosol mass concentrations with routine data from the National Aeronautics and Space Administration (NASA) satellite-borne Moderate Resolution Imaging Spectro-radiometer (MODIS) sensor aboard the sun-synchronous Terra Satellite, the NASA's ground-based AErosol RObotic NETwork (AERONET), and the ground-based Interagency Monitoring of PROtected Visual Environment (IMPROVE) Network. The motivation for this analysis is to determine how best to use these parameters in evaluating model predicted PM 2.5 concentrations. CMAQ surface extinction estimates due to scattering at 550 nm wavelength are compared with the IMPROVE nephelometer data obtained from 28 sites within the United States (US). Sulfate fractional Aerosol Optical Depth (AOD) is found to dominate in the northeastern (NE) part of the US; hence, ground-based measurement of sulfate concentrations have been compared with time-series of columnar AOD as observed by the MODIS instrument and also with the CMAQ-predicted tropospheric column values obtained during the June-July-August (JJA) period of 2001. CMAQ surface extinctions are found to be relatively higher than the IMPROVE nephelometer observations; however, there is a good agreement between CMAQ AOD trends and AEROMET and MODIS data, obtained at the seven AERONET sites located in the Eastern U.S. CMAQ is also found to capture the day-to-day variability in the spatial AOD patterns. Monthly average satellite AOD estimates are found to be higher than the AOD data obtained using the CMAQ-predicted aerosol concentrations. Seasonal variation of the satellite-measured aerosol intensive property 'Angstrom exponent' (a gross indicator of the aerosol size distribution) is presented for four selected sites; one each in the Eastern and Central parts and two in the Western part of Continental United States (CONUS). Variability of Angstrom exponent at these four selected sites are analyzed in conjunction with the variation of summer-time AOD (observed and modeled), mass concentrations (observed and modeled) and modeled SO 4 average concentrations during the summer (JJA) period of the year 2001. Annual time series of Angstrom exponent data at the 4 selected sites show a large east-west variation.
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