10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Evaluation of the Second IMPROVE Equation
ANTHONY PRENNI, Jenny Hand, William Malm, Scott Copeland, Bret Schichtel, National Park Service
Abstract Number: 578 Working Group: Remote/Regional Atmospheric Aerosol
Abstract The Interagency Monitoring of Protected Visual Environments (IMPROVE) network provides the primary method for visibility monitoring at Class I Areas throughout the United States. Monitoring is conducted by collecting PM2.5 and PM10 samples every third day at over 150 sites nationwide, with PM2.5 samples being analyzed for chemical composition. Using these data, light extinction is reconstructed using the second IMPROVE Equation. Reconstructed extinction values are then used to calculate haze levels and estimate visibility. In addition to calculated light extinction from IMPROVE, the National Park Service directly measures aspects of visibility through optical measurements at a subset of IMPROVE sites using Optec NGN-2 integrating nephelometers. The optical measurements serve as a quality assurance check for IMPROVE, in that measured scattering from the nephelometers and reconstructed scattering from the IMPROVE Equation should be equivalent. During its development, the second IMPROVE Equation was shown to accurately estimate light scattering for a broad array of aerosol compositions and loadings. However, periodically revisiting the IMPROVE Equation is needed, as some of the relationships have been empirically-derived and may change with changing atmospheric composition. In this presentation, we compare measured light scattering to reconstructed light scattering from the second IMPROVE Equation at 11 sites over a 15 year period. The data suggest that the relationship between measured and reconstructed light scattering has changed over time, corresponding to systematic changes in the chemical and physical properties of the aerosol. One important implication of this is that trends based on calculated light scattering from IMPROVE measurements generally overestimate decreasing trends in measured light scattering. We present possible reasons for the discrepancies, discuss their implications, and propose a potential approach for correcting the IMPROVE Equation.