The Interagency Monitoring of PROtected Visual Environments (IMPROVE) program provides stewardship of visibility in National Parks and Wilderness Areas. Aerosol light absorption can contribute significantly to visibility reduction and is determined with forward and backward light scattering measurements of aerosol collected on PTFE filters using a hybrid integrating plate and sphere (HIPS) method with a 633 nm light source. Light absorption is also derived from light absorbing carbon (LAC) measured on quartz filters using a thermal optical reflectance (TOR) method. An IMPROVE sampler collecting 24-hr PM2.5 samples on quartz and PTFE filters every-day was established on the roof of the Physics building at the University of Nevada Reno to compare and evaluate the HIPS filter light absorption (fabs) and TOR LAC measurements against an in-situ light absorption (babs) method. In-situ measurements were made with custom photoacoustic instruments operating at 405 nm, 532 nm, 635 nm, 660 hm, 870 nm, and 1047 nm. All but the 1047 nm instrument were also equipped with a reciprocal nephelometer, which measured aerosol light scattering. Reno is impacted from local mobile and other sources. It has clear air quality days when storms move the urban aerosol out, sometimes replaced with windblown dust from distant sources, and is susceptible to smoke from wildland fires and residential wood combustion. This diversity of sources of absorbing aerosols make Reno a good location for the aerosol light absorption intercomparison experiment. This presentation will describe the experiment and show preliminary results for the intercomparison from January through November 2020. This period experienced clean days as well as poor air quality due to urban aerosol, a strong dust storm on February 8th with Honey Lake CA as the aerosol source region, wild fire smoke from California, and the sudden reduction in societal activity caused by the global viral COVID-19 pandemic. The spectral response of the babs measurements shifted on day dominated by wildfire smoke compared to other days. However, the underlying relationship between the fabs and in-situ absorption was stable throughout the entire time period at babs < 5 Mm-1. At higher babs there was evidence of filter loading artifacts causing the fabs to be low. The TOR LAC concentrations did not show evidence of a filter loading artifact, but the LAC to babs increased with increasing babs.
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