10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Long-Term Assessment of Sunset OC/EC with Chemical Speciation Network Measurements
STEVEN G. BROWN, Elizabeth Landis, Hilary Minor, Theresa O'Brien, Joann Rice, Sonoma Technology, Inc
Abstract Number: 1526 Working Group: Carbonaceous Aerosol
Abstract As part of a U.S. Environmental Protection Agency (EPA) project to evaluate the feasibility of the Sunset Semi-Continuous Organic and Elemental Carbon (OC/EC) monitor, EPA sponsored the deployment of this monitor by local air quality agencies in Chicago, Illinois; Houston, Texas; Las Vegas, Nevada; St. Louis, Missouri; Rubidoux, California; and Washington, D.C. Sunset monitors were collocated with existing 24-hr measurements of OC and EC made via filter sampling as part of the EPA’s Chemical Speciation Network (CSN); at Houston, St. Louis, and Washington, they were also collocated with Aethalometer instruments, which measure black carbon (BC). Sunset data were compared to CSN and Aethalometer data to assess whether the Sunset monitors could be deployed in a routine monitoring network application in lieu of instruments that make filter measurements. Agencies encountered significant operational issues with the Sunset monitors, substantially reducing the number of valid data points for the multi-year deployment. After screening for these issues, and excluding results that were suspect, we found that Sunset OC generally compared well with the CSN OC (r2=0.73 across five sites); the Sunset/CSN OC ratio was on average 1.06, with a range among sites of 0.96 to 1.12. Sunset thermal EC and CSN thermal EC did not compare as well, with an overall r2 of 0.22, possibly because 26% of the hourly measurements were below the Sunset EC detection limit. Sunset Optical EC had a much better correlation to CSN thermal EC (r2=0.67 across all sites), with an average Sunset/CSN EC ratio of 0.90 (range of 0.7 to 1.08). There was also a high correlation of Sunset Optical EC with Aethalometer BC (r2=0.77 across all sites), though with a larger bias (average Sunset/Aethalometer ratio of 0.56). There was no systematic difference among the measurements by site location, i.e., Sunset Optical EC was consistently lower than Aethalometer BC at all three sites, with no significant seasonal variation. Collocated Sunset instruments at St. Louis had high precision, with r2 values of 0.93 for OC and 0.91 for Optical EC. Somewhat surprisingly, the diurnal pattern of OC was fairly invariant, while a morning peak was observed for EC at all sites. Overall, operational issues with the Sunset monitors were persistent at all sites, but when the instruments were operating well, collected data were comparable to results from the CSN and the collocated Aethalometers.