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Project Status: complete

Title: Characterization of PM2.5 Episodes in the San Joaquin Valley Based on Data Collected During the NASA DISCOVER-AQ Study in the Winter of 2013

Principal Investigator / Author(s): Cappa, Christopher

Contractor: UC Davis

Contract Number: 14-307

Research Program Area: Atmospheric Processes

Topic Areas: Area Sources, Field Studies, Toxic Air Contaminants


Air quality in the SJV continues to be the worst in the state with wintertime PM2.5 air pollution often exceeding the U.S. EPA 24-hour standard. In addition, models used in air quality planning for this region are inconsistent in their ability to predict high PM2.5 concentrations. The current project investigated this gap in understanding the processes for PM2.5 formation in the SJV through advanced analyses of a unique data set: aircraft and surface measurements collected during NASA's campaign Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ). This program created an extensive set of vertically resolved aircraft measurements of PM2.5 and critical gaseous species for the SJV for January/February 2013. These were complemented by measurements at a network of ground sites, which included a "supersite" at CARB's Fresno-Garland monitoring station that provided detailed high time resolution, measurements of PM2.5 composition and optical properties. Results from these analyses provided a case study of how vertical coupling between chemistry and meteorology can lead to build up episodes of high particulate matter. In particular, the relative contributions of daytime photochemical and nocturnal heterogeneous hydrolysis production of nitrate to total PM mass were resolved. Analysis of detailed chemical composition measurements from the supersite provided estimates of contributions from primary sources and secondary formation, both organic and inorganic, to PM in the SJV. This work showed that both organic aerosol (directly emitted and secondarily produced) and ammonium nitrate, which are out of phase with respect to day and night, are essential in keeping PM2.5 concentrations elevated during pollution events during the winter in the SJV. The results indicate that further controls in emissions of primary PM (vehicle, cooking and wood burning), gas phase secondary organic precursors, and NOx are needed for long-term PM mitigation; and that NOx reductions are likely to have a direct, and significant, impact on nocturnal ammonium nitrate production aloft. This deeper understanding of the process that can lead to high PM2.5 episodes in the SJV will be of immediate value for improving air quality models for this region and in developing effective air quality attainment strategies.


For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 322-3893

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