Transport and dispersion during wintertime particulate matter episodes in the San Joaquin Valley, California

Particulate matter (PM) concentrations in California's San Joaquin Valley (SJV) often exceed the National Ambient Air Quality Standards (NAAQS) for size fractions under 10 and 2.5 micrometers (PM10 and PM2.5, respectively). Data analysis and modeling were performed to characterize the spatial and temporal variability of wintertime transport and dispersion processes and the effect of these processes on PM in the SJV. The data analysis and modeling results will be used to support future meteorological and photochemical modeling efforts and aid in the development of sound emission control strategies for the SJV. Measurements from unique observing platforms, including radar wind profilers (RWP) and radio acoustic sounding systems (RASS), were collected at 18 sites throughout the SJV. These data were used to estimate hourly mixing heights for a three-month period and to create high-resolution diagnostic wind fields, which were used for trajectory and dispersion analyses.

Data analyses show that PM episodes, compared to non-episode days, were characterized by an upper-level ridge of high pressure, light winds throughout the entire depth of the atmospheric boundary layer, and low mixing heights. Peak daytime mixing heights during episodes were about 400 m agl compared to about 800 m agl during non-episodes. These episode/non-episode differences were observed throughout the SJV. Trajectory analyses and dispersion model simulations revealed that wintertime pollutant transport into the SJV from surrounding areas is not a significant contributor to PM in the SJV. More specifically, little inter-basin pollutant transport occurred among major population centers in the SJV; however, intra-basin transport during the episodes was predominately strongest in the northern SJV and Sacramento regions, with regular pollutant transport out of the SJV into the San Francisco Bay Area on episode days.