Evolution of Atmospheric Particles along Trajectories Crossing the Los Angeles Basin

Trajectory analysis shows that the air masses arriving at Riverside, CA, on the afternoons of September 24 and 25, 1996, previously passed near air monitoring sites at Santa Catalina Island, Long Beach, and Fullerton, CA, in succession. At those sites, electrical aerosol analyzers and optical particle counters acquired continuous particle size distribution data, inertial impactor and bulk filter samples were taken with 4-h time resolution for determination of particle size and chemical composition during intensive sampling periods once per day at each site, and aerosol time-of-flight mass spectrometers acquired continuous data on particle size and composition at the single-particle level. These data permit particle evolution to be studied within single air masses as they sequentially pass several monitoring sites over a 2-day period. Air parcels associated with both of the trajectories studied show mineral dust, organic carbon, particulate nitrate and ammonium, and total suspended particulate matter concentrations that increase as transport occurs across the air basin. Large increases in particulate ammonium and nitrate concentrations occur between Fullerton and Riverside due to overnight air stagnation in an area with high gaseous ammonia emissions. The aerosol time-of-flight mass spectrometers show how the externally mixed population of individual particles is modified chemically during transport from Long Beach to Riverside, CA. The coastal aerosol at Long Beach containing sea-salt particles and primary carbon particles is changed substantially as these particles individually accumulate secondary ammonium nitrate and organics during travel across the air basin.