Daily variation in chemical characteristics of urban ultrafine aerosols and inference of their sources.

A summer air quality monitoring campaign focusing on daily variation of ultrafine (<180 nm in diameter) particle chemical characteristics was conducted in a typical urban site in Los Angeles during June-July 2006. Ultrafine particles (UFP) were collected weekly for two 3 h periods each day, one to capture the morning commute (06:00-09:00 PDT) (Pacific Daylight Time) and one to investigate photochemically altered particles (13:00-16:00 PDT). Samples were analyzed for ionic compounds, metals, trace elements, elemental carbon, and organic carbon. In addition, measurements of individual organic species and their variation with time of day at the urban site were conducted. The relative abundances of alkanes, PAH, and hopanes in the morning denote a strong influence of commute traffic emissions on ultrafine particle concentrations. By contrast, afternoon concentrations of oxygenated organic acids and sulfate rose, while other species were diluted by increased mixing height or lost due to increasing temperature. These are clear indicators that secondary photochemical reactions are a major formation mechanism of ultrafine aerosols in the afternoon. The concentrations of organic species originating from vehicular emissions measured in this study compare favorably to those from freeway-adjacent measurements by using CO2 concentrations to adjust for dilution, demonstrating the effectiveness of this tool for relating sites affected by vehicular emissions.