Spatial and temporal variation of chemical composition and mass closure of ambient coarse particulate matter (PM10–2.5) in the Los Angeles area

Abstract To study the seasonal and spatial characteristics, as well as chemical composition of coarse (2.5–10 μm) particulate matter (PM), 10 sampling sites were set up in different areas (urban, semi-rural, desert, near-freeway, community-based, etc) of the Los Angeles Basin. Ambient coarse PM was collected over a year long in 24-hour periods once per week during weekdays, and was analyzed for elemental carbon and organic carbon (EC–OC), water soluble inorganic ions, and total metals and elements. Five categories were used to reconstruct PM mass: 1) crustal materials and other trace elements 2) organic matter 3) elemental carbon 4) sea salt and 5) secondary ions. Overall, crustal materials and other trace elements were the most abundant category, accounting for an average of 47.5 ± 12% of the total reconstructed mass. Secondary ions (sulfate, nitrate and ammonium) and organic matter also contributed significantly at mass fractions of around 22.6% and 19.7%, respectively. Elemental carbon was a less significant component, accounting for less than 2% of total mass across sites. Sea salt particles were more prevalent in spring and summer (12.7%) due to the strong prevailing onshore southwesterly wind in that period. Mass fractions of organic matter, as well as crustal materials and other trace elements were higher in fall and winter, indicating that their contributions were not affected by the lower wind speed and change in wind direction during that period. PM concentrations of sea salt particles decreased from coast to inland along the trajectory of LA Basin, while crustal materials and other trace elements became dominant in inland sites. On the other hand, organic carbon was found to be well-correlated with tracers of soil dust ( R  = 0.74 and 0.72 for Ti and Fe respectively), suggesting that humic substances might be the major constituent of organic matter in coarse mode in the Los Angeles Basin.