Seasonal Trends in Los Angeles Ambient Organic Aerosol Observed by High-Resolution Gas Chromatography

Abstract

Ambient samples of fine organic aerosol collected from four locations in the Los Angeles Basin over a 1-year period have been characterized quantitatively via high-resolution gas chromatography. Bulk chemical characteristics, such as the mass of acidic versus neutral organic material, have been determined for these samples. The seasonal variations of these bulk characteristics are examined over a year-long time period. Due to weather patterns typical of the Los Angeles area, the ambient concentration levels in the western Los Angeles Basin exhibit a strong peak during the fall/winter period that is driven by changes in the concentration of higher molecular weight neutral organic compounds; such compounds are characteristic of motor vehicle exhaust, tire dust, road dust, and vegetative detritus. When meteorological effects are excluded, this seasonal variation in the higher molecular weight neutral organic fraction collected from all four sites is removed completely. Correction for the effect of seasonal changes in atmospheric dilution reveals an increase in source emissions and/or atmospheric production of lower molecular weight acidic and neutral organic aerosols between April and July. Secondary organic aerosol formation, which produces polar organic reaction products appearing in the acidic fraction, plus increased emissions from biological activity at that time of year may account for the observed increase in lower molecular weight organic aerosol inputs during the April-July period.