Gas-Particle Distribution Factors for Organic and Other Pollutants in the Los Angeles Atmosphere

Abstract The distribution of organic pollutants between the gas and particu-late phases was measured for 6 days Including one with the highest ozone level observed in Pasadena In 1973 (7/25). Gas phase pollutants were monitored continuously while particulates were sampled over a one hour interval by filtration. The filters were extracted using a polar and a non-polar solvent; particulate organic carbon was determined using a carbon analyzer, and chemical analysis carried out by fractionation, gas chromatography, infrared, and CHON analysis. The organic carbon fraction (OCF) was always large, up to 43% of the total particulates (TP). Most of the organics were oxygenated compounds of photochemical origin. There was a linear relation between O3, OCF, and the infrared carbonyl band intensities of the extracts. Hourly variations of OCF and TP are discussed with respect to the gas phase pollutants and conversion processes. Although secondary pollutant concentrations were found in the order: organics > nitrates > sulfates, nitrates were more efficient than organics in visibility degradation, based on a statistical evaluation of the data. The secondary aerosol contribution varied with O3, accounting for up to 95% of TP when the O3 peaked. The unusual nitrate peak observed for 7/25 is discussed with the atmospheric chemistry of other nitrogen compounds. We define an organic carbon distribution factor: fc = particulate organic carbon (POC, μg/m3C)/POC + gas phase reactive hydrocarbons (μg/m3C). The gas-particle distribution factors for organics (fc), nitrates (fN) and sulfates (fs) are in inverse order of their aerosol concentrations: fc < fN < fs . The measured fc are low: average 2–3%, highest value 6% for 7/25, and correspond to a conversion rate ≤2% hour−1. Ambient fc are much lower than fc measured for certain specific hydrocarbon precursors.