Size Distributions of Organic Functional Groups in Ambient Aerosol Collected in Houston, Texas

The size distributions of aliphatic carbon, carbonyl, and organonitrate functional groups in ambient aerosol were determined at three sites during an air quality field study conducted during August and September 2000, in Houston, Texas. Samples were collected using a Hering low-pressure impactor and were analyzed, in transmission mode, using Fourier transform infrared spectroscopy. The submicron fractions of the samples exhibited patterns seen in other urban areas. Carbonyl and organonitrate group absorbances showed maxima in the 0.05–0.26 μ m and the 0.5–1.0 μ m size fractions, and based on these absorbances appear to be primarily due to secondary organic aerosol. Aliphatic carbon absorbances in the submicron aerosol showed maxima in the 0.076–0.12 μ m size fraction and the 0.5–1.0 μ m size fraction. Weak correlation between ozone concentrations and aliphatic carbon absorbances suggest that some of these aliphatic functional groups are due to primary emissions. For the supermicron aerosol, all three functional groups exhibit maxima in the 2–4 μ m size fraction. These coarse size fractions observed in Houston were not observed in similar samples collected in Los Angeles. In addition, some of the aerosol samples collected in Houston display organic functional group concentrations, especially for organonitrates, that are significantly higher than the median concentrations. Samples with very high organic functional group concentrations had molar ratios of carbonyl and organonitrate functional groups that are suggestive of either primary emissions or secondary organic aerosol formation processes that are not commonly observed in other urban areas.