Hydrocarbon source apportionment for the 1996 Paso del Norte Ozone Study

The 1996 Paso del Norte ŽPdN. ozone study was conducted to improve current understanding of the significant meteorological and air quality processes that lead to high concentrations of ozone in El Paso, Texas ŽUSA. and Ciudad Ju´arez, Chihuahua ŽMexico.. Two-hour canister samples were collected five times daily at 05.00 07.00 h, 07.00 09.00 h, 09.00 11.00 h, 11.00 13.00 h, and 15.00 17.00 h MST during intensive study periods at one urban and one rural site on each side of the border. An automated gas chromatograph was operated at one site in central El Paso. Source profiles Žthe fractional chemical composition of emissions. from motor vehicles, gasoline, liquefied petroleum gas, and commercial natural gas were combined with source profiles from other studies for input to the Chemical Mass Balance ŽCMB. receptor model to apportion the measured non-methane hydrocarbons ŽNMHC. to sources. On-road vehicle emissions accounted for one-half to two-thirds of the NMHC in Ciudad Ju´arez and El Paso with the highest contributions occurring during the morning and afternoon commute periods. Emissions from diesel exhaust contributed approximately 2 3% of NMHC in Ciudad Ju´arez and less than 2% in El Paso. The average sum of liquid gasoline and gasoline vapor increased during the day in Ciudad Ju´arez from 2% at 06.00 h to approximately 12% at 16.00 h. Diurnal and day-of-the-week patterns in the liquid gasoline contributions are essentially identical to the corresponding patterns for motor vehicle exhaust, which suggest that a large fraction of the liquid gasoline contribution may be associated with tailpipe emissions rather than evaporative emissions from motor vehicles or industrial sources. Including the sum of the two sources put the upper limit for tailpipe contributions at 60 70% of NMHC.