Weekend/Weekday Ozone Observations in the South Coast Air Basin

into a category named "UNID." The source profile data reported in units of ppbC or ppbv are
converted to µg/m 3 prior to calculating the weight percentages using species-specific conversion
factors. One-sigma uncertainties were derived from variations among multiple measurements for
a particular source type or a nominal analytical uncertainty of 15 percent. The assigned
uncertainties are the larger of the two values.
The source composition profiles used in the CMB analysis are shown in Table 4.4-1.
They include gasoline and diesel exhaust, gasoline liquid and vapor, commercial natural gas and
liquefied petroleum gas, surface coatings, consumer products ,and isoprene.
Gasoline Exhaust. A profile for gasoline exhaust was derived for this study from the four
samples collected along the Harbor Freeway (HF1). Each HF1 sample was corrected for
surrounding background VOC concentrations by subtracting the Dodger Stadium sample (DS1),
which preceded each HF1 sample. The composition profile included CO and NOx in proportion
to the sum of PAMS species. The two weekday samples generally reflect vehicle operation in
stop-and-go traffic and the two weekend samples reflect free-flow conditions.
Diesel Exhaust. A profile for diesel exhaust was derived for this study from the three
samples collected at a truck stop near the intersection of I-10 and I-15. These samples were
collected between 2:00 and 5:00 a.m. on Tuesday, October 3. Each truck stop sample was
corrected for background VOC concentrations by subtracting the average of the 2:00-2:45 a.m.
samples that were collected at Industry Hills (IH1) on Monday, 10/2/00 and Wednesday, October
4. The background correction was adjusted in order to yield zero abundance of MTBE in the
background-corrected diesel profile.
Gasoline Liquid and Vapor. Gasoline samples that were collected and analyzed for this
study consist of the two grades (regular and premium) for five brands (ARCO, Union 76, Shell,
Chevron, and Mobil). In addition to profiles for individual samples, composites were derived for
each grade of gasoline from a combination of the five brands. An overall composite liquid
gasoline profile was constructed based on this relative weighting of 68% regular and 32%
premium (Kirchstetter et al., 1999).
The compositions of gasoline headspace vapors were predicted from the measured
composition of liquid gasoline using the method described by Kirchstetter et al. (1999). This
method is based on the proportionality between the equilibrium headspace partial pressure for
each compound identified in gasoline with its mole fraction in liquid gasoline times the vapor
pressure of the pure species. The individual vapor pressures are determined using the Wagner
equation.
Commercial Natural Gas and Liquefied Petroleum Gas. The commercial natural gas
(CNG) and liquefied petroleum gas profiles are based on samples taken in the summer of 1972 at
Los Angeles, CA and in the summer of 1973 at El Monte, CA (Mayrsohn et al., 1976, 1977).
Contribution of aged emissions is a plausible alternative interpretation of these two sources. The
combination of the two profiles accounts for the excess ethane and propane that typically exist in
most urban areas.
Surface Coatings. A large variety of formulations are used in surface coatings, and it is
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