Weekend/Weekday Ozone Observations in the South Coast Air Basin

the freeway, and the daily minimum NOx concentrations are measured during this period.
During summer nights, the winds are calm and typically offshore causing the highest NOx
concentrations to occur overnight.
Like CO, VOC shows no discernible differences between weekdays and weekends that
might be ascribed to changes in emission patterns, and day-to-day variations track the trend in
meteorological conditions. Figures 4.3-7 and 4.3-8 show the diurnal and day-to-day variations in
CO, nonmethane hydrocarbons, and nonmethane organic gases at Azusa and Pico Rivera,
respectively. The effect of Pico Rivera’s microenvironment is also evident on the diurnal
variations of CO and VOC. Accordingly, the VOC/NOx ratios are more variable at Pico Rivera
than at the Azusa monitoring station. Figure 4.3-9 shows that the VOC/NOx ratios at Azusa are
consistently about five during the weekdays with little diurnal variation. Ratios were greater
during the weekends, ranging between 5 and 10. In contrast, the ratios at Pico Rivera are
considerably more variable with ratios around five during the night and ratios in the range of 15
to 20 during the afternoon. The large afternoon ratios are due to low NOx concentrations
resulting from the impact of a well-mixed, aged air mass during this time of day.
4.4 Source Apportionment of VOC
The Chemical Mass Balance (CMB) receptor model consists of a least-squares solution to
a set of linear equations, which expresses each receptor concentration of a chemical species as a
linear sum of products of source profile species and source contributions. The source profile
species (the fractional amount of the species in the VOC emissions from each source type) and
the receptor concentrations, each with uncertainty estimates, serve as input data to the CMB
model. The output consists of the contributions for each source type to the total ambient VOC as
well as to individual VOC species concentrations. Gasoline and diesel exhaust profiles were
derived from samples collected during the field study along the Harbor Freeway (HF1) and at the
truck stop near I-10 and I-15. Profiles for fuels were developed from analysis of gasoline
samples collected during the field study. In addition to the relative contributions of gasoline and
diesel exhaust, the detailed speciation of VOC from the mobile sampling and the time-resolved
VOC speciation at Los Angeles, Azusa, and Pico Rivera monitoring stations provide source
attribution of other sources of VOC by time of day and day of the week. These analyses address
questions regarding the source contributions of VOC carried over from the previous evening and
the relative importance of sources other than gasoline and diesel combustion sources in the
diurnal variations in VOC/NOx ratios.
4.4.1 Source Composition Profiles
The profiles are expressed as weight percentages and are normalized to the sum of the 55
Photochemical Assessment Monitoring Station (PAMS) target NMHCs. The PAMS species
typically account for 70 to 80 percent of the total ambient hydrocarbons at most urban locations.
The PAMS hydrocarbon data are ideally suited for CMB analysis because of the level of
hydrocarbon speciation, consistency among networks in measurement methods and quality
assurance, and the available spatial and temporal resolution of the data. Compounds other than
the 55 PAMS species that are identified by the DRI laboratory are retained in the database
individually and as a subtotal named “OTHER.” Compounds reported as "unknown" are grouped
4-8