Weekend/Weekday Ozone Observations in the South Coast Air Basin

dioxide (NO 2 ) 4 , and carbon monoxide (CO), which serves as an estimate of nonmethane
hydrocarbons (NMHC). Non-methane hydrocarbons (NMHC) were estimated from CO using an
empirical relationship between NMHC and CO from canister samples collected by Desert
Research Institute at three sites in the SoCAB during the summers of 1995 and 1996 (Zielinska
et al. 1999) 5 . The retrospective analysis of ambient data focused on day-of-the-week differences
in the overnight carryover of ozone precursors, the extent of inhibition of ozone formation during
the morning due to titration with NO, and the rate of ozone accumulation from the end of the
inhibition period to time of peak ozone. For the purposes of this analysis, the time at which [NO]
= [O 3 ] was used to mark the end of the ozone inhibition period. Changes in the rate of ozone
accumulation were correlated to historic trends in CO/NOx ratios, which served as partial
surrogates for trends in VOC/NOx ratios.
Current observations of the weekend effect were characterized by examining the summer
1999 and 2000 ambient data from the South Coast Air Quality Management District (SCAQMD)
Photochemical Assessment Monitoring Stations (PAMS) in Azusa, Pico Rivera, and Upland and
from the California Air Resources Board ozone precursor trends site in downtown Los Angeles
(North Main). The analyses focused on relating weekday differences in the diurnal variations of
CO, NMHC, NO, and NO 2 , and NOx with variations in ozone, VOC/NOx ratios, and the ratios
of O 3 to potential ozone (i.e., O 3 plus NOx). Weekday variations of ozone were also related to
VOC reactivity, photochemical aging, and estimated photolysis rate parameter for NO 2 . Current
weekday and weekend observations of the VOC and NOx mixing ratios are superimposed on an
ozone isopleth plot along with similar observations from 1987.
The experimental phase of this study focused on time-resolved measurements of VOC,
NOx, CO, and particulate black carbon to examine relationships between emissions sources and
the diurnal and day-of-the-week variations of ozone precursors and VOC/NOx ratios. The field
study was conducted in the Los Angeles area over a period of 9 days from September 30, 2000 to
October 8, 2000. The field measurements involved two approaches: supplemental measurements
at existing SCAQMD monitoring sites and mobile sampling along freeway and surface street
loops and at regional/background sites. Supplemental measurements made by DRI at the
monitoring stations included hourly C 2 to C 11 volatile organic compounds by automated gas
chromatography with mass spectrometry (Varian 3800 GC interfaced to an Entech model 7100
automated preconcentrator and a Varian Saturn 2000 ion trap mass spectrometer) at Azusa,
continuous black carbon by light absorption with an Anderson RTAA-1000 aethalometer
(approximately 5-minute averages) at Azusa and Pico Rivera, and 3-hour composite Tenax
samples for C 8 to C 18 hydrocarbons beginning at 2, 6, and 9 a.m. PDT on September 30, October
4 NO 2 is determined by the difference of NOx and NO measured by chemiluminescence analyzers. In addition to
NO 2 , analyzers that are commonly used at air quality monitoring stations also convert other reactive nitrogen oxide
species such as PAN to NO, thereby causing interference. NO 2 reported by these instruments must be considered
upper limits. The magnitude of the interference is relatively small in the urban center where NO sources are large,
but can be substantial in downwind and rural areas.
5 Because data were not available prior to the early 1990s, NMHC is estimated from CO using an empirical
relationship between NMHC and CO for data collected in 1995 and 1996. While these estimates are reasonably
valid for determining day-of-the-week variations in NMHC concentrations and NMHC/NOx ratios for any year
within the 18-year period of interest, they are less reliable for estimating long-term trends in NMHC and
NMHC/NOx ratios because the slope of the regression between CO and NMHC may have changed over time with
changing emission control technology.
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