Weekend/Weekday Ozone Observations in the South Coast Air Basin

interfaced to an Entech model 7100 automated preconcentrator, a Varian Saturn 2000 ion trap
mass spectrometer, and a windows-based PC to manage the analytical and data acquisition
operations. The target species included C 2 to C 12 hydrocarbons, methanol, ethanol, and other
alcohols. Also included are C 2 and larger carbonyls including acetone, methyl ethyl ketone
(butanone), benzaldehyde and others. MTBE and other oxygenates used in fuels are included as
is isobutene, an important breakdown product of MTBE. Oxygenated compounds in surface
coatings such as benzaldehyde, benzoic acid, and butylacetate are also included, as are the
biogenic species isoprene, alpha and beta pinene, and limonene. Commonly detected halocarbons
are also included. The detection limits for these species are about 0.2 ppbv for all targeted
species.
Stainless steel SUMMA-polished canisters of 3-L capacity were used for volatile
hydrocarbon (C 2 -C 12 ) collection. These canister samples are suitable for analysis of speciated
hydrocarbons by EPA Method TO-14, as well as for analyses of CO, CO 2 , methane, and
oxygenated species. Prior to sampling, the canisters were cleaned by repeated evacuation and
pressurization with humidified zero air, and certified as described below. The sampling
procedure essentially follows the pressurized sampling method described by EPA Methods TO-
12 and TO-14 and the EPA document "Technical Assistance Document for Sampling and
Analysis of Ozone Precursors" (October 1991, EPA 600/8-91-215). Gas chromatography with
flame ionization detector is the established technique for monitoring volatile hydrocarbons in
ambient air. The DRI analytical procedure for analysis of C 2 -C 12 hydrocarbons is consistent with
the EPA document "Technical Assistance Document for Sampling and Analysis of Ozone
Precursors" (October 1991, EPA 600/8-91-215). The GC/FID response is calibrated in ppbC,
using primary calibration standards traceable to the National Institute of Standards and
Technology (NIST) Standard Reference Materials (SRM) C 8 to C 20 Hydrocarbons by GC-FID
Analysis of Tenax Cartridges.
Heavy hydrocarbons, defined as hydrocarbons in the range of C 8 to C 20 , were collected
using Tenax-TA (Alltech) solid adsorbent. The Tenax samples were analyzed by thermal
desorption-cryogenic pre-concentration, using the Chrompack Thermal Desorption – Cold Trap
Injection unit (Chrompack International BV), followed by high resolution gas chromatography
and Fourier transform infrared detection (IRD) – mass spectrometry detection (MSD) (Hewlett
Packard 5890II GC, 5965 IRD and 5970 MSD)
4.3 Spatial and Temporal Variations in Pollutant Concentrations and VOC/NOx Ratios
This section examines the spatial and temporal variations in NO, NOy, black carbon, CO,
MTBE, VOC and semi-volatile hydrocarbons. We recognized that meteorological conditions
could be a major factor in the day-to-day variations in pollutant concentrations. Our collaborators
in this study at Sonoma Technology, Inc. (STI) examined the meteorological conditions on each
day from September 29, 2000 to October 9, 2000. STI’s analysis showed the presence of a
trough over Southern California on a majority of the days during the study. STI examined eleven
meteorological parameters and applied a subjective rating of favorable, not favorable, or neutral
with respect to the probable influence of each parameter on ambient concentrations of primary
pollutants. The period from Wednesday, October 4 to Saturday, October 7 was not conducive to
high pollutant concentrations. The two Sundays (October 1 and 8) and both Mondays (October 2
and 9) were overall neutral for pollutant buildup. Each weekend day during the study was found
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