Central California Ozone Study (CCOS) - Desert Research Institute

CCOS Field Operations Plan Version 2: 5/31/00
overall ozone production. The SCICHEM was evaluated favorably as part of the 1995
Nashville/Middle Tennessee Ozone Study. Since the composition of the plumes will be different
for central California, these new modules should be evaluate using the CCOS database.
In principle, well-performing air quality modeling systems have the ability to quantify
local and transported contributions to ozone exceedances in a receptor area. However, many of
the interbasin transport couples in the CCOS study region involve complex flow patterns with
strong terrain influences that are difficult to realistically simulate. The CCOS field campaign
provides routine and supplemental measurements at locations where transport can occur. The
proposed upper air network provides a “flux plane” method by which quantitative estimate is
possible, with suitable assumptions. In combination with modeling, data analyses can improve
the evaluation of modeling results and provide additional quantification of transport
contributions. Methods that should be applied include timing of ozone where morning peaks
indicate fumigation of carry-over aloft, peaks near solar noon indicate possible local
contributions, and delayed peaks indicate transport, with later times corresponding to sites
further downwind along the transport path. Other methods include back trajectory wind analysis
and examination of ratios of hydrocarbons of varying reactivity (i.e., xylenes-to-benzene ratio).
In addition, vertical planes intersecting the profiler sites downwind of and perpendicular to the
transport path can be defined and provide estimates of transport through likely transport
corridors. The analyses use surface and aircraft measurements of pollutant concentrations and
surface, wind profiler, and aircraft meteorological data for volume flux estimations.
An important question for ozone control strategies is whether to focus on NOx or VOC
control. To choose the proper control strategy it is necessary to know if ozone production is
limited in a location more by the NOx or VOC available. Models and measurements have been
applied to answer this question. One of the methods would be to determine ozone isopleths from
measurements for each site in an airshed but usually not enough data are available. Therefore
indicators for VOC or NOx limitation include ratios such as HNO 3 /H 2 O 2 and other ratios have
been applied to both measurements and models. Within CCOS, NOx and VOC limitation will be
investigated through the use of measurements, indicators and modeling analysis.
One of the most important tests of model simulations is the ability to simulate accurately
weekday-weekend differences in precursors and ozone. One of the most important applications
of this test is that it helps to probe the relative sensitivity of ozone concentrations to VOC and
NOx. Since the mid-1970’s it has been documented that ozone levels in California’s South
Coast Air Basin (SoCAB) are higher on weekends than on weekdays, in spite of the fact that
ozone pollutant precursors are lower on weekends than on weekdays (Elkus and Wilson, 1977;
Horie et al., 1979; Levitts and Chock, 1975; Zeldin et al., 1989; Blier and Winer, 1998; and
Austin and Tran, 1999). Similar effects have been observed in San Francisco (Altshuler et al.,
1995) and in the northeastern cities of Washington D.C., Philadelphia, and New York (SAIC,
1997). While a substantial “weekend effect” has been observed in these cities, the effect is less
pronounced in Sacramento (Austin and Tran, 1999), and is often reversed in Atlanta (Walker,
1993) where VOC/NOx ratios are typically higher. Several of the above studies show that the
“weekend effect” is generally less pronounced in downwind locations where ambient VOC/NOx
ratios are higher.
Chapter 1: INTRODUCTION 1-4