Weekend/Weekday Ozone Observations in the South Coast Air Basin

show that hydroxyl radical mixing ratios increase with decreasing NO x mixing ratios. Daytime
nitric acid production has decreased between 1987 and 2000 due to the decrease in VOC mixing
ratios. Now the production of HNO 3 would be expected to be lower by about 9 ppb between
weekend and weekday. PAN, HCHO, aldehyde and organic nitrates have been reduced due to
reductions in VOC mixing ratios but their mixing ratios are not strongly affected by the NO x
mixing ratios at current concentrations so a weekend effect is not expected for these species.
Our simulations for EKMA are consistent with a study by Milford et al. (1994) as quoted
by Finlayson-Pitts and Pitts (1999). Milford et al. found that for conditions that are similar to
present-day conditions in the SoCAB on Sundays (i.e., VOC near 300 ppbC and NO x at 20 to 30
ppb) the rate of ozone production increases with decreasing NO x concentrations. The study
showed that an ozone disbenefit results if present-day Sunday NO x concentrations are reduced.
The Milford et al. (1994) study also shows that ozone production rates increase during the day
because NO x is consumed faster than VOC, which increases the VOC to NO x ratio. Given the
present-day concentrations of NO x , increases in its emissions during the day will decrease, not
increase, ozone production rates. However, based on the Milford, et al. study the NO x timing
hypotheses will become an important mechanism at the same point when reductions in NO x
emissions lead to lower ozone production rates. Modeling studies should be made with updated
emissions inventories and models to verify the our modeling results and those of Milford, et al.
(1994).
1.5 Recommendations
The varying emissions that occur between weekday and weekend periods provide a
natural test of the ability of air quality simulation models to simulate accurately weekdayweekend
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. The
Coordinating Research Council has sponsored a study by ENVIRON to conduct proximate
modeling of weekday/weekend ozone episodes in the SoCAB using input data collected during
the SCOS97-NARSTO field study in order to determine whether proposed hypotheses explain
the weekend ozone effect. Similar modeling is planned by the ARB after developing a weekend
emissions inventory for SoCAB. These modeling results should be compared against the
measured temporal and spatial variations of ozone and ozone precursors. These operational
evaluations of modeling results should be accompanied by an evaluation of the accuracy of the
temporal and spatial patterns of precursor emissions on weekdays and weekends. In addition to
probing the underlying physical and chemical processes that drive the weekend effect, a series of
modeling runs should be made using plausible alternative future emissions inventories for 5, 10
and 15 years into the future. These runs should be used to assess the direction and sensitivity of
projected concentrations of ozone, PAN, nitric acid, particulate nitrate, and formaldehyde to
future VOC and NOx controls. These assessments will be particularly important for downwind
areas where emissions transported from the SoCAB could transition from VOC limited to NOxlimited
and for assessing strategies that might achieve the 8-hour ozone standard in downwind
areas but increase peak ozone concentrations in the central basin. Species such NOy, PAN,
HCHO, and speciated hydrocarbons should be measured in these downwind area to provide
corroboration of modeling results.
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