2 Analysis of Variations in Ozone & Ozone Precursors 2 . 1 Diurnal Patterns of Ozone and Precursors in the South Coast Air Basin

This chapter is a summary of the diurnal (hour-by-hour) variations observed in concentrations of ozone, carbon monoxide, and oxides of nitrogen, as well as in NO 2 /NO ratios, in the South Coast Air Basin by day of the week during the summers of 1994 and 1998. The diurnal variations are examined for consistency with each Weekend Effect Hypothesis. The plausibility of a Weekend Effect Hypothesis is improved if it explains (or is consistent with) the observed temporal and spatial variations in the parameters. The diurnal variations of secondary pollutants appear to have changed with the introduction of cleaner burning fuels in 1995. The diurnal patterns observed, within spatial groupings, are consistent with the NO X-Reduction Hypothesis and the NO X-Timing Hypothesis. The diurnal profiles observed tend to oppose the Carryover at Ground Level Hypothesis and the Increased Emissions Hypothesis. The evidence is largely moot concerning the other hypotheses. 2.1.2 Background We examined diurnal patterns of ozone and other pollutants in the South Coast Air Basin (SoCAB) during 1994 and 1998 for weekday-weekend differences. We generated average diurnal profiles of pollutant concentrations by day-of-week groups for each site in the South Coast Air Basin reporting data for the May-October periods in 1994 and 1998. We examined pollutant concentration data for ozone, oxides of nitrogen (NO X), non-methane hydrocarbons (NMHC), and carbon monoxide (CO). Data from 29 ozone monitors, 22 NO X monitors, and 20 CO monitors in the South Coast Air Basin (SoCAB) were available for analysis and the diurnal variations are described below. However, NMHC data were only available from a few sites and for 1994 only. Because the coarse resolution of the NMHC data tended to obscure day-of-week differences, the NMHC results are not presented. However, a diurnal profile of the total hydrocarbon concentrations at Los Angeles-N. Main in 1994 will be presented to provide a general characterization of this ozone precursor. This chapter is a summary of the analytical results for the summers of 1994 and 1998. We selected these years for analysis based on a recent analysis (Austin and Tran, 1999) by staff of the Air Resources Board (ARB). That analysis found an ozone weekend effect at many sites throughout California, with Saturday typically having the highest concentrations in 1992-1994, and Sunday typically having the highest concentrations in 1996-98. We selected the South Coast Air Basin for more