This paper updates the historic trends (1980-2010) in ambient ozone and ozone precursor concentrations in the South Coast Air Basin (SoCAB) and examines the evolution of the ozone-precursor relationship in the Basin. Whereas reductions in NOx (oxide of nitrogen) emissions have decreased nitrate and PM2.5 (particulate matter with an aerodynamic diameter < or = 2.5 microm) concentrations in the Basin during the past decade, ozone levels have increased at the central basin locations since about 2005 following a reversal in the decline of volatile organic compound (VOC)/NOx ratios during the previous two decades. A chemical box model was used to simulate the effects of changes in precursor concentrations on ozone formation using day-of-week-specific initial precursor concentrations that were derived from measurements and'projected to 2020 based on expected emission reductions from 2005 (-10% VOC and -50% NOx). Results show that peak ozone formation rates in 2020 will increase on weekdays by a factor of 3 relative to 2005 and will be comparable to 1995 weekday and 2005 Sunday rates. Ozone production will become precursor limited on Sundays in 2020, but with higher initial rates than 2005. Although a greater NOx reduction scenario in 2020 of -75% will result in even higher initial ozone formation rates, precursor limitation is reached quickly, leading to a further shift westward in the location of peak ozone levels. However ozone levels will likely be lower in downwind areas where transport is more important than local production of ozone. The ambient versus emission inventory reconciliation indicates a factor of 2 underestimation of VOC emissions in 2009 relative to NOx. Other analyses suggest that there is an overall increase in VOC emissions on hot days that is not fully accounted for by emission inventory estimates. Air quality models using emission inventories that underestimate VOC emissions relative to NOx may lead to inaccurate forecasting of the consequence of emission reductions.
Implications: The rate and efficiency of ozone formation and accumulation in the SoCAB is more rapid than would be indicated by air quality model simulations based on the current inventory. Projected reductions in NOx emissions without concurrent reductions in VOC emissions will likely cause ozone to increase during the next decade within central regions of the SoCAB compared with a flat or slightly declining trend in far downwind locations. Air quality statistics that are commonly used to track progress toward attainment, such as basin-wide ozone design value and standard exceedances mask these varying trends within the Basin.