Comparison of simulated ozone generated with growth-and-control vs. uniformly-reduced emission inventories in California

Photochemical models have been commonly applied in California to demonstrate the attainment of the federal 1-hour ozone standard and applications are planned to investigate the attainment of the 8-hour ozone standard. One method used for modeling-based attainment demonstrations is to simulate the target attainment year using an emissions inventory specific to that year. If the model does not show attainment with the attainment-year inventory, then emissions may be further uniformly reduced across the modeling domain until attainment is demonstrated. An alternative to uniform emission reductions is the development of future-year (FY) inventories, using years beyond the target attainment date. Such future-year emissions estimates can be developed based upon growth and control factors, which may differ from uniform emission reductions to the attainment-year inventory in their spatial and temporal distributions and in their chemical speciation. However, this alternative approach is not generally followed because creating FY inventories for multiple years is costly and time consuming and because it is not known a priori what year's inventory will bring ozone into attainment.

This study examines the utility of uniform emission reductions for representing the efficacy of future emission reductions for a high ozone period captured during the summer 2000 Central California Ozone Study (CCOS). The CCOS period offers a unique opportunity to conduct this comparison because modeling emissions inventories were developed for the base-year (BY) 2000 and various inventories were produced as target attainment years in different regions of the modeling domain, including years 2007 for the Bay Area, 2012 for the Sacramento and San Joaquin Valley areas, and 2018 as an alternative for the San Joaquin Valley or Sacramento. These FY inventories included effects such as projected population increases and implementation of additional specific pollution control measures. Variations among FY inventories included spatial and temporal differences as well as differences in speciation due to different levels of controls applied to the various source types.

For each FY inventory, the average percent reduction of NOx and VOC was calculated with respect to the 2000 BY. We found that, if the same percent reductions were applied across-domain to the BY inventory, ozone concentrations at 53 key sites did not differ substantially from those obtained with the FY inventories. As such, uniform emission reductions across the domain represent an effective tool to obtain first estimates of ozone levels when an appropriate FY emission inventory is not available. However, across-domain reductions often appear to be more effective at reducing maximum 8-hour ozone concentrations than FY inventories, and consequently do not offer the desirable property of being conservative.