AN OBSERVATION-BASED MODEL FOR ANALYZING OZONE PRECURSOR RELATIONSHIPS IN THE URBAN ATMOSPHERE

An Observation-Based Model (OEM) is described, which uses in-situ atmo spheric observations to determine the sensitivity of ozone concentrati ons in an urban atmosphere to changes in the emissions of ozone precur sors (i.e., volatile organic compounds and nitrogen oxides). The model is formulated following the concept of Relative Incremental Reactivit y (RIR) developed by Carter and Atkinson. In the OEM, however, observe d concentrations rather than emission inventories are used to drive th e photochemical simulations and thereby ensure that the calculations a re carried out for the proper mix of nitrogen oxides and volatile orga nic compounds. From these calculations, a series of sensitivity factor s, or RIRs, are inferred that can be used to (1) determine whether red ucing emissions of nitrogen oxide or emissions of hydrocarbons would b e most effective in abating ozone in a given urban area, and (2) ident ify the most critical subset of hydrocarbons present in an urban atmos phere causing ozone exceedances. Because the OEM is relatively easy an d inexpensive to operate and makes use of data that are increasingly a vailable, it can be used to analyze a wide array of ozone episodes and , thus, could prove to be a relatively cost-effective tool for the ana lysis of ozone precursor relationships in an urban atmosphere. On the other hand, because the OEM is diagnostic rather than prognostic, it c annot be used in a predictive mode to estimate exactly how much emissi on reduction is needed to reduce ozone concentrations. For this reason , the OEM should be viewed as a complement to, rather than a substitut e for, more sophisticated gridded, emission-based models. To illustrat e the characteristics of the OEM and to demonstrate its applicability, we first compare the results of the OEM to those obtained from a seri es of simulations of the Atlanta metropolitan area using the Urban Air shed Model (UAM), a three-dimensional Eulerian grid model. The OEM is then used to analyze a dataset obtained from the 1990 Atlanta Ozone St udy, an EPA field sampling program conducted during the summer of 1990 . Because of limitations and potential flaws in the 1990 Atlanta datas et, the results of this OEM analysis are largely illustrative rather t han definitive. Nevertheless, a few important issues are elucidated by the analysis. These include (1) the importance of accounting for biog enic hydrocarbons produced from urban vegetation; (2) the potential fl aw in using early-morning VOC-to-NOx ratios to infer whether ozone pro duction is Limited by VOC or NOx; (3) the critical need for high-sensi tivity nitrogen oxide measurements to quantify the sub-ppbv concentrat ions of NO during the afternoon hours; and (4) the need to consider a number of individual ozone episodes in studying an urban atmosphere be cause of the possibility that the degree of VOC- and NOx-limitation ma y vary from one episode to another.