A regional model study of the ozone budget in the eastern United States

In order to better understand the photochemical and meteorological processes controlling regional scale air quality problems such as ozone formation, we have developed a three‐dimensional Eulerian model and applied this model to a high‐pressure period (July 4 to July 7, 1986) over the eastern United States. Meteorological and physical variables from a three‐dimensional primitive‐equation model are used to drive the transport parameters over a grid with 60×60 km 2 horizontal resolution, and 15 unequally spaced vertical layers extending from the ground to roughly 15 km. The treatment and incorporation of the dynamic model, the transport model, surface deposition, emission of anthropogenic and natural O 3 precursors, the chemical mechanism for 35 individual species, solar radiation and the numerical methods are discussed in detail. Model performance is tested by comparing model predicted O 3 concentrations with observations from the U.S. Environmental Protection Agency ozone‐monitoring network. Although a significant correlation between model and observed O 3 is found, systematic discrepancies also exist and are discussed in relation to the basic model formulation, and variability in the observed O 3 . Additionally, a comparison of time‐averaged NO x and anthropogenic nonmethane hydrocarbon (NMHC) concentrations to relatively long term observations provides a qualitative assessment of the model's ability to simulate certain aspects of these O 3 precursors from the few available observations. The model is used as a diagnostic tool to analyze various aspects of regional scale O 3 formation and the budgets of the primary O 3 precursors. Ozone formation over much of the continental model domain is shown to be NO x . limited. On the other hand, for midday NO x levels greater than about 4 or 5 ppbv, O 3 formation is generally suppressed because of the low NMHC to NO x ratios (1 to 7) that are characteristic of the emissions inventory. Regional‐scale budget analyses show that very Hide NO x or NMHC is transported to the free troposphere for the high‐pressure conditions of this study and in the absence of a significant subgrid‐scale vertical mixing process (i.e., efficient cumulus transport). We calculate a net turnover time of about 1.5 days for continental O 3 below 1800 m with in situ photochemical formation being balanced by photochemical loss and transport off the American continent. The results of this work are intended to serve as a baseline for further model development.