The latest version of EPA's Regional Oxidant Model, ROM2.1 was evaluated to determine and to document its accuracy. In this evaluation, model estimates were compared with ambient measurements of hourly surface ozone collected on 26 days during the summer of 1985 in the northeastern United States. The observed and modeled maximum daytime concentrations agreed on average within 2 ppb (79 ppb versus 77 ppb), but the model tended to underestimate the concentrations at the higher extremes of the frequency distribution, particularly in the southern and western portions of the model domain. Estimated and observed spatial patterns for three day maximum ozone generally showed good agreement. ROM2.1 improved noticeably over ROM2.0 with regard to the orientation of the high-ozone plumes in the Northeast Corridor. A unique aspect of the ambient evaluation was an assessment of the model's ability to estimate boundary conditions for the Urban Airshed Model. Near New York City, estimated and observed boundary conditions agreed to within 4 ppb (57 ppb versus 61 ppb). Model performance degraded, however, during some situations with dynamic mesoscale wind flow conditions. A series of diagnostic tests was also done on ROM2.1 to investigate the accuracy of its numerical solution algorithms. When the model was subjected to extremely steep concentration gradients (steeper than those observed in the ambient atmosphere), the model did not conserve mass during a 48 h simulation, deviating by as much as 18% from the initialized value. However, tests with a mass-corrected version of the full simulation model showed that predicted ozone values deviated only slightly (less than 4%) from the original model.

"April 1990." "EPA/600/3-90/046." Microfiche.