By: Dr. DAVID WOJICK PE                                 March 1996 Vol. 1, Number 1

What's An OTAG? (Pronounced Oh'-tag)

The Ozone Transport Assessment Group has been a rather amorphous organization, but it's clearly the biggest thing since the 1990 Amendments. . OTAG began with a letter from the Ozone Transport Commission to EPA stating that the OTC states could not achieve the federal ozone standard because of airborne transport of pollutants from the Midwest and South.

So EPA convened, via The Environmental Council of the States, or ECOS, the air quality commissioners from the 37 eastern most states plus DC -- everybody east of the Mississippi and the first tier of states west of it. They are the heart of OTAG, called the Policy Decision Group. Plus there are various Subgroups and Workgroups, with both formal and ad hoc members, where most of the action is at present. By year's end, OTAG is to recommend to ECOS and EPA control strategies to bring about attainment with the air quality standard for ozone throughout the OTAG region, based on modeling.

OTAG Releases Review Of Recent Ozone Studies,
Report Finds NOx Control Problematic

Review of Recent Ozone Measurement and Modeling Studies in the Eastern United States was prepared by Ralph Morris, Environ Corp., (415) 899-0700. Morris is one of the pioneers of the UAM-V model, OTAG's photochemical model of choice, and is working on a new UAM-X model. Thirty eight studies are presented, with many charts, tables and color plates. This massive report is the prelude to OTAG's own measurement and modeling efforts. Perhaps the most significant conclusion comes at the end of the 'Synthesis of Results', after a discussion of regional versus urban NOx controls:

"Regional NOx controls are generally effective in reducing ozone on a regional scale, however, the effectiveness varies widely by source type and geographic location and such control may produce local adverse effects in urban areas. Urban NOx control may cause an increase in peak ozone concentrations on the local scale, but may be effective in reducing ozone further downwind on the regional scale." (page xxxii)

From this report it seems the problems with NOx control are twofold. First the variability of effectiveness, and second the adverse impact on urban areas, whether the controls are regional or merely local. These problems will be a major focus of the recently announced sensitivity analyses by the OTAG Regional and Urban Scale Modeling Workgroup. However, some experts are skeptical. As one puts it: "I think OTAG is barking up the wrong tree if they think point source NOx controls will significantly reduce regional ozone concentrations." Another modeler, when asked about the adverse effects on urban areas, says simply: "Everybody knows that." However, the policy people seem inclined to ignore the possibility of adverse effects. An early reference to minimizing adverse effects in the OTAG goal statement was dropped. It remains to be seen what the sensitivity analyses reveal.

How Shall We Interpret The OTAG Sensitivity Analyses?

In a sensitivity analysis either NOx or VOC emissions or both are reduced a specified amount and the model is run to see what happens. Experts disagree on the significance of these results. Some argue that since no particular control strategy is being modeled, it is the model not the effect that is being tested. Others argue that since the model has already been accepted as valid, each sensitivity run is a generic 'what-if' scenario. They argue that each run incorporates by implication the set of control strategies that could be used to achieve the reductions. Emission reductions drive controls, they contend, not vice versa. In other words, it's a question of whether the sensitivity analyses are the last step in evaluating the model or the first step in picking a control strategy. Maybe they are either, depending on how they turn out.

Biogenic Emissions: BEIS (rhymes with vice) 1 or 2 ?

Another sticky issue is the choice of a model for natural or 'biogenic' emissions, which can be quite substantial. Early studies use EPA's Biogenic Emissions Information System or BEIS. But research indicates that BEIS significantly underestimates the role of biogenic emissions and a new version, BEIS 2, has been developed. The problem is that UAM-V, the photochemical model of choice by OTAG, as well as EPA's ROM model, do not work well when run with BEIS 2. As one report puts it: "Model performance for most performance measures is deteriorated" when BEIS 2 is used with either model. This is a dilemma for OTAG.


ECOS: Bob Roberts, Executive Director (202) 624-3660 (fax: 3666)

OTAG Policy Decision Group Chair: Mary Gade (217) 782-9540 (fax: 9039)

(This newsletter in not affiliated with ECOS or OTAG)

The OTAG RAG is free to a good home via email or fax, and on the web: (click on PowervisioN)
Send requests or comments to or call 540-858-3503