States Seek Better Ozone Models for Exceptional Events

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By Andrew Childers

May 11 — Faced with more stringent ozone standards, state air officials said they need new tools from the Environmental Protection Agency to model emissions from exceptional events like wildfires and uncontrollable emissions blown in from overseas.

The modeling needs are particularly acute in the West where the EPA's new national ambient air quality standards for ozone of 70 parts per billion approach background levels of the pollutant in some places, state officials said May 11 at the Electric Power Research Institute's ENV-Vision conference in Washington, D.C.

While the EPA allows states to exclude emissions from events like wildfires when they demonstrate compliance with the ozone standards, providing evidence of the link between those events and elevated ozone concentrations can require a significant investment in state time and resources without assistance from the EPA, Tom Moore, air quality program manager for the Western States Air Resources Council, said.

“Exceptional events is kind of an unfunded mandate,” he said.

States also need better tools to model uncontrollable emissions such as ozone precursors that blow into the U.S. from China as they try to meet the ozone standards, Moore said.

The EPA has proposed (RIN:2060-AS02) an update to its exceptional events policy intended to make the process less cumbersome for states. The proposal would allow states to use approved pollution control plans to satisfy the criterion that the event was “not reasonably controllable or preventable” and removing the criterion that states show a regulatory violation wouldn't have occurred “but for” the event (219 DEN A-4, 11/13/15).

EPA Looking at Modeling Improvements

Rohit Mathur, a senior scientist at the EPA, said new modeling of long-term transportation trends indicates that stratospheric transport of precursors like volatile organic compounds and nitrogen oxides have a “pretty significant” contribution to ground level ozone formation in the U.S.

Reviewing data from 1990 through 2010 has shown “an increasing trend in the contribution of long range transport to surface ozone in the U.S.,” he said.

That finding suggests that additional ozone monitors placed at the top of tall buildings could yield valuable data about how transported pollution contributes to ground-level ozone in the U.S., Mathur said.

Moore agreed that additional data on pollution transported to the U.S. would be particularly beneficial to Western states that are working to reduce ambient ozone concentrations.

“We need to understand for planning purposes more about this trend,” he said.

Modeling Personal Exposure Levels

The EPA is also doing more work on modeling personal exposure levels to ozone, including a better understanding of how a changing climate will impact ozone exposure, Lisa Baxter, a branch chief at the EPA, said. Changing climate conditions will affect how populations are distributed, meteorological conditions and air quality, all of which could affect human exposure.

While the EPA is looking at how climate will affect ozone exposure now, Baxter said the agency plans to look at the effect of climate on other air pollutants as well.

To contact the reporter on this story: Andrew Childers in Washington at

To contact the editor responsible for this story: Larry Pearl at