There is good news and better news about ground-level ozone. While dangerous ozone levels have fallen in places that clamp down on emissions from vehicles and industry, a new study from Rice University suggests that a model widely used to predict the impact of remediation efforts has been too conservative.
Particularly in Northeastern US cities, ozone levels dropped even beyond what was anticipated by cutting emissions of nitrogen oxides (NOx) from 2002 to 2006. The study published online by the journal Atmospheric Environment suggests the 'community multiscale air quality' (CMAQ) model misjudged the reduction in ozone by 20 to 60 per cent.
''The models have been underpredicting how much benefit we get from controlling NOx emissions in some instances,'' said Daniel Cohan, an assistant professor of civil and environmental engineering and an author of the study with Rice graduate student Wei Zhou and Sergey Napelenok, a scientist in the Environmental Protection Agency's Atmospheric Modeling and Analysis Division.
''Following major controls of NOx, ozone has come down more quickly than anticipated,'' Cohan said. ''This is good news. But it also poses a challenge because states rely upon models to predict whether they'll attain ozone standards in the future. If the models have key uncertainties that affect their responsiveness, that can affect the states' control strategies.''
Ozone is not emitted directly but instead forms near the ground from precursor emissions of NOx and hydrocarbons. Modelling of this complex chemistry is important to help states comply with federal standards for ozone, which now stand at 75 parts per billion (ppb) and may be tightened by the Obama administration.
A recent Rice study showed a positive correlation between high ozone levels and cardiac arrest (See: Analysis links ozone levels, cardiac arrest)