Feb 10, 2009
Spotlight on ozone in North America
Surface ozone is harmful to health and can affect crop growth. With that in mind, the US is setting increasingly stringent air quality targets but this has raised the question whether air pollution from Mexico and Canada could prevent these from being met. Now Daniel Jacob of Harvard University and colleagues have used a chemical transport model to analyse this effect.
"The most important message is that Mexican and Canadian emissions never cause the background ozone in the US to rise above 60 ppb," Daniel Jacob of Harvard University, US, told environmentalresearchweb. "Thus a standard as low as 60 ppb could be met solely through domestic controls."
According to Jacobs, the US Environmental Protection Agency (EPA) sets its air quality standard for ozone based on an increment of health effect risk above that associated with the background ozone concentration. "This 'policy-relevant background' (PRB) has been defined by EPA as the ozone concentration that would be present in US surface air in the absence of North American anthropogenic emissions [from the US, Canada and Mexico]," he said. "The last revision of the ozone standard – recommending a value for the standard in the range 60-75 ppb – drew criticism that such standards would be unachievable because the PRB can be higher than 60 ppb. I responded to this criticism with work by my former grad student Arlene Fiore showing that the PRB is actually in the range 20-35 ppb with little variability, i.e. it would not impede a standard as low as 60 ppb."
Jacobs says that criticism then shifted to the PRB not accounting for emissions in Canada and Mexico, over which the US has relatively little control. This is where the current study comes in. Surface ozone forms from the oxidation of volatile organic compounds and carbon monoxide in the presence of nitrogen oxides.
Together with colleagues from Harvard University, Argonne National Laboratory in the US, Seoul National University, Korea, the US National Exposure Research Laboratory and Canada’s Dalhousie University, Jacobs used the global chemical transport model GEOS-Chem to quantify the effects of manmade emissions from Canada, Mexico and outside North America on daily maximum 8-hour average ozone concentrations in the US.
For the summer of 2001, the simulations indicated mean background concentrations of 26 +- 8 ppb if manmade emissions from North America were removed and 30 +-8 ppb if only US anthropogenic emissions were accounted for.
"At the same time, we found that Mexican and Canadian emissions often contribute significantly to poor air quality in US areas immediately downwind – southern California, Midwest, Northeast," said Jacob. "It would make sense in these areas to engage our neighbour countries in air quality policy, particularly since they're receptors as well as sources of pollution – there's a shared interest."
On average Canada and Mexico contributed 3 +-4 ppb of ozone in the US during summer, but days exceeding the 75 ppb quality standard in eastern Michigan, western New York, New Jersey and southern California were often associated with a contribution of more than 10 ppb. Occasionally the figure was much higher, for example upstate New York saw a peak contribution of 33 ppb from outside the US on a day with 75 ppb of total ozone, and southern California saw 18 ppb on a day with 68 ppb total ozone.
The team also looked at air quality projections for 2020 – these indicated that in the Northeast US the influence of Canadian pollution will become similar to that from US power plants.
Now the researchers, who reported their work in Atmospheric Environment, plan to examine the potential of satellites to observe air quality over North America and to test their models of pollution transport.
About the author
Liz Kalaugher is editor of environmentalresearchweb.