Rice University, EPA study finds pollution controls may be
working better than anticipated
HOUSTON – (March 11, 2013) – There is good news and better news
about ground-level ozone in American cities. 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 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
percent.
“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. Modeling 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.
In 2002, the EPA implemented a cap-and-trade program known as the
NOx SIP Call to curtail emissions of ozone-forming NOx from
industries in Eastern states. The dramatic reduction in emissions
over the subsequent four years provided a real-world experiment for
the researchers to test how well computer models predict
improvements in air quality.
“We found that even when we tried to model things with the best
available emissions and the best available meteorology, we still had
a gap, especially in the Northeast states, that couldn’t be
explained,” he said.
In the SIP Call regions, the researchers found the simulated drop in
ozone was 4.6 ppb, while the observed drop was 8 ppb, a significant
difference. Faster-than-expected reductions in NOx emissions may
explain some but not all of that gap. The remaining gap may result
from inaccuracies in how the model represents the chemistry and
transport of air pollutants, Cohan said.
“How ozone responds to changes in NOx and hydrocarbons is a
nonlinear chemistry,” Cohan said. “So it’s certainly possible that
even the best models could be slightly inaccurate in defining those
relationships. It tells us that, as modelers, we need to revisit the
formulations, especially the chemistry.”
While it may be preferable for models to be a bit conservative
rather than too aggressive in predicting ozone improvements, Cohan
said, the models are intended to represent air pollution as
accurately as possible. A study by Cohan’s research group last year
showed that regulatory modeling by states tended to slightly
under-predict the ozone reductions that were actually achieved.
“The goal of everyone in the process is to reach attainment in the
most cost-effective manner possible, and we need accurate models to
inform those decisions,” Cohan said.
The National Science Foundation funded the study through a CAREER
grant to Cohan.
SOURCE: Rice University
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