2010 Progress Report - International Joint Commission

Figure 26. Network of Monitoring Sites Used to Create Graphs
of Ambient Ozone, NO x
and VOC Levels
Commitments
Source: US EPA and Environment Canada 2010
The data in Figures 24 and 25 represent measurements
for the ozone season (i.e. May through September).
Although NO x
and VOC concentrations have fl uctuated
over recent years, because VOC concentrations are
infl uenced by temperature, these fl uctuations are
most likely due to varying meteorological conditions.
Overall, the data indicate a downward trend in the
ambient levels of both NO x
and VOCs. The limited
correspondence between composite ozone and
precursor trends could refl ect the regional complexity
of the problem as well as network limitations.
Recently in the United States, there has been much
investigation into the relationship between NO x
emissions reductions under the NO x
SIP Call and
observed concentrations of ambient ozone in the
states participating in the NBP. Generally, a strong
association exists between areas with the greatest
NO x
emission reductions and downwind monitoring
sites measuring the greatest improvements in ozone.
This suggests that, as a result of the NBP, transported
NO x
emissions have been reduced in the East,
contributing to ozone reductions that have occurred
after implementation of the NBP. More information on
the relationship between NO x
emissions and ambient
ozone concentrations in the eastern United States
is available in the NO x
Budget Trading Program
2008 Environmental Results report available at
http://www.epa.gov/airmarkets/progress/NBP_3.html.
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