Environmental Engineering Reference
In-Depth Information
4.3 On the Use of a Dynamic Evaluation Approach
to Assess Multi-year Change in Modeled and Observed
Urban NO
x
Concentrations
James M. Godowitch, George A. Pouliot, and S. Trivikrama Rao
Atmospheric Modeling and Analysis Division, National Exposure Research Laboratory,
United States Environmental Protection Agency, Research Triangle Park, NC, USA
Abstract
Model results and measurements were analyzed to determine the extent
of change in concentrations of nitrogen oxides (NO
x
) during morning weekday
high traffic periods from different summer seasons that could be related to change
in mobile source emissions. The dynamic evaluation technique was applied to
compare the relative (%) changes in modeled and observed 3-h morning NO
x
at
numerous urban locations. The average changes in modeled and observed NO
x
levels between 2002 and 2005 were −16% and −15%, respectively, which are
close to the decline (−17%) in NO
x
emissions estimated by a mobile emissions model.
Keywords
Model evaluation, dynamic evaluation, NO
x
concentrations, NO
x
mobile emissions
1. Introduction
Since nitrogen oxides (NO
x
= NO + NO
2
) are key precursor species in the
photochemical production of tropospheric ozone, numerous control programs have
been implemented to reduce NO
x
emissions from various source types (USEPA,
2005). The major source categories of anthropogenic NO
x
emissions include the
electrical utility sector, industrial point sources, on-road and non-road mobile
sources. However, in populated areas, on-road mobile emissions are the dominant
source of NO
x
. It is important from an accountability viewpoint to determine
whether emission reductions achieve improvements in air quality. To investigate
whether recent estimated reductions in mobile emissions have translated into
changes in NO
x
concentrations, observed and modeled NO
x
concentrations during
high traffic weekday 6:00-9:00 periods are analyzed from different summer years.
The emerging dynamic evaluation approach was applied to focus on the model's
ability to simulate concentration response to emission changes and how well it
compares to changes in observations. In this regard, selected results of the relative
(%) changes in modeled and observed morning NO
x
concentrations between the
summers of 2002 and 2005 are highlighted.
