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Examination of the impact of recent laboratory evidence of photoexcited NO2
chemistry on simulated summer-time regional air quality Golam Sarwar, Robert Pinder, Wyat Appel, and Rohit Mathur
National Exposure Research Laboratory, U.S. Environmental Protection AgencyResearch Triangle Park, NC 27711, USA
1. Introduction
Li et al. (2008) recently suggested
that excited nitrogen dioxide, produced
from nitrogen dioxide (NO2) by visible
sunlight, can react with water vapor to
form hydroxyl radical (OH) and
nitrous acid (HONO) (reactions 1-3).
Wennberg and Dabdub (2008)
incorporated the chemistry into an air
quality model and reported that it
increased ozone (O3) by up to 55 ppbv
for Los Angeles in Fall of 1987.
Current US emissions are significantly
different than those in 1987. This
study examines the impact of the
excited NO2 chemistry on current day
air quality in the US.
NO2 + h ( 420 nm) NO2*
(1)
NO2* + M NO2 + M (2)
NO2* + H2O OH + HONO (3)
2. Methodology
The Community Multiscale Air
Quality (CMAQ) modeling system
(version 4.7) (Byun and Schere, 2006)
with the 2005 version of the Carbon
Bond mechanism was used. The 2001
and 2002 US National Emissions
Inventories were used in the study.
The Biogenic Emissions Inventory
System (version 3.13) was used to
prepare biogenic emissions for the
study (Schwede et al., 2005).
Two different modeling domains
were used: one for the eastern US and
the other for the western US. Two
simulations were performed for July
2001 for eastern US domain and two
other simulations were performed for
July 2002 for the western US domain.
One simulation included the CB05
chemical mechanism and the other
simulation included the CB05 mechanism
augmented with the excited NO2 chemistry.
The formation of excited NO2 by visible
light was calculated using absorption cross-
section data from Rothman et al. (2003).
3. Results
The largest differences in hourly predicted
O3 between the two model versions during
the month are shown in Figure 1. The
inclusion of the excited NO2 chemistry
enhanced O3 by more than 1 ppbv in several
urban areas in the eastern US: Houston,
New York/New Jersey, Chicago, and New
Orleans. It increased O3 by more than 1
ppbv in five urban areas in the western US:
Los Angeles, San Francisco, Portland,
Seattle, and Phoenix. The largest increases
in hourly predicted O3 were 6 ppbv in
Houston, 17 ppbv in New York/New Jersey,
14 ppbv in Chicago, 11 ppbv in Los
Angeles, 4 ppbv in San Francisco, 7 ppbv in
Seattle, 7 ppbv in Portland, and 5 ppbv in
Phoenix.
A comparison of predicted maximum 8-hr
average O3 with observed data from Air
Quality System (AQS) for Houston, New
York/New Jersey, and Los Angeles is
shown in Figure 2. The difference in
predicted maximum 8-hr average O3
between the two simulations was small,
resulting in a small change in the model bias
and error for predicted O3.
For example, VOC levels of up to
about 1,500 ppbC and 3,200 ppbC had
been measured in Summer and Fall of
1987 in Los Angeles (Fujita et al.,
1992). Thus, the impacts of excited NO2
on O3 under current US emissions
conditions are perhaps much less than
those suggested by Wennberg and
Dabdub (2008). Additionally, we used a
horizontal grid resolution of 12-km.
Wennberg and Dabdub did not report
the grid resolution used in their study.
Differences in horizontal grid resolution
and different modeling time period may
have also affected the results.
4. Summary
Excited NO2 chemistry increases O3
in several urban areas in the US for
current emissions; however these
increases are small. Ambient VOC and
NOx levels in Los Angeles in 1987 were
much greater than the current levels;
thus the impact of excited NO2
chemistry on O3 were greater. The
inclusion of the excited NO2 chemistry
in air quality models can potentially
increase O3 by a large margin in areas
with high NOx and VOC conditions.
5. ReferencesByun, D., Schere, K. L., 2006. Review of the governing
equations, computational algorithms, and other components of the
Models-3 Community Multiscale Air Quality (CMAQ) modeling
system. Applied Mechanics Reviews, 59, 51–77.
Fujita, E. M., B. E. Croes, C. L. Bennett, D. R. Lawson, F. W.
Lurmann, and H. H. Main, 1992. Comparison of emission inventory
and ambient concentration ratios of CO, NMHC, and NOx in
California's South Coast Air Basin, J. Air Waste Manage. Assoc., 42,
264-276.
Li, S.L., J. Matthews, and A. Sinha, 2008. Atmospheric hydroxyl
radical production from electronically excited NO2 and H2O, Science,
319, 1657-1660.
Schwede, D., Pouliot, G., and Pierce, T., 2005. Changes to the
biogenic emissions inventory system version 3 (BEIS3), 4th Annual
CMAS Models-3 Users' Conference, September 26-28, 2005, UNC-
Chapel Hill, NC. Available at
http://www.cmascenter.org/html/2005_conference/abstracts/2_7.pdf
Rothman, L.S., et al., 2003. The HITRAN Molecular
Spectroscopic Database: Edition of 2003, Journal of Quantitative
Spectroscopy & Radiative Transfer, 82, 5.
Wennberg, P.O. and D. Dabdub, 2008. Rethinking ozone
production, Science, 319, 1624-1625.
To better understand the impact of the excited
NO2, box model simulations were performed with
and without the excited NO2 chemistry by using
different initial NOX and VOC conditions.
Isopleths of the increases in average predicted O3
are shown in Figure 3. The increases in O3 are
large when both NOx and VOC are high.
Increases in O3 at other VOC and NOx
conditions are small. While NOx levels in CMAQ
simulations were high, VOC levels were generally
low. The highest hourly VOC levels for Los
Angeles were about 300-400 ppbC, for Houston
200-400 ppbC, and for New York/New Jersey
500-700 ppbC. VOC levels at other times were
even lower. Thus, increases in O3 with the excited
NO2 chemistry in CMAQ were generally small.
The impact of the excited NO2 chemistry on O3
in Los Angeles is less than that reported by
Wennberg and Dabdub (2008). We performed a
model simulation for 2002 while Wennberg and
Dabdub performed a simulation for 1987. VOC
and NOx levels in Los Angeles were greater in
1987 than in 2002.
Figure 1. Predicted largest increases in O3 due to excited NO2 chemistry (a) eastern US (b) western US
(a)
(b)
(a)
(b)
Figure 2. CMAQ prediction vs. AQS maximum 8-hr O3 for a) Houston b) New York/New Jersey c) Los Angeles
Figure 3. Increase in average O3 (ppbv) due to the excited NO2 chemistry
(a)
(b)
(c)
(a)
(b)
(c)