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)