Summary of Ozone Transport

Ad Hoc Air Trajectory Workgroup



The purpose of this document is to present the finding and conclusions from studies conducted by the OTAG Air Quality Analysis workgroup which applies to the transport of ozone and its precursors. The findings and conclusions are presented in a form of questions and answers addressing specific OTAG related issues and concepts important for the understanding of ozone and ozone precursor transport. This report will be updated as new results from the Air Quality and Modeling workgroups become available. Summaries and evaluations of specific research results are located on the OTAG AQA web site and elsewhere.

Back to Contents

1) What aspect of transport is the most relevant to OTAG?

The mission of OTAG is to identify control strategies and implementation options for the reduction of regional ozone with specific goals to reduce ozone and ozone precursor aloft throughout the OTAG region and at the boundaries of non attainment areas (Strategies & Control Subgroup, 1996). This mission statement implies that the aspect of transport of most concern is the transport of regional ozone and precursors across the boundaries and into non-attainment regions. Regional ozone and precursors are those at the boundary of the non-attainment regions not due to the non attainment region itself. Therefore, significant ozone transport occurs when the ozone at the boundary of non attainment regions is high and there is a persistent wind moving the ozone across the boundary.

Back to Contents

2) What are examples of regional scale ozone and precursors transport?

Ozone transport has been clearly demonstrated within urban and major power plant plumes using aircraft to track the plumes over the course of a day (White, 1977; White et al. 1976). During transport, these plumes are dispersed horizontally and mixed vertically in the first 1-2 km of the atmosphere. Over the distance of a few hundred kilometers these processes cause the individual plumes to merge and losing their individual identities becoming part of the regional ozone (Figure 1).

The transport of regional scale ozone was illustrated by Schichtel and Husar (1996) in animations where atmospheric transport simulations were merged with measured ozone data throughout the Eastern US. Figure 2, presents several images from the animations depicting the transport of regional ozone from the Industrial Midwest to the east. Over the course of two days, 6/8 - 6/9/91 an airmass over the Industrial Midwest was moving slowing in a clockwise direction with wind speeds between 1-3 m/s. The 2 PM ozone concentrations increased from ~70 ppb throughout the region to greater than 100 ppb in parts of the Ohio River Valley. From the 9th to the 10th of June, the ozone laden airmass began to be transport east - northeast, and by the afternoon of June 10 the airmass was being transported by a well defined flow with wind speeds between 4-7 m/s over most of the Industrial Midwest and greater than 12 m/s in parts of the Great Lake region. The 2 PM ozone concentrations in parts of western Pennsylvania increased over 40 ppb from the previous day's ozone and exceeded 100 ppb. The easterly flow continued throughout the following day, and the 2 PM ozone concentrations decreased to less than 80 ppb over most of the Industrial Midwest and western Pennsylvania. However, the concentrations were still greater than 100 ppb along the Atlantic coast from Boston MA to Washington DC.

A) B)
C) D)
Figure 2. The merging of a simulation of the atmospheric flow (particles) and measured ozone data. The ozone has been spatial interpolated from over 600 monitoring sites. The Arrows represent the direction and speed of transport.

Back to Contents

3) What are the nonattainment regions where inter regional transport of ozone and its precursors are significant?

Inter regional transport of ozone and ozone precursors to the Northeast is more likely to occur then in other parts of the Eastern US. This was demonstrated in a study by Schichtel and Husar (1996) using forward plumes to construct "transport vectors" over the Eastern US during periods of high and low ozone in a Southeast, Industrial Midwest, and New England region (Figure 3). The transport vectors in both the Industrial Midwest and South East during the high ozone events are short and meandering indicated short transport distances. However, in New England the transport vectors are longer and all aligned. This indicates a well defined flow capable of much longer range transport from the west-southwest into New England then was seen for high ozone condition in the Industrial Midwest and Southeast during.

The spatial variation in transport ranges is supported by the back trajectory of Wishinski and Poirot (1996). They assessed the regions most likely to be upwind of selected receptor sites during periods of high ozone, and found that the relative influence (and distance) of transport generally increased from Southwest to Northeast for low elevation sites (Figure 4).

These conclusions are also supported from air quality data analyses. Husar (1996) plotted the absolute magnitude of ozone for west to east cross sections over the Eastern US (Figure 5). The west to east cross section from Kansas to Maryland across the industrial states seems to indicate generally rising ozone levels toward the east, as if there was a cumulative addition of one ozone source on top of the other as air travels from west to east. However, this pattern is not seen in the southern west to east cross section from S. Texas to N. Florida (Figure 5B), where the ozone levels between the urban centers are about 45 ppb over the entire cross section.

The lack of significant transport during high ozone events in the South is also supported by a study from Edgerton and Hartsell (1996). They examined O3, NOy, CO, and SO2 data at three rural sites in the Southeast, and came to the conclusion that by virtue of the detectability of NOy plumes and associated tracer species the ozone and precursor species were predominately of local origin rather than distant origin.

A) B) C)
Figure 3. Transport vectors during the 20% highest ozone concentrations during the summers, June - August 1991 - 1995 in a A) Southeast, B) Industrial Midwest, and C) New England region. The box in each figure shows the region the ozone was averaged over.

Figure 4. Incremental Probability plots at A) Gregg Co., Texas, B) Boone Co., Kentucky, and C) Port Clyde Maine. The incremental probability plots display the regions that the receptor airmass was more likely to reside prior to its arrival at the receptor during the highest 10% of the ozone concentrations compared to the likelihood of residence during the entire time period from the summers, June - August, 1989 - 1995.

A) B)
Figure 5. Cross sectional charts of average summertime daily maximum ozone concentration. A) Kansas to Maryland and B Texas to N. Florida.

Back to Contents

4) What is the characteristic lifetime of ozone?

The characteristic lifetime of ozone is not known. However, it possible to begin to place bounds on this lifetime. It has been shown that in the rural southeastern US there is a strong relationship between ozone and NOy (Trainer et al., 1993; Olszyna et al., 1994; Edgerton and Hartsell, 1996). This relationship implies that ozone formation is NOX limited in the rural South and that ozone is formed rapidly and fairly close to the sources of NOX. Therefore, a lower bound of the ozone lifetime in the southeastern US would be the lifetime of NOx, which is estimated to be between 1-2 days (EPA, 1993).

Back to Contents

5) How does the scale of transport vary in space and time?

Sources influence more distant receptors in the predominate direction of transport. which is to the east of most sources in the Eastern US except in eastern Texas and Louisiana where it is to the north-northeast. During the summer months, June - August, the characteristic distance of transport is smallest in the Southeast and increases the further north the source region. During high summer ozone conditions in the Southeast and Industrial Midwest, the scales of transport decrease by about 50% from average, but remain about the same for high ozone events in New England (Figure 6). These results are demonstrated in a study by Schichtel and Husar (1996) who examined the scales of transport from a purely transport standpoint. The smaller transport scales in the south are also supported from the back trajectory analysis applied by Wishinski and Poirot (1996) (Figure 3). Porter et al., (1996) calculated "zones of influence" from spatial correlations of ozone data over the time period 1983-1995, and found that these zones were elongated along the direction of predominate transport.

Back to Contents

6) What bounds can be placed on the scale of transport?

The scale of transport calculated from Porter et al., (1996) zones of influence is approximate 550 km along the axis of predominate transport. From the source regions of influence calculated by Schichtel and Husar (1996) the average scale of transport in the region north of Tennessee is approximately 300 - 600 km for a one day ozone lifetime and 450 - 1000 km for a two day ozone lifetime. In the South, the scale of transport is approximately 200 - 400 km for a one day ozone lifetime and 300 - 800 km for a two ozone day lifetime (Figure 6).

A) B)
C) D)
Figure 6. Source regions of influence (SRI) for pollutants with one and two day lifetimes during June - August of 1991 - 1995. Plot A) contain SRI's for six sites located through out the Eastern US calculated from data over the entire time period. Plots B, C, and D contain the SRI's at three sites for the days associated with the highest 20% of the daily average maximum ozone over a B) Southeastern, C) Industrial Midwest, and D) New England region. The averaging regions are shown in
Figure 3.

Back to Contents

7) Do some source regions influence more non attainment regions than others?

The Industrial Midwest with its high stationary source NOx emissions appears to be a common source area for many receptor areas. Wishinski and Poirot (1996) demonstrated this using back trajectory analyses that showed this source region to be persistently upwind of many different receptors when ozone was high (Figure 7). The potentially significant influence of the Industrial Midwest to neighboring regions can also be seen in Figure 6 where the Ohio River Valley has broad source regions of influence during average and high ozone conditions in neighboring regions of the Eastern US.

Figure 7.

Back to Contents


Eric S. Edgerton, E.S. and Hartsell, B.E. (1996) Ozone/NOy/Tracer Relationships at Three SOS-SCION Sites. Web Address:

Husar, R.B. (1996) Spatial Pattern of Daily Maximum Ozone over the OTAG Region. Web Address:

Olszyna, K. J., E. M. Bailey, R. Simonaitis and J. F. Meagher. (1994) O3 and NOy Relationships at a Rural Site. J. Geophys. Res., 99, 14557-14563.

Porter, P.S., S.T. Rao, Igor Zurbenko, E. Zalewsky, R.F. Henry and J.Y. Ku. (1996) Statistical Characteristics of Spectrally-Decomposed Ambient Ozone Time Series Data. Web Address:

Schichtel, B.S. and Husar, R.B. (1996) Source Regions of Influence for High and Low Ozone Conditions in the Eastern US. Web Address:

Strategies & Control Subgroup. (1996) OTAG policy paper: operational definitions of OTAG's goal. Web Address: " OPERATIONAL.html"

Trainer, M. et al. (1993) Correlation of Ozone with NOy in Photochemically Aged Air. J. Geophys. Res. 98, 2917-2925.

Wishinski, P and Poirot, R. (1996) Air Trajectory Residence Time Analysis Investigation of Ozone Transport Pathways: 1989-95. Web Address:

Back to Contents

Submit your comments, feedback, questions, and ideas pertaining this page. Your input will be automatically added to the existing annotations. In order to add a new comment, you must be registered with the OTAG/AQA Peoples Page.