Climatology of Ozone Transport over the Eastern US: Page 1 of6 

3.3 CLIMATOLOGY of Ozone Synoptic scale Transport in the EASTERN US 

Bret Schichtel and Rudolf Husar
 

Washington University St. Louis Missouri
 

Richard Poirot and Paul Wishinski
 

Vermont Department of Environmental Conservation
 

Presented at the American Meteorological Society's 78th annual meeting, Jan. 11-161998, Phoenix AZ
 

1.0 introduction 

OTAG (Ozone Transport Assessment Group) was a 2-year project to develop consensus solutions to the 

tropospheric ozone problems in the Eastern US. A central issue of OTAG was assessing the extent of regional 

scale ozone transport and its contribution to non attainment areas. To aid in this endeavor forward and backward 

trajectory analyses were conducted over 5-7 summers between 1989 - 1995 in the Eastern US (Poirot and 

Wishinski, 1995; Poirot and Wishinski, 1996a,b,c; Schichtel and Husar, 1996a; Schichtel and Husar, 1997). These 

analyses were used to identify regional scale transport directions, speeds, and potential pollutant transport 

pathways during high and low ozone conditions, as well as the OTAG modeling episodes. While these techniques 

and meteorological data drivers are able to characterize regional scale transport, they are poorly suited for, and not 

intended for identification of local flows or near-by source influences. 

2.0 Meteorological Data 

The analyses used trajectories calculated from the National Meteorological Centers Nested Grid Model (NGM) 

(Rolph, 1992). The NGM database, contains three dimension wind vectors with a horizontal resolution of -160 km 

and ten vertical layers up to seven kilometers. The data are at a two hour time step from 1989 - 1995. 

3.0 Trajectory analysis methods 

3.1 Transport Wind Vectors and Source Regions of Influence 

Using the CAPITA Monte Carlo Model (Schichtel and Husar, 1996b) five summers, 1991-1995, of forward plumes 

were calculated for each NGM grid cell over the Eastern US. A forward plume identifies the downwind location of 

mass that was continuously released from a source at an instance in time. It can be though of as a snapshot of a 

plume from a smoke stack. The plumes were sorted based upon locally high and low ozone days, the 90th and 10th 

percentile of the average daily maximum ozone for each source region, i.e. each NGM grid cell, respectively. Each 

plume subset was aggregated together assuming a pollutant lifetime of 1 or 2 days and a constant emission rate of 

1 gls to create transfer matrices, average relative source contributions to each NGM grid cell. 

Transport wind vectors and source regions of Influence (SRI) were calculated from the transfer matrices (Schichtel 

and Husar, 1996; Schichtel and Husar, 1997). The source region of influence is the smallest region around the 

source encompassing 63% of the ambient mass. The transport wind vectors is a vector pointing from the source to 

the centroid of the SRI. It is a measure of the resultant mass transport direction and speed from the source. For 

example, assuming constant speeds, a source with transport predominately in one direction will have a longer 

transport vector than a source with transport almost equally likely in all directions. 

3.2 Residence Time Analysis 

Seven summers, June - August, 1989-95 of backward trajectories for 23 ozone monitoring sites across the OTAG 

region were calculated using the HY-SPLIT model (Draxler, 1992) and NGM winds. The trajectory results were 

aggregated over 1440 80*80 km squares using an ozone concentration based residence time analysis (Poirot and 

Wishinski, 1995; Poirot and Wishinski, 1996a,b,c). For each receptor location, an "Every Day Probability Field" was 

calculated using all summer trajectories. The trajectory "residence time hours" were summed for each of the 1440 

grid squares, and then expressed as a probability by dividing the square's hours by the total hours in all squares. 

The trajectories were then sorted according to whether the resulting ozone was "low" (below median) or 

"high" (above median). Additional trajectory subsets were developed for "very high" ozone levels - as defined as 

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the upper 90th and 95th percentile of ozone concentrations at the receptor. For each of these trajectory subsets, the 

"everyday probability field" was subtracted from the "ozone-sorted probability field". The resulting "incremental 

probability" showed locations more likely than normal to be upwind prior to the specified ozone level. 

4.0 Results 

4.1 High and Low Ozone Transport Characteristics 

The Transport wind vectors and SRls during the locally high ozone days are presented in Figure 1a. The high 

ozone in the South and the Midwest were associated with short and/or meandering transport wind vectors, and 

smaller SRls. However, in the northern and western part of the domain there was stronger more persistent 

transport from the southwesterly and southerly directions respectively. These results suggest that the high ozone 

concentration in the South and Midwest result from near field transport, while in the north and westerly section the 

transport conditions were conducive to regional scale transport. Similar results were found from the incremental 

probability analysis, where the incremental probability areas increased from the south to the north during high 

ozone periods (Figure 2). This indicates that in the north airmasses traveled longer distances in route to the 

receptor in a given period of time than in the south. 

It was also found that the incremental probabilities for many individual receptors had unique transport pathways 

associated with elevated ozone concentrations, >50th percentile (Figure 3). For example elevated ozone at Mark 

Twain, MO, was associated with transport from the south and east; from the west and north at Lithia Springs, GA; 

and from the south and southwest at Bennington VT. However, when the incremental probabilities from multiple 

receptors were grouped together (Figure 4a) the Central Eastern US became a common region airmass traversed 

in route to receptors throughout the Eastern US. This is also evident in the transport wind vectors during locally 

high ozone days (Figure 1a), where the locations surrounding the central industrial region have transport wind 

vectors pointing outward from it. In contrast, both the transport wind vectors (Figure 1b) and incremental probability 

plots (Figure 4b) show that low ozone concentrations are associated with swift transport from outside the (i.e., 

Canada, Atlantic, and Gulf of Mexico) into the Eastern US. 

4.1 aTAG Modeling Episode Transport Characteristics 

Using the transport wind vectors and source regions of influences, the transport conditions during the 1991, 1993, 

and 1995 aTAG episodes were characterized. It was found that a broad region of stagnation existed for each 

episode. The south had stagnant conditions during all three episodes, while it extended further north into the 

Midwest for the '91 and '95 episodes. The average aTAG episode transport, with the exception of the Southeast, 

east of Texas, was characteristic of region-wide high ozone concentrations, with slow meandering transport over 

Kentucky, Tennessee, and West Virginia and strong clockwise transport around this region of stagnation (Figure 

5a). Where region-wide high ozone concentrations were the 90th percentile of the daily maximum ozone averaged 

over the entire OTAG domain. However, the OTAG episode transport differed from that during locally high ozone 

concentrations. The transport speeds were higher during the aTAG modeling episodes and the resultant direction 

of transport over the Illinois-Pennsylvania corridor was from the west, rather than the typical southwesterly flow 

(Figure 5b). In the Southeast, the average transport was characteristic of the slow meandering transport during the 

locally high ozone days (Figure 5b). 

5.0 Conclusions 

These long-term, trajectory-based results suggest that the central Midwest is an important source region for 

regional-scale transport of moderately high ozone levels to surrounding receptor areas in many directions. This 

regional transport results in elevated background concentrations which, combined with local-scale influences (not 

investigated here), can contribute to exceedances of ozone standards. This feature of a central source region 

affecting receptors in many directions is characteristic of locally high ozone periods. It is not likely to be observed 

from the OTAG modeling episodes which had transport characteristic of regionally high ozone periods. 

6.0 References 

Draxler R.R. 1992. Hybrid single-particle Lagrangian integrated trajec10ries (HY-SPLlT): Version 3.0 - user's guide and model description. NOAA 

Technical Memorandum ERL ARL-195. Air Resources Laboratory, Silver Spring Maryland. . 

Poirol, R. and P. Wishinski. 1995, Air trajectory residence time analysis investigation of ozone transport pathways: 1989-95 (hltp:llcapjta.wustl.edu/otagl 

ReportsNITRAJ5/StatusReport1 html) 

Poirot, R. and P. Wishinski. 1996a, VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology, Status Report to OTAG Air Quality Analysis 

Workgroup: 8/15/96 (hllp:llcapita.wustl.edu/otag/ReportsNITRAJ51 StatusReport1.html) 

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Poirot, R. and P. Wishinski. 1996b, VT DEC air trajectory analysis of long-term ozone climatology, Status Report to OTAG Air Quality Analysis 

Workgroup: 11/7/96 (hltp:llcapita.wustl.eduJotag/reportsl Status Dec96/Status Dec96.html) 

Poirot, R. and P. Wishinski. 1996c, VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology, Status Report to OTAG Air Quality Analysis 

Workgroup: 12/3/96 (hltp;lIcapita.wustl.edulotaglreoortsl Status Dec96/Stalus Dec96.html) 

Rolph G.D. 1992. NGM Archive. NCDC Report TD-6140, July, National Climatic Data Center. 

Schichtel. B. and Husar, R. 1996a. Source regions of influence for high and low ozone conditions in the Eastern US. 

(hltp:llcapita.wustl.edu/otag/reportslsril sri hI03.htm) 

Schichtel, B.A. and Husar, R.B. 1996b. Regional simulation of atmospheric pollutants with the CAPITA Monte Carlo model. J. Air & Waste Manage. 

Assoc. 47, 331-343. 

Schichtel, BA and Husar, R.B. 1997. Update on the Characterization of Transport over the Eastern US. 

hltp:llcapita,wustl.edu/Otag/ReportslSriconVSricont.html 

AS 

Transport Wind Vectors & Regions of Influence Transport Wind Vectors & Regions of Influence
 

High (90%-i1e) Local Ozone Days
 Low (10%-i1e) Local Ozone Days 

Figure 1. Transport wind vectors and source regions of influences during high (90th percentile) local ozone 

days (a) and low (10th percentile) local ozone days (b). 

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Upper iOlj OZ!Y1B vs Everyd".,. 

Upper iOlj OZ!Y1B vs Everyday 

Greqq Co., Texas Boone County. Kentucky + liIFF in PR08ABlUTY for GRlD sct..lAFES + OIFF in PAOBABlLlTY for GRlD sct..lAFES 

Port Clyde, Maine + OIFF in PR08ABlUTY for GRlD S(JJ6f£S 

Uppgr iOlj 0Z!Y1B va Everyday 

Figure 2. Incremental probability plots for Gregg Co., Texas, Boone County, Kentucky, and Port Clyde, 

Maine, from left to right, for the 90th percentile of ozone concentrations. 

Mark T\loin State Pork, Missouri Lithia Springs. Georgia Benninqton, Vermont + OIFF in PROOABlUTY for GAlD S(JJ6f£S + OIFF In PA08ABlUT'I' for GRlD S(JJ6f£S + OIFe in PR08ABlUTY for GAlD ~S

Upper SOlj OZar<! VS EvgrydBy Upper SOlj OZar<! vs Evoryday Uppgr SO~ OZar<! vs Evoryday 

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Figure 3. Incremental probability plots for Mark Twain, Missouri, Lithia Springs Georgia, Bennington 

Vermont, from left to right, for the upper 50th percentile of ozone concentrations. 

AS
 

d with 23 OTAG Receptor Site 

High Ozone 

Figure 4 a) Frequency of back trajectories for 22 receptor sites for low ozone days (lower 50th percentiles) 

(a) and high ozone days (upper 50th percentiles) (b). 

AS 

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Climatology of Ozone Transport over the Eastern US: Page 60f6 

Transport Wind Vectors Transport Wind Vectors 

'91, '93, '95 OTAG Episodes & High [90%-ile) Regional Ozone Days '91, '93, '95 OTAG Episodes & High (90'.4-lIe) local Ozone Days 

-

Region" Ozone H~

­local Ozone 

'91, '9J, '9~-OTAG Episodes I '91, 'IJ, '11;Episodes

&.­

cAl 

1\- 4, 

Figure 5. Comparison of transport winds during the 1991, '93, and '95 OTAG domain episodes and the 

regionally high (a) and locally high (b) ozone conditions. 

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