Update on the Characterization of Transport over the Eastern US

by Bret A. Schichtel and Rudolf B. Husar

February 1, 1997


Contents


Background

This report presents the continuation of the work to characterize the summer time transport over the Eastern US during high and low ozone conditions for the years 1991 - 1995. In addition, the transport condition for the 1991, 1993, and 1995 OTAG episodes are evaluated and compared to the average summer time conditions. The initial work is presented in a report by Schichtel and Husar (1996) at http://capita.wustl.edu/otag/reports/sri/sri_hlo3.htm.

Transport Vectors

The transport conditions are assessed using the source region of influence (SRI) which identifies the region surrounding a source that its emissions are most likely to impact assuming a characteristic pollutant lifetime (Figure 1). The SRIs are generated by the aggregation of multiple forward "plumes" with a specified characteristic lifetimes. Consequently, the size and shape of the SRIs are dependent upon the characteristic lifetime of the pollutant and the transport speed and direction away from the source. The SRIs can be calculated for specific condition by filtering out all time periods that due not meet a specified criteria, such as time periods when the ozone concentrations is above or below some value. See Schichtel and Husar (1996) for the full methodology of calculating the SRIs.

The SRIs in Figure 1 are elongated to the northeast of the St. Louis source. This elongation is due to a higher fraction of the emitted mass being transported further away from the source in the northeast direction then say in the southwest direction. This increase in the distance of mass transport is due to a combination of higher frequency of transport in the direction of the elongation and the speed of transport. This elongation has been quantified by the transport vector (Figure 1) which is a vector in the direction from the source to the centroid of the SRI. The magnitude of the vector is proportional to the distance between the source and the SRI's centroid. A short vector indicates that the source emissions nearly equally impact in all directions around the source, while a long vectors shows that the transport direction and speed are balanced such that the source emissions have a larger impact in the direction of the vector. The transport vector is similar to an average wind vector, but it incorporates the variations of wind speed and direction with height and along the path of transport, and thus can be thought of as the result transport.

The transport vector is a convenient means of simplifying the information contained within the SRI. However, lost in this representation is any indication of the overall size of the SRI, as well as the fact that the source can and does impact regions not in the resultant direction of the transport vector.

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Residence Time

The speed that emissions are transported away from the source is an important property for determining the impact of a source on its surroundings. The higher the speed, the less time the pollutants reside near the source, and thus the further the pollutants are transport before being removed. However, the higher transport speeds also result in less time for the pollutants to accumulate from the continuos emissions at a source, thus the concentrations near the source are smaller. These influences of the transport speed on the impact of a source on its surroundings are evident by the area encompassed within the SRI, where the higher average transport speeds result in larger areas.

The area encompassed by the SRI was used to calculate characteristic transport distances and speeds from each source. This area was assumed to be circular with the average transport distance equal to its radius. From this radius, the characteristic transport speed was estimated by dividing by the lifetime of the pollutant. The lifetime was used because the average time of transport of an air parcel from the source to the boundary of the SRI is approximately equal to the lifetime of the pollutant. From an air pollution perspective, the time the air parcel resides in a given area is more meaning full than the transport speed. Therefore, the transport speed is inverted creating a normalized residence time.

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Results

Average Transport Condition, June - August 1991-95. The SRIs averaged over the five summers from 1991 - 95 for eleven sources over the OTAG domain are presented in Figure 2. The resulting transport vectors and normalized residence times are presented Figure 3. As shown in Figure 2, there is substantial transport in all directions over the Eastern US, but there is a general resultant transport vector over the entire region (Figure 3). This resultant transport from Texas to Nebraska is to the north. As one moves eastward the resultant transport is more to the east, and east of the Mississippi it is primarily to the east. The residence time over the Eastern US increases by about 50% from the north (~0.2 s/m) to the south (~0.3 s/m).

Transport Conditions During High and Low Ozone Concentrations, June - August 1991-95. To evaluate the transport conditions during high and low ozone concentrations, SRIs were constructed for each source from the plumes during the 10% of the days with the highest and lowest daily maximum ozone concentrations. The ozone concentrations at each source were estimated by spatially interpolating the ozone concentrations from the AIRS and CASnet monitoring networks to the source location.

The transport vectors and normalized residence times during the high and low ozone periods are presented in Figures 4 and 5. During the high ozone episodes, the normalized residence times increased for all sources compared to the average summer values. The largest normalized residence times are in the central and southern part of the Eastern US from southern Ohio to the Gulf of Mexico, including East Texas, where they are greater than 0.35 s/m. These high normalized residence times are associated with short and meandering transport vectors. These transport conditions are indicative of stagnating airmasses. In the Great Planes, Great Lake States and New England, the residence times are less than 0.3 s/m and the transport vectors are longer than on average indicating more persistent ventilating transport from a given direction. In the Great Plans the Transport is from the south while in New England it is from the southeast.

During the low ozone days (Figure 5), the residence times are slightly lower than on average. Also, the transport vectors are longer than average and indicate flow from outside the OTAG domain, coming from Canada, Atlantic Ocean, and Gulf of Mexico.

Transport Condition during the 1991, 1993, and 1995 OTAG Episodes. The SRIs, transport vectors and normalized residence times over the Eastern US for the 1991, 1993, and 1995 OTAG episodes as well as the three episodes combined, are presented in Figure 6 & 7. The episodes differs markedly from the average summer transport pattern with each episode distinguished by a region of high normalized residence time. During the 1991 episode, the highest average normalized residence time is located in the deep Southeastern US and the Industrial Midwest (> 0.4 s/m). The resultant transport over the Eastern US is clockwise around this region of stagnation. The transport around the region of stagnation is in the same direction as the average summer conditions, but it is more persistent leading to longer transport vectors than for the average summer conditions. The flow pattern during 1995 is similar to 1991, but the region of highest normalized residence time is from eastern Kentucky to South Carolina and southeastern Texas, where it can exceed 0.5 s/m.

The 1993 episode is characterized by high normalized residence time in the South from Louisiana to South Carolina with values greater than 0.5 s/m. There is also region of very low normalized residence times ( < 0.2 s/m) extending from Wisconsin to Ohio. The resultant transport (Figure 7) in this region is from the south, but as can be seen from the SRIs in Figure 6, there is substantial transport from north - northwest of this region. The 1993 episode is the only episode with lower normalized residence times in any region of the Eastern US than the average transport conditions.

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Conclusions

1) High ozone conditions in the central and south parts of the OTAG domain are associated with stagnating transport condition, i.e. high residence times (0.35 to 0.45 s/m) and meandering transport directions.

2) In the northern regions of the OTAG domain, the Dakota's to New England, the transport conditions are associated with lower normalized residence times (0.25 to 0.3 s/m) and persistent transport from the interior of the OTAG domain. These conditions are conducive to influences from more distance sources than in the Midwest and Southern regions of the OTAG domain.

3) Transport condition associated with low ozone concentrations are low normalized residence times over the entire OTAG domain (0.2 to 0.3 s/m) and transport from outside of the domain, i.e. transport from Canada, Atlantic Ocean, and Gulf of Mexico.

4) The '91, '93, and '95 OTAG episodes are each characterized by one or two stagnation regions with high normalized residence times (between 0.3 and 0.6 s/m) and short meandering transport vectors. The south had stagnant condition during all three episodes, while this stagnation region extended the furthest north into the industrial Midwest during the 1991 episode.

5) The transport conditions during the 1993 episode were substantially different from either the 1991 and 1995 episodes and the average transport conditions. During the 1993 episode, the northern region of the OTAG domain had very low normalized residence times ( < 0.2 s/m) and resultant transport to the north.

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Figures

Figure 1. The source region of influence for a St. Louis MO source during the summers of 1991 - 1995.

Figure 2. Source regions of influence for eleven sources evenly distributed over the Eastern US for pollutants with a one day lifetimes during the summer months of 1991 - 1995.

Figure 3. The transport vectors and normalized residence times over the Eastern US for pollutants with a one day lifetimes during the summer months of 1991 - 1995.

Figure 4. The transport vectors and normalized residence times for the highest 10% of the daily maximum ozone concentrations during the summer months of 1991 - 1995 at each source. The results are based upon a pollutant with a one day lifetime. The ozone concentrations at each source were spatially interpolated to the source's location from the AIRS and CASnet monitoring networks.

Figure 5. The transport vectors and normalized residence times for the lowest 10% of the daily maximum ozone concentrations during the summer months of 1991 - 1995 at each source. The results are based upon a pollutant with a one day lifetime. The ozone concentrations at each source were spatially interpolated to the source's location from the AIRS and CASnet monitoring networks.

Figure 6. Source regions of influence for eleven sources evenly distributed over the Eastern US for pollutants with a one day lifetimes during the A) 1991, b) 1993, c) 1995 OTAG episodes and D) the combined '91, '93, and '95 episodes.

 

Figure 7. Transport vectors and normalized residence times over the Eastern US for pollutants with a one day lifetimes during the A) 1991, b) 1993, c) 1995 OTAG episodes and D) the combined '91, '93, and '95 episodes.

 

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