Air Trajectory Residence Time Analysis Investigation of Ozone Transport Pathways: 1989-95

Paul Wishinski and Rich Poirot, VT Department of Environmental Conservation, 2/20/95 Summary Draft


"Trajectory Analysis" has been identified on the task list of the OTAG Air Quality Analysis Workgroup. VT DEC had agreed to devote some in-kind resources to assist in conducting this task. We have a copy of the NOAA HY-SPLIT trajectory model (Draxler, 1992), and NOAA NGM meteorological data sets for the years 1989 through 1995 for Eastern North America (East of 105 degrees Longitude). We have modified the original (PC) HY-SPLIT fortran code to run on a UNIX workstation, and have developed graphic output routines to display forward or backward trajectory results on a UNIX platform. Our current operating mode for backward trajectory calculates 4 trajectories per day (with arrival times of 3 AM, 9 AM, 3 PM and 9 PM, EST) for up to six individual receptor locations. Individual back trajectories with a maximum duration of 108 hours are stored for later display and analysis.

For analysis of large numbers of back trajectories, we employ a technique known as "Residence Time Analysis" (adopted from Ashbaugh, 1984). Our particular techniques for residence time analysis are described in Poirot and Wishinski (1985) and Wishinski and Poirot (1986). The general approach is to grid the trajectory domain, and keep track of the "residence time hours" of each trajectory over each grid square in its path. We can then examine the spatial characteristics of the long-term "trajectory climatology".

Initial Site Selection

For an initial group of six receptor sites, we selected the following group of high elevation sites, all of which have reasonably complete ozone data sets for the past 7 summers (1989-95 - for which we have NGM data.

LatitudeLongitude Elev. (m)AIRS Site #
Whiteface Mtn., NY44.36573.902 1480360310002
Mt. Greylock (Adams), MA42.63773.169 1140250034002
Roof of World Trade Ctr., NYC40.71174.013 503360610063
Shenandoah NP (Big Mead.), VA38.52278.436 1073511130003
Greenbriar County, WV37.81980.513 829540240001
Gt. Sm. Mt. NP (Look Rock), TN35.63183.944 793470090101

Fig. 1,2 Locations of Selected "High Elevation" Sites and Average JJA, 1989-95 Diurnal Patterns

Figure 1

Figure 2

These high "elevation sites" were selected for the following reasons:

- They are (with exception of World Trade Center Rooftop) inherently remote, and relatively free from local source influences (much of what affects these sites is obviously transported).

- They exhibit minimal diurnal variation, as they are relatively free from nocturnal ozone destruction (by NOx scavenging and dry deposition) which is characteristic of most lower elevation sites.

- They provide a good opportunity to characterize nocturnal (and daytime) ozone transport (or lack thereof) in layers above the surface (where nocturnal decoupling, lack of scavenging, and higher wind speeds may make transport most important).

Unfortunately, sites in excess of 500 meters above sea level are primarily limited to the Appalachian Mountain range, and are not widely distributed throughout the OTAG domain. On the other hand, these sites are generally located to the West of the East Coast urban corridor, and may therefore be good locations to characterize transport flows to/from the corridor to other OTAG subregions. Future analyses will focus on sites in other areas of the OTAG domain.

Figure 3

Trajectory Calculations

For each selected site, backward trajectories, with a maximum duration of 108 hours, were calculated 4 times each day, for the months June through August, 1989 through 1995. An example individual trajectory arriving at Whiteface Mtn., NY at 3 PM on 7/7/94 is displayed in Figure 3. The Whiteface ozone concentration at this arrival time was 51 ppb - about average for this site for the last 7 Summers.

The individual trajectory segments (between dots) represent 3-hour time intervals. When the segments are packed closely together, the air parcel is moving slowly. When the segments are farther apart (over Illinois, for example) the air parcel movement is more rapid.

Figure 3 also displays the grid of 80 x 80 km squares employed in our Residence Time Analysis calculations. For each trajectory, we keep track of the "residence time" over each grid square in the trajectory's path (white squares in this figure).

Figure 4

Figure 4 displays 8 back trajectories from Whiteface Mountain which arrived at 9 AM with an ozone concentration of greater than 90 ppb. These were the 8 highest 9 AM concentrations at Whiteface (for which we also have trajectory data) during the months of June through August, 1989 through 1995. As such, these might be considered to represent extreme events which occur on average of about once a year (although there were seven 9 AM ozone values > 90 ppb in 1988 - for which we have no trajectory data).

Figure 5

These morning ozone levels, in excess of 75% of the standard are unquestionably transported, from ozone production at least a day (or more) earlier. The associated trajectories suggest a wide range of regional influences, including recirculation within the OTR region (red), several episodes moving along the Ohio River Valley (black and blue), and several more rapidly moving events which have passed over northern, western and southwestern regions of the OTAG domain.

Figure 5 shows all back trajectories arriving at Whiteface at 3 AM, 9 AM, 3 PM and 9 PM during June - August, 1989 - 1995. (Aside from draining your ink jet printer with one picture), this figure provides useful information in two areas:

(1)It clearly shows the eastern and western limits of the meteorological data employed in these model runs. Many trajectories exceed these limits, and are subsequently truncated.

(2) It shows that the summertime air on Whiteface Mtn. Has on various occasions previously resided over (and might be potentially influenced by emissions from) any and all locations in the OTAG domain. Equivalent plots for the other 5 selected sites are quite Similar to Figure 5.

Figure 6

There are 2,120 trajectories included in Figure 5, which represents 82% of the possible 2,576 trajectories (4 trajectories/day x 92 days/summer x 7 years) during this time period. The difference is caused by time periods for which either NGM meteorological data or local ozone data is missing. Collectively, these trajectories have resided for 153,737 hours over the model domain - which represents 67% of the possible 228,960 hours (2,120 trajectories x maximum of 4.5 days per trajectory). The difference is caused by truncated trajectories which exceed the model domain, or for which missing NGM data is encountered within the model domain.

In Figure 6, the total 153,737 trajectory residence time hours for Whiteface Mtn. for the past 7 Summers have been aggregated into the grid squares displayed in Figure 4. Contours have been selected for plotting so that they bound the smallest numbers of grid squares accounting for 25% , 50%, and 75% of the total residence time hours. The area shaded pink, grey and tan each include 25% (38,434 hours) of the total residence time hours for this scenario, and represent areas of decreasing residence time probabilities away from the receptor. The least probable 25% of scenario residence time hours are in the unshaded, white area of the map.

This residence time probability plot may be thought of as providing an answer to the following question: "Where is the summertime air at Whiteface Mountain most likely to have previously resided"? Perhaps the answer ("Somewhere to the west") did not require this level of effort, but now we have a specific statistical definition based on a long-term "trajectory climatology", and can begin asking other questions of the trajectory database.

For the residence time plots in Figures 7 and 8, the Whiteface Mtn. trajectories have been sorted into two groups, depending on whether the resultant ozone concentration was low or high. The definition of high or low in this case is based on determining the ozone level for which half the total cumulative, seven-summer ozone dose is contributed by hours with higher concentrations, and half by hours with lower concentrations. In this case the 50% cut point is an ozone concentration of 51 ppb. Figure 7 shows the residence time probability for "low" Whiteface Mtn. ozone concentrations and is based on 1323 trajectories, each with resultant ozone levels of less than 51 ppb. Figure 8 shows similar residence time probabilities, but in this case, based on 791 trajectories with resultant ozone concentrations of 51 ppb or higher (roughly 40% or more of the standard). Figures 9 and 10 are similar to figure 8, but are based on increasingly higher ozone thresholds. Figure 9 is based on 323 trajectories with resultant ozone concentrations greater than 62.5 ppb (50% of standard) which collectively contributed 25% of the total 7-summer ozone dose at Whiteface Mtn.. The upper 10 % of the ozone dose at Whiteface was associated with 103 trajectories with ozone levels greater than 75.5 ppb. Thus, Figure 10 shows the most probable locations of air arriving at Whiteface Mtn. With ozone levels in excess of 60% of standards.

Figure 7

Whiteface Mtn., NY Residence time plots depict residence time probabilities for trajectories associated with lower 50% of 7-Summer cumulative ozone dose (Fig.7), upper 50% (fig. 8), upper 25% (Fig. 9) and upper 10 % of cumulative 1989-95 ozone dose (Fig. 10).

Figure 8

Figure 9

Figure 10

Figures 11 through 15 are similar to Figures 7 through 10, except that they are based on trajectories and ozone concentrations from Look Rock Mtn., TN. They show residence time probabilities associated with ozone levels of < 52.5 ppb (Fig. 11), > 52.5 ppb (Fig. 12), > 62.2 ppb (Fig. 13) and > 71 ppb (Fig. 14).

Figure 11

Figure 12

Figure 13

Figure 14

By comparison to the Whiteface Mtn., the Look Rock Mtn. residence time plots show much less of a westerly orientation - and are more uniformly distributed in all directions from the receptor. Also, the differences at Look Rock between the plots of relatively low (< 52.5 ppb) and relatively high (> 52.5 ppb) ozone are less distinctly different from each other - showing large regions around the receptor which are associated with both high and low ozone levels. There is a distinct south-southwesterly orientation to the Look Rock trajectories with low resultant ozone levels, while the higher concentrations at Look Rock appear to be more frequently associated with transport from the north-northwest.

Figures 15 and 16 display average ozone levels at Whiteface (Figure 15) and Look Rock (Figure 16) as a function of prior trajectory location. The shaded areas in both plots represent grid squares where the residence time-weighted average of all trajectories passing through that square and arriving at the receptor is greater than 50 ppb (approximately the average ozone concentration at both receptor sites). Grid squares are only included for these plots if they contained at least 100 hours of trajectory residence times (which is approximately the average number of residence time hours for each of the 1440 grid squares in our OTAG Domain map for our seven-summer data set). So the shaded areas are both frequently upwind of the receptors, and are also areas which are associated with higher than average ozone levels at the receptors. Trajectories passing through areas shaded green result in average ozone concentrations of > 55 ppb at the respective receptor sites. Areas shaded pink result in ozone greater than 52.5 ppb. Areas shaded dark tan, purple and red are associated with average receptor ozone concentrations of 55 ppb, 57.5 ppb and 60 ppb, respectively.

Figure 15

Figure 16

There are some interesting similarities and differences between Figures 15 and 16. One obvious difference is that substantially larger areas are associated with higher average ozone levels (> 57.5 and >60 ppb) at Whiteface Mtn. than at Look Rock. As noted with the residence time plots, a broader range of areas are associated with higher ozone levels at Look Rock, and many of these areas were associated with both high and low ozone levels at this site (presumably due to influences of other meteorological factors - such as precipitation, stagnation, cloud cover, etc.). The highest Whiteface ozone levels are associated with flows from the southwest, while the highest ozone levels at Look Rock are associated with flows from the northwest. Consequently, there is a large area of overlap in the regions associated with the highest ozone levels on these two mountaintop sites, located approximately 800 miles apart.

Figure 17 is based on all the back trajectory calculations for the summers of 1989 through 1995 for each of four high elevation sites at Whiteface Mtn., NY, Mt. Greylock, MA, Big Meadows at Shenandoah National Park, VA and Look Rock Mtn. in Great Smokey Mountain National Park, TN. This represents a total of 7,782 trajectories, which have collectively resided for a total of 575,779 hours over our gridded Eastern US (approximately OTAG) domain. Trajectories arriving at each of these sites have (as in Figure 4 for Whiteface Mtn.) Passed over all locations in the OTAG region.

Figure 17

For each of these 4 sites, we calculated an average ozone value for trajectories passing through each grid square and arriving at the receptor location. Thus, for each grid square there are 4 average ozone values - one associated with each of the 4 receptor locations. Then for each grid square, we selected the lowest of the 4 average ozone

values and plotted the results in figure 17. Trajectories passing through the areas shaded green have resulted in average concentrations of at

least 45 ppb at all 4 mountain

top sites. Trajectories passing through the areas shaded purple and red have resulted in average ozone levels of greater than 52.5 and 55 ppb at all 4 mountain sites, respectively. Thus there appears to be a large region in the Midwest which is consistently upwind prior to high, (transport-dominated) ozone levels at mountain tops throughout the Appalachians. This ozone transported aloft is approximately half the ozone standard on average, and can exceed standards on occasion - minimizing the effectiveness of local control strategies in downwind locations during typical afternoon episode periods when these elevated layers of high ozone mix downward to the surface.

Future Applications and Issues

The ensemble trajectory analyses presented here are based on a meteorological model, without consideration of emissions or atmospheric chemical reactions. The preliminary results do not demonstrate cause and effect, but do identify areas with strong statistical associations with airmasses subsequently resulting in high measured ozone concentrations at downwind monitoring sites - at relatively high elevation. The high elevation sites provide ideal "lamp posts" for examining ozone transport, but such lamp posts are limited primarily to the Appalachian Mountain range, and do not illuminate the entire OTAG region. For future applications we would like to include receptor sites more widely distributed in other parts of the OTAG domain, and are seeking recommendations from other OTAG participants. For lower elevation sites with strong diurnal ozone patterns, we believe it will be important to examine different hours of the day separately (and perhaps the night time ozone values will contain little useful information.

Also, in running trajectories for these higher elevation sites, we have eliminated (or minimized) the importance of choosing among complex alternative options in the way the HY-Split Model is run. One option in particular (which has not been thoroughly tested to our knowledge) allows the modeler to specify any number of "sub-layers" within the models lowest "sigma layer" (below approximately 300 meters). The user can specify the height of these sub-layer(s), and the model will interpolate a series of new windfields at the specified elevations, by interacting with the gridded model terrain with a modified "Ekman Spiral" approach. Based on our limited sensitivity testing use of these interpolated sublayers has little effect on the trajectory paths for higher elevation sites - which range (with exception of the World Trade Center rooftop) from about 800 to 1500 meters). For lower elevation sites, the use of sub-layers can become quite important. For example, Figure 18 shows two different HY-SPLIT back trajectory results for a low elevation (near sea level) receptor site in Rye, NH. Both trajectories arrived at 3 PM on July 20, 1991, with an ozone concentration of 156 ppb.

Figure 18

The grey (northerly) trajectory was calculated in the standard HY-SPLIT default mode - without interpolated sub-layers. The orange ( southerly) trajectory was calculated with interpolated sub-layers at 50, 100, and 250 meters above the surface. These heights were selected for approximate consistency with the vertical layer structure for a 7-layer UAMV run according to OTAG Modeling Protocol. Clearly the resultant trajectoriy paths are quite different, as the terrain interpolated sub-layers tend to slow the trajectory down, and turn it in a counter-clockwise direction. The trajectories tend to suggest potential transport (and local) influence from different source regions (although for other episodes at this site, trajectories with and without sub-layers are very similar to each other). In a general sense, this "more than one answer" result is probably quite realistic for many ozone episode periods. That is, many ozone exceedances are influenced by local-scale emissions and short-range transport, mesoscale transport from nearby urban areas and adjacent states, and synoptic scale transport from more distant source regions.

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