Ozone sources and transport in the northeastern United States

Ozone Sources and Transport in the Northeastern United States Chester W. Spicer”, Darrell W. Joseph, Philip R. Sticksel, and Gerald F. Ward Battelle, Columbus Laboratories, 505 King Avenue, Columbus, Ohio 4320 1

In order to devise oxidant control strategies which accommodate pollutant transport, it will be necessary to quantify oxidant transport in terms of concentration, dimensions of transported air masses, and the distance of transport under various meteorological conditions. These topics are discussed in this paper in terms of data collected in southern New England during the 1975 Northeast Oxidant Transport Study. Several potential sources of surface-level ozone and their contributions to the overall ozone burden are discussed. The sources include natural or “background” ozone, ozone associated with high-pressure systems (regional ozone), ozone formed from local anthropogenic emissions, and ozone generated in urban plumes downwind of cities. Ozone associated with high-pressure systems tends to blanket large regions of the country and, when superimposed on the background and locally generated ozone, can result in concentrations which exceed the federal oxidant standard. Ozone formed in urban plumes is superimposed on ozone from other sources and forms “hot spots” in the regional blanket of ozone. Emphasis in this paper is placed on characterization of ozone plumes from urban areas.

In recent years elevated ozone concentrations have been observed in many rural areas that were previously thought to be relatively immune from the symptoms of photochemical smog. The finding of ozone in rural atmospheres, far from the generally accepted sources of photochemical smog precursors (Le., urban areas), has very important implications in terms of the strategies devised to control smog formation. Since various strategies for the control of ozone may differ in efficiency, with some having important social and economic consequences, it is important that the strategies be conceived with a thorough understanding of the origins of both rural and urban ozone. A number of recent field investigations have dealt with the sources of ozone and the impact of ozone transport in the Middle West (1-6) and Far West (7-9). Results of these and other studies indicate transport of ozone and its precursors across regional boundaries is an important source of ozone in downwind rural and even urban areas. Analysis of data collected in the northeastern United States (10-13) suggested that transport of ozone into and within the Northeast is also a significant factor in determining peak oxidant concentrations in this area. T o study these phenomena in the Northeast, and continue its long-term investigations of ozone formation and movement, the EPA organized the 1975 Northeast Oxidant Transport Study. The study involved the coordination and participation of a number of research groups including Battelle-Columbus Laboratories, Washington State University, EPA-RTP (Research Triangle Park), EPA-LV (Las Vegas), EPA-Region I, and the Interstate Sanitation Commission. A number of state and local air pollution agencies also provided invaluable data and assistance during the field study. This paper is based on data collected during that study. Descriptions of the study and preliminary data reports were published early in 1976 by the major study participants (14-20). In addition, the proceedings of a symposium held in January, 1976, dealing with the preliminary results of the 1975 study have been published (21). These reports should be consulted for a detailed description of the study design and tabulations of the data. 0013-936X/79/0913-0975$01.OO/O

@ 1979 American Chemical Society

Results and Discussion In order to devise oxidant control strategies that accommodate pollutant transport, it would be useful to quantify oxidant transport in terms of concentrations, dimensions of transported air masses, and the distance of transport under various meteorological conditions. These topics will be discussed in this paper. Of primary concern here will be transport within urban plumes over distances approaching 200 miles. T h e much longer range transport associated with high-pressure weather systems has been discussed in earlier reports (2, 3, 22) and will only be mentioned briefly. Our studies suggest that the concentration of ozone a t any given location is the result of the superposition of ozone originating from a number of different sources. These sources include natural or “background” ozone (e.g., from natural emissions and stratospheric origin), ozone generated from local anthropogenic emissions, ozone formed from precursors accumulated in high-pressure cells and transported many hundreds of miles over several days (regional ozone), and ozone formed in urban plumes downwind of cities. Ozone associated with high-pressure systems tends to blanket large regions of the country, and, when superimposed on the background and locally generated ozone, can easily result in concentrations that exceed the federal oxidant standard. Ozone formed in urban plumes is superimposed on ozone from these other sources and forms “hot spots” in the regional blanket of ozone. The contribution of these various sources to the total ozone burden will be discussed in terms of the Northeast Oxidant Study data. Urban Plumes. T o determine the maximum distance O3 can be transported within an urban plume, urban plumes were investigated under conditions where the plume extended out over the ocean. This should he a rather ideal transport situation, owing to the smooth terrain and lack of scavenging emissions . Figure 1shows the ozone concentration during an aircraft study of the New York urban plume on the evening of August 9, 1975. The resultant meteorological winds were from the northwest, as shown by the arrow on the map. Ozone concentrations are plotted vs. distance for each segment of the flight pattern. T h e return leg of the flight (c-d) was flown a t 3500 ft AGL (above ground level), because a vertical profile 1000 f l A b o v e Ground Level

B

100

0 500

25

SO 75 IO0 D i r l o n c c , m,ies

e25

150

100011 A b o v e Ground Level

e

8

100

Figure 1. Flight pattern and ozone results for New York urban plume flight during the evening hours of Aug 9, 1975

Volume 13, Number 8, August 1979 975

I

.~..,... \

Figure 2. (a-f) Ozone distribution in southern New England on July 18, 1975

a t point c indicated the maximum ozone levels were a t about t h a t altitude. I t is clear from the figure that, as the aircraft moved from point a to point b, the ozone concentration increased dramatically. The maximum ozone along the path a-b is found directly downwind of the urban center. This segment of the flight may be viewed as a constant-altitude, partial cross section of ozone concentration within the urban plume. The ozone concentrations along flight segments b-c and c-d are well above the 0.08 ppm federal standard and suggest that generation and transport of ozone within urban plumes extend for at least 150 miles. A subsequent study of the Boston urban plume as it was transported over the ocean on westerly winds extends this maximum transport distance of urban plumes under these ideal conditions to at least 250 miles. Even at such distances, ozone concentrations approaching twice the federal standard were observed. As stated earlier. the conditions for 976

Environmental Science & Technology

ozone transport are rather ideal over the ocean owing to the lack of scavenging emissions and smoother terrain (minimal deposition) than found over land. Vertical profiles a t the extreme downwind points of both the New York and Boston urban plume studies demonstrated t h a t the 0 3 maximum occurred a t the base of a temperature inversion well above the surface, between 3000 and 4000 ft. A visible haze layer was observed a t the same altitude as the ozone maximum. We have frequently observed urban plumes rising above the surface over land, and these flights suggest t h a t the same phenomenon occurs over the ocean. Further insight into the transport of ozone within urban plumes can be gained by examining surface ozone data collected downwind of major metropolitan areas. The metropolitan New York plume is clearly defined in the ozone distribution maps of southern New England for July 18 shown in Figure 2. These and subsequent ozone distribution maps

Table 1. Winds at 1000 ft on July 18, 1975 time

0700

1300

1430 1900

iocatlon

Chatham, Mass. Springfield, Mass. Putnam, Conn. Chatham, Mass. Springfield, Mass. Putnam, Conn. Avery Point, Conn. Chatham, Mass. Putnam, Conn.

speed, mPh

direction, deg

8 2 8 23 15 14 17 29 14

210 180 27 1 250 177 243 260 245 220

were prepared using 0 3 data from more than 30 ground monitoring stations as well as aircraft 0 3 data collected by Battelle-Columbus (14) and Washington State University ( 1 5 ) .Air mass trajectories indicate that the air moving into southern New England on the afternoon of July 18 has passed over the New York metropolitan area (northern New Jersey, New York City, southwestern Connecticut) during the morning rush hours. T h e urban plume which moves into Connecticut a t 1200 E S T moves through southern New England on southwest winds during the rest of the afternoon, and by 2100 E S T extends northeast of Boston. It is initially surprising that an air mass moves rapidly enough to traverse the approximately 200 miles between New York and Boston during the course of a single day, as these maps seem t o suggest. Table I shows the 1000-ft wind directions and speeds a t several southern New England locations during July 18. Higher altitude winds are generally even higher in speed. During much of the day the winds averaged a t least 15 mph within the important surface to 5000-ft transport layer. At this speed, a polluted air mass could easily travel 200 miles over a single day. The dissimilar ozone trends a t two of the major ground monitoring stations on July 18 provide an interesting demonstration of the effect of an urban plume. T h e two stations were located in Simsburg and Groton, Conn. (These stations were operated by Battelle’s Columbus Laboratories and Washington State University, respectively.) The relatively rural Simsbury station is shown on the ozone distribution maps of Figure 2a northwest of Hartford, while Groton is slightly east of the mouth of the Connecticut river. These two locations are identified by circles in the 0900 E S T map for July 18 (Figure 2a). Referring to Figure 2, it is clear that the 0 3 concentration remains comparatively low during the afternoon a t Simsbury, even while concentrations of 200 ppb were recorded a t Hartford only 15 miles to the southeast. By 2100 EST, however, higher concentrations of 0,j had reached Simsbury. Air mass trajectories (23) show Groton receiving urban air most of the day but Simsbury not until evening. The late arrival of the urban air at Simsbury may explain the 0 3 behavior. A plot of the fluorocarbon-11 (F-11) profiles at Simsbury and Groton shown in Figure 3 confirms this. (Fluorocarbon-11emissions are related to population density and are used here to distinguish rural air from air which has passed over urban areas. A later section will discuss the F-11 data in more detail.) The F-11concentration increases to very high levels during late morning and early afternoon a t Groton, indicating a direct influx of polluted urban air. As might be expected, the concentration of 0 3 also reached very high values a t 1,500 that afternoon. (Due to the reaction time necessary to generate 0 3 , we should not always expect the F-11 and 0 3 peaks to occur simultaneously.) Based on the F-11 profile in Figure 3 , Simsbury did not receive its infusion of urban air until 1900-2000 EST. For this reason, the Simsbury 0 3 peak did not occur until dusk (132 ppb at 2000 EST). Since the reactions forming 0 3 are completed by this time, the peak 0 3

400

c

Simsbury

h

E t 100

9

.OOt

tt

I

1

\

2w

IcO

o

I

I

9

IO

l II

I 12

I3

I 14

1

1

I

I

15 I6 17 I8 19 Time of D a y , (EST!

~ 20

I PI

22

~ 23

~ 24

Figure 3. Fluorocarbon-1 1 profiles for July 18, 1975

and F-11 concentrations occur simultaneously at 2000 EST a t Simsbury. The occurrence of such high levels of 0:3 after dark in a rural area like Simsbury, combined with the simultaneous peak in the urban air tracer, F-11, is very strong evidence of the transport of urban pollution to rural areas. The meteorological situation on July 19, 1975, was similar to July 18. Wind flow was still from the southwest throughout most of the day, but the wind speed was somewhat greater. Ozone distribution maps for July 19 are shown in Figure 4. The ozone patterns at 0900 E S T show the effect of the previous evening’s high ozone in the northeastern part of the region. It is uncertain how much of this ozone is truly residual, that is, surviving from the previous evening, and how much is a result of an increase in the morning rate of ozone formation due to a more favorable N02/NO ratio caused by nighttime reaction of residual 0 3 with NO. The diurnal ozone profiles from such sites as Fitchburg and Lowell, Mass., indicate that significant surface concentrations of 03 did exist overnight a t these locations. Since the overnight concentration of ozone within stable layers aloft was probably even higher than the surface concentration, it seems plausible that much of this morning ozone is actually left over from the previous evening. By noon, there is a definite intrusion of ozone-rich air into Connecticut from the southwest. The greatest concentrations exist around Bridgeport, but it is clear that the entire southern portion of the Connecticut River Valley from New Haven t o Hartford is affected. At 1500 E S T the highest levels of O3 are in the vicinity of Hartford, about 40 miles northeast of Bridgeport. However, high concentrations of 0 3 (>lo0 ppb) exist within a band from southwestern Connecticut t o northeastern Massachusetts. The fact that wind speeds averaged more than 30 mph throughout the day is entirely consistent with the hypothesis that this band of high ozone represents the smeared-out urban plume from the urban complexes in New Jersey, New York, and southwestern Connecticut. Table I1 shows representative wind information Volume 13, Number 8, August 1979

977

l

I

C


Table II. Winds at 1000 ft on July 19, 1975 lime

location

0700 0900 1700

Chatham, Mass. Avery Point, Conn. Avery Point, Conn. Chatham, Mass.

1900

speed, mph

direcllon, deg

30 25 33

250 247 247 230

41

for July 19. The band of high 0:j is somewhat smaller and has moved north and slightly eastward by 1800 EST. The highest 0 3 a t this time is a t Fitchburg, Mass. Since the photochemical reactions which generate O3 are essentially terminated by this time, some scavenging and decay of 0 3 are undoubtedly occurring; thus, the band of high concentrations is shrinking. By 978


2100, all of the surface stations report concentrations less than 100 ppb. Highest concentrations in the region are in the rural areas of central Massachusetts and northwestern Connecticut, and are clearly the residue of the ozone band observed entering these areas a t 1800. By midnight, concentrations throughout the region are less than 60 ppb. The 0 3 concentration within the urban plume is superimposed on the 0 3 that would form in the absence of the plume (about 70 ppb in this case judging from the concentrations on either side of the plume). The ozone which exists outside the urban plume may result from local precursor emissions, long-range transport (i.e., associated with a high-pressure system), natural sources, or some combination of the three. The interaction and superposition of 0:3and precursors from these sources were discussed in our 1974 Middle West Study report ( 4 ) .


Some feeling for the spatial extent of urban plumes can be gained from the ozone maps for July 18 and 19. On July 18 the maximum diameter (perpendicular to the wind flow) of the urban plume ( 0 3 > 100 ppb) was between 50 and 80 miles. The length of the plume was in excess of 100 miles. On the 19th, with considerably higher wind speeds, the plume diameter was only 30-40 miles, with a length greater than 175 miles. Of course, the spacing between the urban centers in New Jersey, New York, and southern Connecticut contributing to this plume undoubtedly contributes to its overall length. Additional information on ozone transport within urban plumes may be gained from an examination of the ozone distribution maps for July 23 and 24, 1975, shown in Figures 5 and 6. A high-pressure system moved eastward through southern New England on July 23 resulting in a southwest to westsouthwest surface flow pattern. By July 24 the high began to

dissipate and winds became more southernly. Representative wind data a t 1000 ft are presented in Table 111. Air mass trajectories for these 2 days (23)show that air arrivingin Simsbury and Groton during the early part of July 23 had passed through the fairly rural south-central areas of New York State. By midafternoon, however, the flow had shifted to the southwest, and the 1900 E S T trajectory a t Groton passed directly over the New Jersey-New York-Connecticut urban complex known as the New York metropolitan area. The air arriving in Simsbury passed well north'of the urban complex. Sets of forward trajectories for New York City and Philadelphia were reported by Wolff et al. (20) and are included here as Figures 7 and 8. (The circled numbers shown along the trajectory paths indicate the air mass position every 6 h after the starting times.) The New York City trajectories show that air leaving the city after 0800 moves into Connecticut from the southwest. Air leaving the metropolitan area during the Volume 13, Number 8, August 1979 979

I

/


morning peak traffic period (trajectory B) arrives in the Bridgeport-New Haven area by midafternoon. Air from Philadelphia (Figure 8) has little impact on Connecticut during July 23, but does enter the state early on the morning of the 24th after passing through the New York metropolitan area. Figures 5a-c show the ozone distribution maps for July 23. Ozone a t 0900 is less than about 60 ppb throughout southern New England. At noon, very high levels of 0.7 are observed entering the region from the southwest. By 1500 a plume of elevated 0.7 (>lo0 ppb) is found from southwestern Connecticut to eastern Massachusetts. The highest concentr'ations, and these were the highest levels observed during the 1975 Northeast Oxidant Study, are found near New Haven. Recall that the New York forward trajectories predicted that the air over the city during the morning rush hours would arrive in New Haven by midafternoon. This corresponds 980


precisely with the New Haven ozone maximum. Undoubtedly New Haven's emissions also contribute to the ozone maximum, although the data are not sufficient to permit one to distinguish its contribution from New York's. The 0 3 and precursors moving into the source area (New Jersey-New York) in the morning may also contribute to the ultimate ozone burden within the urban plume. On some days this contribution can be significant, for example, when the Philadelphia urban plume overlaps the New York metropolitan area. The trajectories suggest this is not occurring on the afternoon of July 23. Other sources of 0 3 and precursors, such as regional 0 3 associated with high-pressure cells, also contribute to the ultimate concentration within the plume. However, judging from the 0 3 levels outside of the urban plume during the afternoon of July 23 (70-85 ppb), it is clear that the dominant source of the very high (>200 ppb) 0 3 within the plume is

Table 111. Winds at 1000 ft on July 23-24, 1975 tlme

location

speed, mPh

/;I I

dlrection, deg

0700 0800 0822 0830 1300 1355 1400 1900

Winds at 1000 ft on July 23, 1975 Chatham, Mass 14 Springfield, Mass. 8 Putnam, Conn. 7 Avery Point, Conn. 16 Chatham, Mass. 17 Avery Point, Conn. 17 Springfield, Mass. 17 Chatham, Mass. 20

270 255 267 305 250 262 205 260

0700 0809 0183 1340 1400 1900

Winds at 1000 ft on July 24, 1975 Chatham, Mass. 28 Springfield, Mass. 15 Putnam, Conn. 13 Avery Point, Conn. 15 Springfield, Mass. 25 Chatham, Mass. 36

250 228 230 240 184 205

MAS S A C H U S E T T S

TI .ojectory Starling 1‘ime ( E S T )

+ cp G

0100 0700 1300 1900

A 0 C D

0

c \

4

upwind urban emissions. By 1800 E S T the photochemical reactions producing 0:jare terminated, yet extensive portions of Connecticut, Rhode Island, and parts of Massachusetts are still experiencing high O:].T h e center of the O:{ distribution extends across the Connecticut Valley from New Haven to the northeast corner of the state. Note that Groton experiences high levels of 0:i during the afternoon, levels about twice those found a t Simsbury. These observations are consistent with the trajectory analysis mentioned earlier, which showed Groton receiving direct input of urban air, in contrast to Simsbury, which experienced relatively clean rural emissions. By 2100 E S T the urban plume extends from the eastern Connecticut valley up to northeastern Massachusetts. Three hours later, a t midnight, high concentrations exist only in eastern Massachuset.ts north and west of Boston. This is about the distance that the wind speed data from Table I11 predict the morning New I’ork air mass would travel in the intervening 15-16 h. We will shortly use aircraft data to track this air mass further north and east of Massachusetts. Before doing so, however, we will first discuss the July 24 0 : j distribution maps. The Simsbury and Groton air mass trajectories for July 24, 1975 (23),indicate that the air arriving a t Simshury during most of the day had passed over the major metropolitan complexes of New York and Philadelphia, and to some extent even Washington and Baltimore. T h e air arriving in Groton during the daytime and evening hours has passed near Baltimore, up the Atlantic coast of New Jersey, and across eastern Long Island. Groton apparently received very little input from New York and Philadelphia on ,July 24. T h e forward trajectories from Philadelphia, shown earlier in Figure 8, are in substantial agreement. Air which left Philadelphia at 1300 E S T on July 23 passes over New York in the middle of the night and enters Connecticut in the early morning hours of July 24. On a day such as this it is very possible that an overlapping of urban plumes within the Washington-Boston corridor could occur. Thus, O:{entering southern New England on July 24 could be the result of emissions from several upwind urban areas. These overlapping urban plumes will be superimposed on any regional O;{which might result from emissions several hundred miles upwind (e.g., the Middle West). T h e ozone distribution maps for ,July 24 are pictured in Figures 6a-c. At 0900 we can see what are probably the remains of the previous day’s high ozone band. Ozone from 1 day frequently survives overnight by being trapped aloft above

+

0

Ozone sources and transport in the northeastern United States

Recommend Documents