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Stratospheric ozone in the lower troposphere—II. Assessment of downward flux and ground-level impact
A!moqheric Enuirmenf
OW4981/83
Vol. I?. No. IO, pp. 1979-1993, 1983
$3.00 + 0.00
0 1983 Fergamon Press Ltd.
Printed inGreatBbain.
STRATOSPHERIC OZONE IN THE LOWER TROPOSPHERE-II. ASSESSMENT OF DOWNWARD AND GROUND-LEVEL IMPACT WILLIAM VIEZEE, WARREN B. JOHNSON
and
FLUX
HANWANT B. SINGH
Atmospheric Science Center, SRI International, Menlo Park, CA 94025, U.S.A. Ahstraet-Aircraft m~surements of four stratospheric O,-intrusion events (two during spring and two during fall) are used in conjunction with concurrent meteorological analyses to estimate the downward 0, flux in the upper and lower troposphere. The aircraft measurements used are among those reported earlier in Part I of this paper (Johnson and Viezee, 1981).The calculated upper tropospheric fluxes for the four cases show good agreement with earlier estimates by Danielsen and Mohnen (1977) of 0% fluxes associated with tropopause folding events. The estimated lower-tropospheric 0, fluxes for the two spring events and for one fall event suggest that more than half of the 0, mass injected into the upper troposphere by these stratospheric intrusions is probably mixed and diluted into the troposphere above 700 mb (3 km ASL). Large, direct impacts of stratospheric 0, intrusions at ground-level are thus unlikely. A review and analysis of the limited number of published observations of high 0, in stratospheric intrusions, and of anomalously high 0, at ground level attributed to stratospheric intrusions, also suggests that direct ground-level impacts may be infrequent (less than 1 per cent of the time), and most likely are associated with 0, concent~tions (v/v) of 100 ppb or less. Additional observational studies are required to conclusively quantify the ground-level impact of stratospheric 0,.
1. INTRODUmION
In an earlier pubii~tion (Johnson and Viezee, 1981), aircraft measurements of atmospheric 0, made over the central United States in the spring and fall of 1978 were described in terms of the structure of stratospheric O3 intrusions into the middle and lower troposphere. The meteorological conditions associated with the intrusion events were presented, and inferences were made as to mechanisms by which the stratospheric O3 can reach ground level. This paper is the sequel to the publication by Johnson and Viezee (1981). It addresses, specifically, the downward flux of the observed stratospheric 0, in the troposhere, and the probable impact of stratospheric/tropospheric intrusions on ground-level air quality*.
2. DOWNWARD FLUX OF STRATOSPHERIC OZONE
2.1. Upper troposphere Using the in-situ SRI aircraft measurements made during the 1978 field program in conjunction with concurrent meteorological analyses (Viezee et a/., 1979), estimates have been calculated of the downward flux of stratospheric 0, into the upper troposphere associated with four intrusion events: two each ob-
* Results have been obtained from research projects sponsored by the Coordinating Research Council, Inc., under Contract CRC-APRAC Project No. CAPA-15-76 (l-77); and by the Environmental Science Research Laboratory of the U.S. Environmental Protection Agency under Grant CR 809330010.
served in the spring and in the fall. The four cases were selected on the basis of their detailed structure of relatively high O3 concentrations at the maximum flight altitude sampled (7.3 km ASL). The flux estimates were calculated by expressing the downward mass flux of 0, through an upper level of the intrusion as: -.F = M(O,)*W.A (1) where F = downward flux of stratospheric O3 (moiecules per unit time) through upper-level intrusion area, M(0,) = molecules of 0, per unit volume averaged over intrusion area, W= downward vertical velocity averaged over intrusion area and A = horizontal area of intrusion. Equation (1) assumes that the downward 0, flux is controlled primarily by the organized mean circulation within the intrusion, rather than by small-scale, turbulent fluct~tions. This assumption is necessary because of the limited observational data available for our study. Although Shapiro (1978, 1980) has indicated that smaller-scale tuibulent components may have a significant role in the dynamics of an intrusion, the ConsistentIy well-defined structure of the intrusions measured during our field program lends support to our assumption. Upper-air meteorological analyses required for the flux computations were obtained from the U.S. National ~eteorologi~l Center (NMC). Since the maximum flight altitude of 400 mb (7.3 km ASL) is not a standard level that is routinely analyzed by the NMC,
1979
1980
WILLIAM VIEZEE rt a/.
the 300 mb (9.0 km ASL) isobaric charts were used in the computations. 2.1.1. Spring events of 13 May and 19 May 1978. Figure 1 shows the selected spring event of 13 May 1978, 14: 34-19: 19 CST. This event represents the most intense stratospheric O3 intrusion observed during the 1978 SRI aircraft program. The twodimensional cross section of 0, [Fig. l(c)] was documented between Muscle Shoals, Alabama and Meridian, Mississippi, and extends from the maximum flight altitude of 7.3 km ASL down to 3.3 km ASL. 0, concentrations (v/v) at the upper altitude are as high as 280ppb, while at the lowest flight altitude a thin lamina of 0, is observed with concentrations (v/v) of lo&120 ppb. A detailed analysis and interpretation of the vertical cross section of measured O,, and the conditions associated with meteorological it [Fig. 1(b)] is presented by Johnson and Viezee (1981). Figure l(a) shows the pattern of downward vertical motion at 300 mb near the time of the aircraft observations (13 May 18:00 CST or 14 May 0O:OOGMT), computed from the standard NMC meteorological analyses and using the adiabatic method (Holten, 1979). Application of the adiabatic technique to the upper tropospheric level is justified because the descending air parcels of a stratospheric air intrusion are characterized by isentropic (adiabatic) flow (Danielsen, 1968). If it is assumed that the area of computed downward motion represents the total stratospheric-intrusion area at about 9 km ASL, then stratospheric air of high 0, concentration and low dewpoint descends through this area into the upper troposphere. A value for the O3 density in this area is estimated by extrapolating to the 9 km level the trend of increasing 0, with altitude which was observed by the aircraft in the intrusion below 7.3 km [Fig. 1(c)l. This extrapolated 0, density (valid only along the flight path) is applied to the entire intrusion area. Using this procedure, the downward 0, flux into the upper troposphere below 300 mb can be computed from Equation (1). A similar procedure to compute the downward 0, flux into the upper troposphere below 300 mb is applied to the second spring event of 19 May 1978,13:1 l-17:40 CST. Results are tabulated in Table 1. The downward 0, flux for the intrusion event of 13 May is about twice as large as that associated with the second event of 19 May, primarily because of a larger estimated intrusion area at 300 mb. This difference in the downward flux between the two spring events is compatible with a corresponding difference in intensity of the upper-tropospheric low-pressure troughs, as discussed by Johnson and Viezee (1981). Using the daily series of 500 mb analyses from the NMC for 1978, the number of migratory low-pressure troughs of the type associated with the two spring events [Fig. 1(b)] were counted for the entire northern hemisphere, and for the area of North America between longitudes 7o”W and 16o”W. An average number of 4.5 travelling cyclones per day for the
northern hemisphere, and an average number of 1.0 per day for the North American continent and the eastern Pacific Ocean, was determined for the month of May. It is assumed that the area of the intrusion and its intensity at 300 mb do not significantly change over a period of one day. Thus, the spring fluxes of Table 1 suggest a cumulative daily 0, flux from the stratosphere into the troposphere (during May) ranging from 1.9 to 3.5 x 1O34 molecules for the northern hemisphere; and from 0.4 to 0.8 x 1O34 molecules for the area of North America and the eastern Pacific Ocean, between 7o”W and 16o”W. 2.1.2. Fall events of 5 October and 14 October 1978. Figure 2 shows the fall event of 14 October 1978, 11:54-16:52 CST. This event is selected for illustration because the two-dimensional O3 structure [Fig. 2(c)] observed during the aircraft flight from southern Illinois to northern Mississippi was mapped in considerable detail in the upper troposphere above 500 mb (5.5 km ASL). Values of O3 concentration as high as 175 ppb and 201 ppb are present along the maximum flight altitude (7.3 km ASL). The 500-mb low-pressure trough [Fig. 2(b)] is typical of those that were found to be associated with stratospheric 0, intrusions. Figure 2(a) shows the pattern of downward vertical motion at the 300 mb level computed by the adiabatic method. As in the spring cases of 13 and 19 May, it is assumed that OS-rich stratospheric air descends through this area into the upper troposphere. Following a similar procedure as outlined above for the two spring events, computations leading to the downward 0, flux are listed in Table 1. Also tabulated are values of intrusion area, vertical motion and downward O3 flux, for the fall event of 5 October 1978, 17: 15-21:37 CST. This event is discussed in detail by Johnson and Viezee (1981) and was observed to penetrate into the lower troposphere down to 2 km ASL, which is near the top of the boundary layer. The downward 0, flux for the event of 14 October is twice as large as that for the case of 5 October because of the larger vertical (descending) motion through the intrusion area. From the daily series of NMC 500 mb analyses for October 1978, an average number of 4.0 low-pressure troughs per day was determined for the entire northern hemisphere; and an average number of 1.0 troughs per day was determined for North America between longtitudes 7o”W and 16o”W. If it is assumed as before that the area of the intrusion [Fig. 2(a)] and its intensity do not appreciably change over a period of one day, then cumulative daily downward flux of ozone into the upper troposphere during October can range from 0.5 to 1.0 x 1O34 molecules for the northern hemisphere, and from 0.1 to 0.3 x 1O34 molecules for the area between 7o”W and 16O”W. 2.1.3. Summary of upper tropospheric fluxes. Figure 3 summarizes the estimates of the stratospheric/
Stratospheric
(al
SOONBAR
PATTERN
OF DESCENDING
(et CROSSWIND Fig.
1. Spring
event
of
ozone in the lower troposphere-II
STRUCTURE
VERTICAL
MOTION
(cm sec”l NEGATIVE-DOWNWARD)
OF OZONE CONCENTRATION
observed ozone intrusion stratospheric/tropospheric
1981
(13 May 1978, 14:34-19: ozone flux is estimated.
(ppb) 19 CST)
for
which
the
WILLIAM VIEZEE et al.
1982
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t
(a) 300.MBAR
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PATTERN
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STRUCTURE
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OF OZONE CONCENTRATION
observed ozone intrusion (14 October, 1978, 11:5+16: stratospheric/tropospheric ozone flux is estimated.
(ppb) 52 CST)
for
which
the
Stratospheric ozone in the lower troposphere-II
SRI DATA -DANlELSEN
119’ A
MOHNEN
(197
8
1983
vertical column of the atmosphere is measured by surface-based instruments, such as the Dobson spectrophotometer. Approximately 70’;/ of the total column 0, content occurs in the stratosphere below 30 km, while only about 7 ‘x occurs in the troposphere. 2.1.4. Interpretation of trapospheric upper fluxes An annual average contribution of strato-
JFMAMJJASOND MONTH
Fig. 3. Estimated annual variation of the monthly stratospheric/tropospheric total ozone flux from tropopause folding events in the northern hemisphere.
tropospheric 0, fluxes obtained from the observational data of the four intrusion events. Values for May based on the spring events, and for October based on the fall events, are compared with the variation of the monthly stratospheric 0, flux into the northern hemispheric troposphere published by Danielsen and Mohnen (1977). Fluxes are expressed in units of 1O35 O3 molecules per month per tropopause folding event (TFE). Danielsen and Mohnen (1977) maintain that mass transport associated with tropopause folding events (stratospheric intrusions) is the dominant mechanism for injecting stratospheric ozone into the troposphere. Their opinion is supported by extensive analyses of tropopause folding events, and by comparing 90Sr activities in extruded stratospheric air with observed ground deposition rates (including the seasonal cycle) in the United States. The outstanding feature of Fig. 3 is the spring maximum and the fall and early winter minimum in the O3 fluxes, which is evident in both our case-study calculations, and the data of Danielsen and Mohnen (1977). This spring-to-fall variation closely resembles the seasonal variation that has been observed in the background of natural O,, both in the free troposphere and near ground level (e.g. Singh et al., 1978; 1980). Also, there is a striking similarity between the seasonal trend in the stratospheric/tropospheric ozone flux of Fig. 2, and the seasonal change in the stratospheric reservoir of natural 0, as shown in Fig. 4. This figure (Wu, 1973) compares the annual variation of the monthly mean total 0, content at Mauna Loa, Hawaii, (19.32”N, 15536”W); Tallahassee, Florida (30.26”N, 84.2o”W); and Green Bay, Wisconsin (44.29”N, 88.08”W). The total amount of 0, in a
spheric 0, to the troposphere of about 14 ppb is obtained from the flux data of Danielsen and Mohnen (1977). This is derived from a simple ‘box’ model for the troposphere that extends from the equator to the pole, and from the surface to 300 mb (9.0 km ASL). This estimate assumes a mean 0, residence time of about 1 month (Singh et al., 1980). Our average values of stratospheric O3 flux into the upper troposphere (estimated from the two May events and the two October events), are very similar to those given by Danielsen and Mohnen. Thus, the 14 ppb annual average tropospheric background of 0, from the stratosphere estimated from their data is not changed significantly by considering our data analyses and calculations. On the basis of ground-level measurements, Singh et al. (1978) found that the yearly average background of natural O3 is about 30 ppb. Since natural O3 can originate from sources other than the stratosphere (such as through photochemical processes involving atmospheric hydrocarbons emitted from natural sources), the estimate of 14 ppb for O3 of stratospheric origin is consistent with Singh’s value of 30 ppb for all natural 0, in the troposphere. Clearly, these results are very approximate, considering the simplified nature of our approach, and the fact that only two cases for each spring and fall season are analyzed in detail. A more sophisticated modeling approach, applied to additional cases, is required. Also, measurements of the spatial extent of intrusions (beyond the two-dimensional plane to which our aircraft observations are limited) need to be incorporated in order to improve the flux computations. 2.2. Lower troposphere There is great interest in determining how much of the natural O3 transported by a stratospheric intrusion into the upper troposphere arrives in the planetary boundary layer, from where it can impact ground-level 0, concentrations. Several investigators maintain that stratospheric air reaches ground level in about two days by way of the surface high-pressure system that follows travelling upper tropospheric low-pressure troughs (e.g. Reiter and Mahlman, 1965; Danielsen, 1980; Wolff et al., 1979). If this concept is correct, it will be difficult to quantify (on the basis of measurements) the stratospheric component of the near-surface 0, budget, since high-pressure areas also are favourable for air stagnation and surface transport of anthropogenie 03. To further clarify the impact of a stratospheric O3 intrusion on the lower troposphere (700mb and below), a better understanding is required of the three-
1984
WILLIAM
VIEZEE
have combined the aircraft-observed 0, intrusions of the two spring events (13 May and 19 May 1978), and the fall event of 5 October 1978, with concurrent threedimensional air-parcel trajectories. These trajectories are computed from operational wind forecasts generated by the Limited-Area Fine Mesh numerical prediction model of the National Meteorological Center (Reap, 1972). Computations are made backward in time at 6-h intervals, for a total of 24 h, starting from a forecast point and proceeding to the origin point. The starting points are at the 700-mb level, the 850-mb level and the surface. The trajectories are routinely computed for 73 locations across the United States. The air-parcel positions (latitude/longitude) at 6-h intervals are transmitted twice daily at 12OOGMT and 0000 GMT, via NWS facsimile and teletype circuits,
dimensional circulation associated with such intrusions. Three-dimensional air-parcel trajectories are particularly important. Thus far, trajectory analyses made in connection with studies of tropopause folds have been based on the assumption that a given air parcel moves on a constant isentropic surface, i.e. that the total energy of the parcel remains constant (adiabatic flow). This assumption becomes untenable when approaching the planetary boundary layer, where diabatic effects from radiative processes and turbulent mixing become important. In this section, we present some preliminary results of additional research on the impact of stratospheric O3 in the lower troposphere. 2.2.1. Intrusion area in the lower troposphere. We 440
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JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
Fig. 4. Monthly mean total ozone for the 1 l-year period between January 1960 and December 1970 at Mauna Loa, Hawaii; Tallahassee, Florida; and Greenbay, Wisconsin (Wu, 1973).
Stratospheric ozone in the lower troposphere-II under the heading “FOUS50-57 Bulletins”. Thus, using these readily available operational data, an analysis can be made of the predicted areas of ascending and descending air parcels for three levels in the lower troposphere. Figure 5 shows a sample of predicted 24-h trajectories that terminate at 700 mb at the receptor locations of Houston, Texas (Trajectory l), Mobile, Alabama (Trajectory 2), and Birmingham, Alabama (Trajectory 3), near the time (13 May, 18:OOCST) that the 0, intrusion of Fig. l(c) was observed. The aircraft flight path between Muscle Shoals, Alabama and Meridian, Mississippi, along which the two-dimensional cross section of ozone structure was observed, is indicated in Fig. 5. Trajectory 1 is outside the area of aircraft observations, and is disregarded. Trajectory 3 intercepts the vertical cross section at 694 mb (3 km ASL) about 46 km south of Muscle Shoals. According to the 0, cross-section of Fig. l(c), this intercept point is below and outside the observed 0, intrusion. Thus, no stratospheric characteristics can be assigned to the air parcel of this trajectory. Trajectory 2 intercepts the vertical plane of the cross-section at 690 mb (3 km
Fig. 5. 24-hour
three-dimensional
1985
ASL) about 92 km south of Meridian. No aircraft observations are available near the location of this intercept point, and, thus, no direct information is available on the stratospheric origin of the air parcels of Trajectory 2. Figure 5, however, only presents three specific trajectories that illustrate the existence and characteristics of a synoptic-scale, descending air mass into the region of aircraft observation. Additional trajectories can be obtained through interpolation. Using this interpolation technique between the airparcel pressure-level data of Trajectories 2 and 3, the 24-h path of the air parcels that intercept the lowaltitude lamina of observed stratospheric 0, (80 to 100 ppb) on 13 May, 18:OOCST can be obtained. In other words, the available three-dimensional trajectory data enable an estimate of the geographic location and atmospheric height where the stratospheric 0, (observed at low altitude near Meridian, Mississippi) originated. Our interpolation of the trajectory data suggests that the observed stratospheric 0, originated in South Dakota and Nebraska at altitudes at and above 550 mb, 24 h earlier on 12 May, 18:Cil CST. We now proceed by using all available 24-h trajec-
air parcel trajectories (12 May 18 : 00 CST--13 May 18 : 00 CST) relevant stratospheric ozcme intrusion of Fig. 1.
to the observed
WILLIAM VIEZEE et il.
1986
tory data for the period 12 May, 18:OUCST to 13 May, 18:00 CST that terminate in the region of the aircraftobserved low-altitude (700mb) O3 intrusion of Fig. l(c). The objective is to identify the total area in which the descending air parcels are predicted to originate (from the 550mb level or above) in South Dakota and Nebraska. This area represents our best estimate of the horizontal extent of the stratospheric intrusion near 700 mb. Figure 6 shows the total area of descending air-parcel trajectories at the 700-mb level which was predicted for 13 May, 18:00 CST using the data from Bulletins FOUS55, 56 and 57. Isolines are labeled in terms of the pressure level (millibars) from which the air parcels originate 24 h earlier (12 May, 18:OOCST). The shaded part represents the area within which the air parcels are predicted to originate in South Dakota and Nebraska, from atmospheric heights of 550 mb and above. According to our analysis technique, air parcels within this 700 mb (300 m ASL) area are presumed to be of stratospheric origin, and
associated with the stratospheric
0,
j
500
intrusion
ob-
served between Muscle Shoals, Alabama and Meridian, Mississippi. A most interesting result of our analysis technique is that Fig.6 shows the presumed shape of the stratospheric air intrusion at the 700-mb level. The analysis suggests that the 0, structure [observed by the SRI aircraft in the two-dimensional plane of Fig. l(c)] extends much further southward from Meridian, Mississippi, and covers a large area along the Gulf Coast from Houston, Texas, to Tallahassee, Florida. Stratospheric 0, descends through this area into the lower troposphere. A similar analysis as described above for the spring event of 13 May 1978 was made for the case of 19 May 1978, 13:11-17:40CST, and for the fall event of 5 October 1978, 17:15-21:37 CST. These three cases were analyzed because the intrusions were observed to penetrate into the lower troposphere to levels near 700mb where the available trajectory data are applied (Johnson and Viezee, 1981). Figure 7 summarizes the presumed intrusion areas for the three cases super-
_: _.
KM
Fig. 6. Area of descending air parcel trajectories near the time that the stratospheric
through the 700-mb level into the lower troposphere as predicted ozone intrusion of Fig. 1 was observed (13 May, 18 : 00 CST).
by LFM-II
Stratospheric ozone in the lower troposphere-II
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1988
WILLIAM V~EZEE et ai.
posed on the 700~mb contour charts. The pressure levelsfrom which the descending air parcels originate 24 h earlier are indicated in mb. The most intense areas of predicted descending air of stratospheric origin (darkest shading) are located behind the surface weather front. This feature is consistent with the concept that frontal downdrafts or post-frontal subsidence can transport stratospheric 0, to ground level (e.g. Lamb, 1977; Derwent r6 ul., 1978; Johnson and Viezee, I98 1)~ 2.22. Luwer-rroposphetic mm fluxes. According to Equation (11,the downward flux of O3 through the intrusion areas illustrated in Fig. 7 equals the product of the area, the area-averaged 0, concentration, and the area-averaged vertical velocity. These values are listed in Table 2 for each case. The 0, concentration in the intrusion area jcalumr~ 3) is obtained either from the direct observations by Johnson and Viezee (198l), or is estimated from an extrapolation to 700 mb following the downward slope of the aircraft-observed O3 iamina [see Fig. l(c)]. The descending motion averaged over the intrusion area (column 5) is computed from the trajectory data that provide the pressure level of the descending air parcels at intervals of 6 h. The intrusion event observed on I3 May 1978 is the most intense. The predicted downward flux of 0, through the 7OO-mb level associated with this spring event (2.5 x 10” molecules s- ‘) is three times as Iarge as that computed for 19 May. The fall event of 5 October is associated with the relatively largest area of descending motion at 7OOmb (8.5 x 10” m2). The computed descent rate, however, is small (0.5 cm s- ’ f, and the observed 0, concentration is only 7&90 ppb. Consequently, the downward O3 flux is only about 0.65 x 102* moleculess- ’ for this fall event.
2.2.3. twzpfications of atone flux caicuhtions. Comparison between the 0, flux calculations in the upper troposphere (Table l), and in the lower troposphere (Table 2) for the 2 spring events (13 and 19 May), and the fall event of 5 October, permits an estimate of the amount of stratospheric O3 that may end up in the lower traposphere below 700 mb. For example, in the case of 13 May 1978, the intrusion area at 300 mb is approximately 1.8 times larger than the estimated area at 700 mb, while the downward vertical motion is twice as large. Consequently, the computed downward flux of 0, at 300 mb is 3.6 times larger than that at 700 mb. These computations imply that, for this particular spring intrusion event, less than l/3 of the stratospheric 0, mass injected into the upper troposphere may be transported directly into the lower troposphere below 700 mb. In other words, more than 2/3 of the stratospheric 0, appears to be mixed and diluted into the troposphere above 700 mb; from there it can stiI1 impact the lower troposphere, but only by indirect circulations. After comparing the upper and lower tropospheric
1989
Stratospheric ozone in the lower troposphere--II Table 2. Downward flux of ozone predicted at 700 mb for three aircraft-observed events of stratospheric ozone intrusion into the lower troposphere
Intrusion event (1978)
Time period of observation (CS-U
Observed or estimated ozone concentration at 700 mb (ppb)
Predicted area of intrusion at 700 mb (10” m’)
13 May 19 May 5 October
14:35-19:19 13:11-17:40 19:15-22:23
8GlOO 8fklOO 7&90
7.7 6.1 8.5
0, fluxes for the other two evenu, our analysis, although very limited in scope, does not suggest a large, direct ground-level impact. In the next section we summarize the results from other published studies which bear on this question. 3. REVIEW OF EXISTING OBSERVATIONALDATA 3.1. Observations oj stratospheric ozone intrusions into the troposphere The structure of stratospheric air intrusions which transport natural ozone into the troposphere by direct circulation and the associated meteorological conditions, are described in detail by Danielsen (1968), Danielsen et al. (1970) Shapiro (1974) and Johnson and Viezee (198 1) on the basis of aircraft observations and by Mahlman (1973) on the basis of an early numerical experiment. These intrusions or tropopause folds have been observed mostly during spring (e.g. Danielsen et al., 1970; Danielsen and Mohnen, 1977; Shapiro, 1974, 1978, 1980; Johnson and Viezee, 1981). Observations during summer are documented by Danielsen (1980), Danielsen and Hipskind (1980) and during fall by Reiter and Mahlman (1965) and by Johnson and Viezee (1981). Table 3 lists published observations of relatively high 0, concentrations in the troposphere specifically associated with stratospheric/tropospheric air intrusions during spring (February, March, April and May), during summer (August) and during fall (October). The 0, concentrations that are tabulated represent the highest values at the lowest tropospheric levels reported by the referenced source. The observations cover various locations across the United States, from the Mississippi Valley westward to the eastern Pacific Ocean. To our knowledge, the only published observational evidence of the injection of stratospheric O3 into the free troposphere is provided by these United States data, those obtained across Canada by the Gametag flights of 1977 (Danielsen and Hipskind, 1980), and the early observations over England by Briggs and Roach (1963). 3.2. Published events of stratospheric ozone at ground level Table 4 lists 10 publications that document observations and analyses of high 0, concentrations at
Averaer downward motion in intrusion area (cm s-r)
Predicted downward ozone flux at 700 mb (10Z8molss-‘)
-2.0 -0.8 -0.5
2.2-2.8 0.709 o.m.7
ground level that were attributed to short-term episodes of stratospheric air intrusions. It is noteworthy that none of these observations was associated with simultaneous aircraft flights which showed the presence, structure and intensity of the intrusion aloft. Instead, the listed values were attributed to an impact from stratospheric 0, on the basis of either meteorological analyses and air-parcel trajectories or simultaneous measurements of beryllium-7 (e.g. Haagenson et a/., 1981; Viezee et al., 1982; Husain et al., 1977; Dutkiewicz and Hussain, 1979; Kelley et al., 1981). Values of reported ground-level impact range from a maximum of 415 ppb (over a lo-min period) at an elevated observatory in Germany (Atmannspacher and Hartmannsgruber, 1973) to a minimum value of 56 ppb (over a l-h period) observed by Kelley et al. (1981) at Pierre, South Dakota, during the summer. The length of the data record which was reportedly examined for unusual occurrences of high groundlevel 0, is listed in Table 4 for each case. This information suggests that the events were sporadic, and generally can be expected to occur less than 1 I’,,of the time. 3.3. Summary analysis of ground-level impact In Fig. 8, the data of Table 3 are plotted as a function of height in the troposphere, irrespective of season and geographic location. Of specific interest is the apparent decrease in observed stratospheric 0, concentration with decreasing height in the troposphere. This is shown by a power curve fit to the 29 data points of the form y = axb where x is tropospheric height (mb) and y is stratospheric 0, concentration (ppb): O,(ppb)
= 7.63 x 10’ [z(mb)] _ 1.40.
This decrease is evident in all observational data published in the literature, and apparently reflects the effects of diffusion and turbulent mixing on the descending stratospheric air. The large deviations from the mean curve are probably indicative of a wide range in intensity of the observed tropopause folding events, as well as of seasonal effects. It is evident, from the data of Table 3 and the summary analysis of Fig. 8, that none of the investigators has actually followed a stratospheric ozone intrusion down to ground level. The lowest altitude at which measurements of 0, of documented stratospheric origin have been reported is 1.9 km ASL (800 mb), where Johnson and Viezee
England (50”N, 1”W to 56”N, 1”W) Duluth/Minneapolis, Minnesota
Geographic location
370 443 373 444
550,450 450 700,600,500 600,500,400 500 650, 500 625, 500 750, 500 600
Colorado Colorado/Oklahoma
Colorado/Utah Colorado
Californ~a/Oregon Northwest Texas California (Los Angeles) Churchill, Canada
Eastern Pacific Ocean Mississippi/Alabama
Minnesota
Tennessee
Minnesota
April 1971 April 1975
April 1976 March 1976 March 1978 August 1977
August 1977 May 1978
May 1978
October 1978
October 1978
450,400,300
Colorado/Wyoming
600,450
365,315
350
April 1969
February 1967 San Francisco. California
April 1966
May 1961
Date
Tropospheric level (mb)
loo
70,95
100, 150
80 to 100 100, 190
Aircraft
Aircraft
Aircraft
Aircraft Aircraft
Aircraft Aircraft Aircraft Aircraft
Aircraft
300 80, 180 80 90, 100, 150 63, 100, 150
Aircraft
Aircraft Aircraft
Aircraft
Balloon
Balloon
Aircraft
Measurement technique -
320
98 225
120, 180, 300
SO, 150
300, 145
120
Stratospheric 0, concentration (ppb) Source
Lovili and Miller, (1968) Danielsen et al. (1970) Shapiro (1974) Danielsen and Mohnen (1977) Danielsen and Mohnen (1977) Danielsen and Mohnen (1977) Shapiro (1978) Shapiro (1978) Shapiro (1980) Danielsen and Hipskind ( 1980) Danielsen (1980) Johnson and Viezee (1981) Johnson and Viezee (1981) Johnson and Viezee (1981) Viezee et al. (1979)
(1968)
Briggs and Roach (1963) Kroening et al.
Table 3. Published observations of stratospheric ozone in the troposphere during extrusion events
15 March
8
6 7
19 March 24.25, 28 and 1 July 4 March July 1978
1978
1977 June 1977 1978
11, 12 July 1975
9
4 10
19 November 1972 6 March 1974 S/9 January 1975
1971
1964
26 February
3 March
Date
3 4 5
1
Case No.
location
episodes
Kisatchie Louisiana
National
Forest,
Denver, Colorado Pierre, South Dakota
Quincy, Florida (near Tallahassee) Observatory Hohenpeissenberg (1OOOm MSL), SW of Munich, Germany Santa Rosa, California Harwell, Oxon. U.K. Zugspitze Mountain near GarmischPartenkirchen, Germany (3000m MSL) Whiteface Mountain, New York (1500m MSL) Sibton, Suffolk, U.K. Whiteface Mountain, New York
Geographic
Table 4. Published
0,
82 $56 < 46 lo(r105
10&l 10 < 41
< 31
415 250 200-230 llG115 16(X193
10@300
(ppb)
Ground-level 0, concentration
of stratospheric
to ground
lh lh 24 h av. 2h
2h 24 h av.
24 h av.
10 min 50 min Ih 2h 4h
3h
Duration of observed event
transport
1971
1973
Spring
1978
1975-1978 July-September
1978
45 y discontinuous June and July 1977
July 1975
November 1972 45 y discontinuous Aug. 1973-Feb. 1976
Dec. 197&May
July 1963-July
Length of data record examined
level
Viezee et al. (1982)
et al. (1981) (1981)
al. (1978) and Husain,
et al. (1977) Derwent et Dutkiewicz (1979) Haagenson Kelly et al.
Husain
Atmannspacher and Hartmannsgruber (1973) Lamb (1977) Derwent et al. (1978) Singh et al., (1980)
Davis and Jensen (1976)
Source
WILLIAM
1992
VIEZEE
et al.
350
400
450 2
; ” Y
500 550 600 650 700 750 2
800 850
2
03
200 CONCENTRATION
1
300 -
400
ppb
Fig. 8. Comparison between observations of high ozone concentrations in the free troposphere associated with stratospheric air intrusions (solid dots with power curve fit), and episodes of high O3 concentrations observed at ground level (numbered blocks) that are purported to be of stratospheric origin. Data sources are listed in
Tables 3 and 4.
(1981) measured 60 ppb (see Paper I). Figure 8 also includes the suspected episodes of stratospheric O3 at ground-level listed in Table 4. The numbered episodes refer to the case numbers in Table 4. (Cases 9 and 10 are not included in Fig. 8 because they involve 24-h average concentrations.) The ground-level concentrations of episode cases 4,6,7 and 8 are comparable with the concentrations actually observed in low altitude stratospheric 0, intrusions. For such concentrations of stratospheric O3 to occur at ground level, the stratospheric O3 concentrations (v/v) of 100 ppb at the 650 and 700 mb levels, would have to be transported directly to the ground over short-time periods by small-scale atmospheric circulations such as convection and turbulence, without significant additional dilution. Case 5 was observed on the Zugspitze Mountain in Germany, at an elevation of 3000 m ASL. It is plausible that the high 0, densities observed in stratospheric intrusions at 500 mb, and above, could directly impact such elevated locations. The data from Cases 1, 2 and 3 seem out of place because the ground-level concentrations are much higher than those observed in intrusions below the 500 mb level. If instrumental uncertainties are ruled out, these cases could be associated with intrusion events having low-altitude intensities that have not yet been observed; or with ground-level 0, concentrations contaminated by sources other than stratospheric. Obviously, more studies are required to better quantify the ground-level impact of stratospheric 0, intrusions. As a first approximation, however, the limited observational data summarized in Fig. 8 suggest that, except for high mountain locations, the ground-level impacts of stratospheric 0, are likely to
be no larger than 1OOppb. Measurements of higher concentrations, particularly at low-altitude locations, should be analyzed carefully for possible contamination from surface transport or local photochemistry. Acknowledgements-We wish to acknowledge the support and technical guidance received from the members of the CAPAProject Committee of the Coordinating Research Council, Inc. (Mr. Timothy C. Belian, Project Manager; Mr. Bruce S. Bailey, Committee Chairman) and from Dr. A. Paul Altshuller, Senior Science Advisor, Environmental Sciences Research Laboratory, U.S. Environmental Protection Agency. Special appreciation is due to Mr. Ronald M. Reap, Techniques Development Laboratory, National Weather Service, National Oceanic and Atmospheric Administration, who supplied the FOUS50-57 Bulletin data required to construct the predicted three-dimensional air-parcel trajectories used in our study.
REFERENCES Attmannspacher W. and Hartmannsgruber R. (1973) On extremely high values of ozone near the ground. Pure Appl. Geophys. (Pageoph), 106108,1091-1096. Briggs J. and Roach W. T. (1963) Aircraft observations near jet streams. Q. JI R. met. Sot. 89, 2255247. Danielsen E. F. (1968) Stratospheric-tropospheric exchange based on radioactivity, ozone and potential vorticity. J. atmos. Sci. 25, 502-518. Danielsen E. F., Bleck R. Shedlovsky J. Wartburg A. Haagenson P. and Pollock W. (1970) Observed distribution of radioactivity, ozone and potential vorticity associated with tropopause folding. J. geophys. Res. 75, 2353-2361. Danielsen E. F. and Mohnen V. A. (1977) Project dust storm report: ozone transport, in situ measurements and meteorological analyses of tropopause folding. J. geophys. Res. 82, 5867-5877.
Stratospheric
ozone in the lower troposphere-II
Danielsen E. F. (1980) Stratospheric source for unexpectedly large values of ozone measured over the Pacific Ocean during Gametag, August 1977. J. geophys. Res. 85, 401412. Danielsen E. F. and Hipskind R. A. (1980) Stratospheric-Tropospheric exchange at polar latitudes in simmer. J. geophys. Res. 85, 393-400: Davis D. R. and Jensen R. E. (1976) Low level ozone and weather systems. Proc. APCA Specialty Conference on Ozone/Oxidants interactions with the Total Environment, Dallas, Texas, March 1976, pp. 242-251. Derwent R. G., Eggleton A. E. J., Williams M. L. and Bell C. A. (1978) Elevated ozone levels from natural sources. Atmospheric Environment 12, 2173-2177. Dutkiewicz V. A. and Husain L. (1979) Determination of stratospheric ozone at ground level using ‘Be/ozone ratios. Geophys. Res. Lett. 6, 171-174. Haagenson P. L., Shapiro M. A., Middleton P. and Laird A. R. (1981) A case study relating high ground-level ozone to enhanced photochemistry and isentropic transport from the stratosphere. J. geophys. Res. 86, 5231-5237. Husain L., Coffey P. E., Meyers R. E. and Cederwall R. T. (1977) Ozone transport from stratosphere to troposphere. Geophys. Res. Lett. 4, 363-370. Johnson W. B. and Viezee W. (1981) Stratospheric ozone in the lower troposphere-I. Presentation and interpretation of aircraft measurements. Atmospheric Environment 15, 1309-1323. Kelly N. A., Wolf G. T. and Ferman M. A. (1982) Background pollutant measurements in air masses affecting the eastern United States-I. Air masses arriving from the northwest. Atmospheric Environment 16, 1077-1088. Kroening J. L., Rosen, J. M. and Mantis H. T. (1968) On the transport of material across the tropopause. J. geophys. Rex 73, 5463-5465. Lamb R. G. (1977) A case study of stratospheric ozone affecting ground-level oxidant concentrations. J. appl. Met. 16, 780-794. Lovill J. E. and Miller A. (1968) The vertical distribution of ozone over the San Francisco Bay area. J. geophys. Res. 73, 5073-5079. Mahlman J. D. (1973) On the maintenance of the polar front
1993
jet stream. J. atmos. Sci. 30, 544556. Reap R. M. (1972) An operational three-dimensional trajectory model. J. appl. Met. 11, 1193-1202. Reiter E. R. and Mahlman J. D. (1965) Heavy radioactive fallout over the southern United States, November 1962. J. geophys. Res. 70, 45014519. Shapiro M. A. (1974) A multiple structured frontal zone-jet stream system as revealed by meteorologically instrumented aircraft. Mon. Weath. Reu. 102, 244253. Shapiro M. A. (1978) Further evidence of the mesoscale and turbulent structure of upper level jet stream-frontal zone systems. Mon. Weath. Rev. 106, 110&1111. Shapiro M. A. (1980) Turbulent mixing within tropopause folds as a mechanism for the exchange of chemical constituents between the stratosphere and troposphere. J. atmos. Sci. 37, 99+1004. Singh H. B., Ludwig F. L. and Johnson W. B. (1978) Tropospheric ozone: concentrations and variabilities in clean remote atmospheres. Atmospheric Encironmenr 12, 2185-2196. Singh H. B., Viezee W. Johnson W. B. and Ludwig F. L. (1980) The impact of stratospheric ozone on tropospheric air quality. J. Air Pollut. Control Ass. 30, 1009%1017. Viezee W., Johnson W. B. and Singh H. B. (1979) Airborne measurements of stratospheric ozone intrusions into the troposphere over the United States. Final Report, SRI Proj. 6690, CRC-APRAC Project No. CAPA- 5-76 (l-77), Coordinating Research Council, Inc. 219 Perimeter Center Parkway, Atlanta, GA 30364. Viezee W., Singh H. B. and Shigeishi H. (1982) The impact of stratospheric ozone on tropospheric air quality ~ Implications from an analysis of existing field data. Final Report, SRI Project 1140, CRC Contract No. CAPA-15-76 (l-80), Coordinating Research Council, 219 Perimeter Center Parkway, Atlanta, GA 30364. Wolff G. T.. Ferman M. A. and Monson P. R. (19791 The distributidn of beryllium-7 within high-pressure‘systems in the eastern United States. Geophys. Res. Lett. 6, 637-639. Wu Mao-Fou (1973) Observation and analysis of gaseous trace constituents in the stratosphere. Department of Transportation, DOT-TST-74-7 (Report No. ERT P-93), Washington, D.C. 20590.
OW4981/83
Vol. I?. No. IO, pp. 1979-1993, 1983
$3.00 + 0.00
0 1983 Fergamon Press Ltd.
Printed inGreatBbain.
STRATOSPHERIC OZONE IN THE LOWER TROPOSPHERE-II. ASSESSMENT OF DOWNWARD AND GROUND-LEVEL IMPACT WILLIAM VIEZEE, WARREN B. JOHNSON
and
FLUX
HANWANT B. SINGH
Atmospheric Science Center, SRI International, Menlo Park, CA 94025, U.S.A. Ahstraet-Aircraft m~surements of four stratospheric O,-intrusion events (two during spring and two during fall) are used in conjunction with concurrent meteorological analyses to estimate the downward 0, flux in the upper and lower troposphere. The aircraft measurements used are among those reported earlier in Part I of this paper (Johnson and Viezee, 1981).The calculated upper tropospheric fluxes for the four cases show good agreement with earlier estimates by Danielsen and Mohnen (1977) of 0% fluxes associated with tropopause folding events. The estimated lower-tropospheric 0, fluxes for the two spring events and for one fall event suggest that more than half of the 0, mass injected into the upper troposphere by these stratospheric intrusions is probably mixed and diluted into the troposphere above 700 mb (3 km ASL). Large, direct impacts of stratospheric 0, intrusions at ground-level are thus unlikely. A review and analysis of the limited number of published observations of high 0, in stratospheric intrusions, and of anomalously high 0, at ground level attributed to stratospheric intrusions, also suggests that direct ground-level impacts may be infrequent (less than 1 per cent of the time), and most likely are associated with 0, concent~tions (v/v) of 100 ppb or less. Additional observational studies are required to conclusively quantify the ground-level impact of stratospheric 0,.
1. INTRODUmION
In an earlier pubii~tion (Johnson and Viezee, 1981), aircraft measurements of atmospheric 0, made over the central United States in the spring and fall of 1978 were described in terms of the structure of stratospheric O3 intrusions into the middle and lower troposphere. The meteorological conditions associated with the intrusion events were presented, and inferences were made as to mechanisms by which the stratospheric O3 can reach ground level. This paper is the sequel to the publication by Johnson and Viezee (1981). It addresses, specifically, the downward flux of the observed stratospheric 0, in the troposhere, and the probable impact of stratospheric/tropospheric intrusions on ground-level air quality*.
2. DOWNWARD FLUX OF STRATOSPHERIC OZONE
2.1. Upper troposphere Using the in-situ SRI aircraft measurements made during the 1978 field program in conjunction with concurrent meteorological analyses (Viezee et a/., 1979), estimates have been calculated of the downward flux of stratospheric 0, into the upper troposphere associated with four intrusion events: two each ob-
* Results have been obtained from research projects sponsored by the Coordinating Research Council, Inc., under Contract CRC-APRAC Project No. CAPA-15-76 (l-77); and by the Environmental Science Research Laboratory of the U.S. Environmental Protection Agency under Grant CR 809330010.
served in the spring and in the fall. The four cases were selected on the basis of their detailed structure of relatively high O3 concentrations at the maximum flight altitude sampled (7.3 km ASL). The flux estimates were calculated by expressing the downward mass flux of 0, through an upper level of the intrusion as: -.F = M(O,)*W.A (1) where F = downward flux of stratospheric O3 (moiecules per unit time) through upper-level intrusion area, M(0,) = molecules of 0, per unit volume averaged over intrusion area, W= downward vertical velocity averaged over intrusion area and A = horizontal area of intrusion. Equation (1) assumes that the downward 0, flux is controlled primarily by the organized mean circulation within the intrusion, rather than by small-scale, turbulent fluct~tions. This assumption is necessary because of the limited observational data available for our study. Although Shapiro (1978, 1980) has indicated that smaller-scale tuibulent components may have a significant role in the dynamics of an intrusion, the ConsistentIy well-defined structure of the intrusions measured during our field program lends support to our assumption. Upper-air meteorological analyses required for the flux computations were obtained from the U.S. National ~eteorologi~l Center (NMC). Since the maximum flight altitude of 400 mb (7.3 km ASL) is not a standard level that is routinely analyzed by the NMC,
1979
1980
WILLIAM VIEZEE rt a/.
the 300 mb (9.0 km ASL) isobaric charts were used in the computations. 2.1.1. Spring events of 13 May and 19 May 1978. Figure 1 shows the selected spring event of 13 May 1978, 14: 34-19: 19 CST. This event represents the most intense stratospheric O3 intrusion observed during the 1978 SRI aircraft program. The twodimensional cross section of 0, [Fig. l(c)] was documented between Muscle Shoals, Alabama and Meridian, Mississippi, and extends from the maximum flight altitude of 7.3 km ASL down to 3.3 km ASL. 0, concentrations (v/v) at the upper altitude are as high as 280ppb, while at the lowest flight altitude a thin lamina of 0, is observed with concentrations (v/v) of lo&120 ppb. A detailed analysis and interpretation of the vertical cross section of measured O,, and the conditions associated with meteorological it [Fig. 1(b)] is presented by Johnson and Viezee (1981). Figure l(a) shows the pattern of downward vertical motion at 300 mb near the time of the aircraft observations (13 May 18:00 CST or 14 May 0O:OOGMT), computed from the standard NMC meteorological analyses and using the adiabatic method (Holten, 1979). Application of the adiabatic technique to the upper tropospheric level is justified because the descending air parcels of a stratospheric air intrusion are characterized by isentropic (adiabatic) flow (Danielsen, 1968). If it is assumed that the area of computed downward motion represents the total stratospheric-intrusion area at about 9 km ASL, then stratospheric air of high 0, concentration and low dewpoint descends through this area into the upper troposphere. A value for the O3 density in this area is estimated by extrapolating to the 9 km level the trend of increasing 0, with altitude which was observed by the aircraft in the intrusion below 7.3 km [Fig. 1(c)l. This extrapolated 0, density (valid only along the flight path) is applied to the entire intrusion area. Using this procedure, the downward 0, flux into the upper troposphere below 300 mb can be computed from Equation (1). A similar procedure to compute the downward 0, flux into the upper troposphere below 300 mb is applied to the second spring event of 19 May 1978,13:1 l-17:40 CST. Results are tabulated in Table 1. The downward 0, flux for the intrusion event of 13 May is about twice as large as that associated with the second event of 19 May, primarily because of a larger estimated intrusion area at 300 mb. This difference in the downward flux between the two spring events is compatible with a corresponding difference in intensity of the upper-tropospheric low-pressure troughs, as discussed by Johnson and Viezee (1981). Using the daily series of 500 mb analyses from the NMC for 1978, the number of migratory low-pressure troughs of the type associated with the two spring events [Fig. 1(b)] were counted for the entire northern hemisphere, and for the area of North America between longitudes 7o”W and 16o”W. An average number of 4.5 travelling cyclones per day for the
northern hemisphere, and an average number of 1.0 per day for the North American continent and the eastern Pacific Ocean, was determined for the month of May. It is assumed that the area of the intrusion and its intensity at 300 mb do not significantly change over a period of one day. Thus, the spring fluxes of Table 1 suggest a cumulative daily 0, flux from the stratosphere into the troposphere (during May) ranging from 1.9 to 3.5 x 1O34 molecules for the northern hemisphere; and from 0.4 to 0.8 x 1O34 molecules for the area of North America and the eastern Pacific Ocean, between 7o”W and 16o”W. 2.1.2. Fall events of 5 October and 14 October 1978. Figure 2 shows the fall event of 14 October 1978, 11:54-16:52 CST. This event is selected for illustration because the two-dimensional O3 structure [Fig. 2(c)] observed during the aircraft flight from southern Illinois to northern Mississippi was mapped in considerable detail in the upper troposphere above 500 mb (5.5 km ASL). Values of O3 concentration as high as 175 ppb and 201 ppb are present along the maximum flight altitude (7.3 km ASL). The 500-mb low-pressure trough [Fig. 2(b)] is typical of those that were found to be associated with stratospheric 0, intrusions. Figure 2(a) shows the pattern of downward vertical motion at the 300 mb level computed by the adiabatic method. As in the spring cases of 13 and 19 May, it is assumed that OS-rich stratospheric air descends through this area into the upper troposphere. Following a similar procedure as outlined above for the two spring events, computations leading to the downward 0, flux are listed in Table 1. Also tabulated are values of intrusion area, vertical motion and downward O3 flux, for the fall event of 5 October 1978, 17: 15-21:37 CST. This event is discussed in detail by Johnson and Viezee (1981) and was observed to penetrate into the lower troposphere down to 2 km ASL, which is near the top of the boundary layer. The downward 0, flux for the event of 14 October is twice as large as that for the case of 5 October because of the larger vertical (descending) motion through the intrusion area. From the daily series of NMC 500 mb analyses for October 1978, an average number of 4.0 low-pressure troughs per day was determined for the entire northern hemisphere; and an average number of 1.0 troughs per day was determined for North America between longtitudes 7o”W and 16o”W. If it is assumed as before that the area of the intrusion [Fig. 2(a)] and its intensity do not appreciably change over a period of one day, then cumulative daily downward flux of ozone into the upper troposphere during October can range from 0.5 to 1.0 x 1O34 molecules for the northern hemisphere, and from 0.1 to 0.3 x 1O34 molecules for the area between 7o”W and 16O”W. 2.1.3. Summary of upper tropospheric fluxes. Figure 3 summarizes the estimates of the stratospheric/
Stratospheric
(al
SOONBAR
PATTERN
OF DESCENDING
(et CROSSWIND Fig.
1. Spring
event
of
ozone in the lower troposphere-II
STRUCTURE
VERTICAL
MOTION
(cm sec”l NEGATIVE-DOWNWARD)
OF OZONE CONCENTRATION
observed ozone intrusion stratospheric/tropospheric
1981
(13 May 1978, 14:34-19: ozone flux is estimated.
(ppb) 19 CST)
for
which
the
WILLIAM VIEZEE et al.
1982
-j-_;
I i,
t
(a) 300.MBAR
--,--:
’
PATTERN
I-- ---
i-
OF DESCENDING
VERTICAL
MOTION
(cm sets’ NEGATIVE-DOWNWARD)
14OCTlO?R14:c.J QT ‘0NT”IIHSII)“rdAMIC METtnSl
-
(b) BOO-MBAR LOW-PRESSURE
TROUGH
26
6.5
24
7.3
120 +
6.1
% 16
4.9
I
4
-60 -148
(cl
Fig.
2. Fall
event
I
I
CROSSWIND of
-40 -74
0 0
60
40 DISTANCE
-
74 DISTANCE
-km
STRUCTURE
-
MEMPH,S t ,:: / ...’
MALDEN
40 L
I
120
160
1.2 0
nm
146
222
296
OF OZONE CONCENTRATION
observed ozone intrusion (14 October, 1978, 11:5+16: stratospheric/tropospheric ozone flux is estimated.
(ppb) 52 CST)
for
which
the
Stratospheric ozone in the lower troposphere-II
SRI DATA -DANlELSEN
119’ A
MOHNEN
(197
8
1983
vertical column of the atmosphere is measured by surface-based instruments, such as the Dobson spectrophotometer. Approximately 70’;/ of the total column 0, content occurs in the stratosphere below 30 km, while only about 7 ‘x occurs in the troposphere. 2.1.4. Interpretation of trapospheric upper fluxes An annual average contribution of strato-
JFMAMJJASOND MONTH
Fig. 3. Estimated annual variation of the monthly stratospheric/tropospheric total ozone flux from tropopause folding events in the northern hemisphere.
tropospheric 0, fluxes obtained from the observational data of the four intrusion events. Values for May based on the spring events, and for October based on the fall events, are compared with the variation of the monthly stratospheric 0, flux into the northern hemispheric troposphere published by Danielsen and Mohnen (1977). Fluxes are expressed in units of 1O35 O3 molecules per month per tropopause folding event (TFE). Danielsen and Mohnen (1977) maintain that mass transport associated with tropopause folding events (stratospheric intrusions) is the dominant mechanism for injecting stratospheric ozone into the troposphere. Their opinion is supported by extensive analyses of tropopause folding events, and by comparing 90Sr activities in extruded stratospheric air with observed ground deposition rates (including the seasonal cycle) in the United States. The outstanding feature of Fig. 3 is the spring maximum and the fall and early winter minimum in the O3 fluxes, which is evident in both our case-study calculations, and the data of Danielsen and Mohnen (1977). This spring-to-fall variation closely resembles the seasonal variation that has been observed in the background of natural O,, both in the free troposphere and near ground level (e.g. Singh et al., 1978; 1980). Also, there is a striking similarity between the seasonal trend in the stratospheric/tropospheric ozone flux of Fig. 2, and the seasonal change in the stratospheric reservoir of natural 0, as shown in Fig. 4. This figure (Wu, 1973) compares the annual variation of the monthly mean total 0, content at Mauna Loa, Hawaii, (19.32”N, 15536”W); Tallahassee, Florida (30.26”N, 84.2o”W); and Green Bay, Wisconsin (44.29”N, 88.08”W). The total amount of 0, in a
spheric 0, to the troposphere of about 14 ppb is obtained from the flux data of Danielsen and Mohnen (1977). This is derived from a simple ‘box’ model for the troposphere that extends from the equator to the pole, and from the surface to 300 mb (9.0 km ASL). This estimate assumes a mean 0, residence time of about 1 month (Singh et al., 1980). Our average values of stratospheric O3 flux into the upper troposphere (estimated from the two May events and the two October events), are very similar to those given by Danielsen and Mohnen. Thus, the 14 ppb annual average tropospheric background of 0, from the stratosphere estimated from their data is not changed significantly by considering our data analyses and calculations. On the basis of ground-level measurements, Singh et al. (1978) found that the yearly average background of natural O3 is about 30 ppb. Since natural O3 can originate from sources other than the stratosphere (such as through photochemical processes involving atmospheric hydrocarbons emitted from natural sources), the estimate of 14 ppb for O3 of stratospheric origin is consistent with Singh’s value of 30 ppb for all natural 0, in the troposphere. Clearly, these results are very approximate, considering the simplified nature of our approach, and the fact that only two cases for each spring and fall season are analyzed in detail. A more sophisticated modeling approach, applied to additional cases, is required. Also, measurements of the spatial extent of intrusions (beyond the two-dimensional plane to which our aircraft observations are limited) need to be incorporated in order to improve the flux computations. 2.2. Lower troposphere There is great interest in determining how much of the natural O3 transported by a stratospheric intrusion into the upper troposphere arrives in the planetary boundary layer, from where it can impact ground-level 0, concentrations. Several investigators maintain that stratospheric air reaches ground level in about two days by way of the surface high-pressure system that follows travelling upper tropospheric low-pressure troughs (e.g. Reiter and Mahlman, 1965; Danielsen, 1980; Wolff et al., 1979). If this concept is correct, it will be difficult to quantify (on the basis of measurements) the stratospheric component of the near-surface 0, budget, since high-pressure areas also are favourable for air stagnation and surface transport of anthropogenie 03. To further clarify the impact of a stratospheric O3 intrusion on the lower troposphere (700mb and below), a better understanding is required of the three-
1984
WILLIAM
VIEZEE
have combined the aircraft-observed 0, intrusions of the two spring events (13 May and 19 May 1978), and the fall event of 5 October 1978, with concurrent threedimensional air-parcel trajectories. These trajectories are computed from operational wind forecasts generated by the Limited-Area Fine Mesh numerical prediction model of the National Meteorological Center (Reap, 1972). Computations are made backward in time at 6-h intervals, for a total of 24 h, starting from a forecast point and proceeding to the origin point. The starting points are at the 700-mb level, the 850-mb level and the surface. The trajectories are routinely computed for 73 locations across the United States. The air-parcel positions (latitude/longitude) at 6-h intervals are transmitted twice daily at 12OOGMT and 0000 GMT, via NWS facsimile and teletype circuits,
dimensional circulation associated with such intrusions. Three-dimensional air-parcel trajectories are particularly important. Thus far, trajectory analyses made in connection with studies of tropopause folds have been based on the assumption that a given air parcel moves on a constant isentropic surface, i.e. that the total energy of the parcel remains constant (adiabatic flow). This assumption becomes untenable when approaching the planetary boundary layer, where diabatic effects from radiative processes and turbulent mixing become important. In this section, we present some preliminary results of additional research on the impact of stratospheric O3 in the lower troposphere. 2.2.1. Intrusion area in the lower troposphere. We 440
I
I
et al.
I
I
I
I
420 GREEN 44 29
BAY N, 88.08”
W
400
380
r’-‘-r
TALLAHASSEE 30.26”
I-----I
N 84 20 W
I L.,__.
: I
I
I
I L,.---
I.__-__/----’
:
I
330 .__-J
MAUNA 19.32”
LOA N
155.35”
W
280
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
Fig. 4. Monthly mean total ozone for the 1 l-year period between January 1960 and December 1970 at Mauna Loa, Hawaii; Tallahassee, Florida; and Greenbay, Wisconsin (Wu, 1973).
Stratospheric ozone in the lower troposphere-II under the heading “FOUS50-57 Bulletins”. Thus, using these readily available operational data, an analysis can be made of the predicted areas of ascending and descending air parcels for three levels in the lower troposphere. Figure 5 shows a sample of predicted 24-h trajectories that terminate at 700 mb at the receptor locations of Houston, Texas (Trajectory l), Mobile, Alabama (Trajectory 2), and Birmingham, Alabama (Trajectory 3), near the time (13 May, 18:OOCST) that the 0, intrusion of Fig. l(c) was observed. The aircraft flight path between Muscle Shoals, Alabama and Meridian, Mississippi, along which the two-dimensional cross section of ozone structure was observed, is indicated in Fig. 5. Trajectory 1 is outside the area of aircraft observations, and is disregarded. Trajectory 3 intercepts the vertical cross section at 694 mb (3 km ASL) about 46 km south of Muscle Shoals. According to the 0, cross-section of Fig. l(c), this intercept point is below and outside the observed 0, intrusion. Thus, no stratospheric characteristics can be assigned to the air parcel of this trajectory. Trajectory 2 intercepts the vertical plane of the cross-section at 690 mb (3 km
Fig. 5. 24-hour
three-dimensional
1985
ASL) about 92 km south of Meridian. No aircraft observations are available near the location of this intercept point, and, thus, no direct information is available on the stratospheric origin of the air parcels of Trajectory 2. Figure 5, however, only presents three specific trajectories that illustrate the existence and characteristics of a synoptic-scale, descending air mass into the region of aircraft observation. Additional trajectories can be obtained through interpolation. Using this interpolation technique between the airparcel pressure-level data of Trajectories 2 and 3, the 24-h path of the air parcels that intercept the lowaltitude lamina of observed stratospheric 0, (80 to 100 ppb) on 13 May, 18:OOCST can be obtained. In other words, the available three-dimensional trajectory data enable an estimate of the geographic location and atmospheric height where the stratospheric 0, (observed at low altitude near Meridian, Mississippi) originated. Our interpolation of the trajectory data suggests that the observed stratospheric 0, originated in South Dakota and Nebraska at altitudes at and above 550 mb, 24 h earlier on 12 May, 18:Cil CST. We now proceed by using all available 24-h trajec-
air parcel trajectories (12 May 18 : 00 CST--13 May 18 : 00 CST) relevant stratospheric ozcme intrusion of Fig. 1.
to the observed
WILLIAM VIEZEE et il.
1986
tory data for the period 12 May, 18:OUCST to 13 May, 18:00 CST that terminate in the region of the aircraftobserved low-altitude (700mb) O3 intrusion of Fig. l(c). The objective is to identify the total area in which the descending air parcels are predicted to originate (from the 550mb level or above) in South Dakota and Nebraska. This area represents our best estimate of the horizontal extent of the stratospheric intrusion near 700 mb. Figure 6 shows the total area of descending air-parcel trajectories at the 700-mb level which was predicted for 13 May, 18:00 CST using the data from Bulletins FOUS55, 56 and 57. Isolines are labeled in terms of the pressure level (millibars) from which the air parcels originate 24 h earlier (12 May, 18:OOCST). The shaded part represents the area within which the air parcels are predicted to originate in South Dakota and Nebraska, from atmospheric heights of 550 mb and above. According to our analysis technique, air parcels within this 700 mb (300 m ASL) area are presumed to be of stratospheric origin, and
associated with the stratospheric
0,
j
500
intrusion
ob-
served between Muscle Shoals, Alabama and Meridian, Mississippi. A most interesting result of our analysis technique is that Fig.6 shows the presumed shape of the stratospheric air intrusion at the 700-mb level. The analysis suggests that the 0, structure [observed by the SRI aircraft in the two-dimensional plane of Fig. l(c)] extends much further southward from Meridian, Mississippi, and covers a large area along the Gulf Coast from Houston, Texas, to Tallahassee, Florida. Stratospheric 0, descends through this area into the lower troposphere. A similar analysis as described above for the spring event of 13 May 1978 was made for the case of 19 May 1978, 13:11-17:40CST, and for the fall event of 5 October 1978, 17:15-21:37 CST. These three cases were analyzed because the intrusions were observed to penetrate into the lower troposphere to levels near 700mb where the available trajectory data are applied (Johnson and Viezee, 1981). Figure 7 summarizes the presumed intrusion areas for the three cases super-
_: _.
KM
Fig. 6. Area of descending air parcel trajectories near the time that the stratospheric
through the 700-mb level into the lower troposphere as predicted ozone intrusion of Fig. 1 was observed (13 May, 18 : 00 CST).
by LFM-II
Stratospheric ozone in the lower troposphere-II
1987
1988
WILLIAM V~EZEE et ai.
posed on the 700~mb contour charts. The pressure levelsfrom which the descending air parcels originate 24 h earlier are indicated in mb. The most intense areas of predicted descending air of stratospheric origin (darkest shading) are located behind the surface weather front. This feature is consistent with the concept that frontal downdrafts or post-frontal subsidence can transport stratospheric 0, to ground level (e.g. Lamb, 1977; Derwent r6 ul., 1978; Johnson and Viezee, I98 1)~ 2.22. Luwer-rroposphetic mm fluxes. According to Equation (11,the downward flux of O3 through the intrusion areas illustrated in Fig. 7 equals the product of the area, the area-averaged 0, concentration, and the area-averaged vertical velocity. These values are listed in Table 2 for each case. The 0, concentration in the intrusion area jcalumr~ 3) is obtained either from the direct observations by Johnson and Viezee (198l), or is estimated from an extrapolation to 700 mb following the downward slope of the aircraft-observed O3 iamina [see Fig. l(c)]. The descending motion averaged over the intrusion area (column 5) is computed from the trajectory data that provide the pressure level of the descending air parcels at intervals of 6 h. The intrusion event observed on I3 May 1978 is the most intense. The predicted downward flux of 0, through the 7OO-mb level associated with this spring event (2.5 x 10” molecules s- ‘) is three times as Iarge as that computed for 19 May. The fall event of 5 October is associated with the relatively largest area of descending motion at 7OOmb (8.5 x 10” m2). The computed descent rate, however, is small (0.5 cm s- ’ f, and the observed 0, concentration is only 7&90 ppb. Consequently, the downward O3 flux is only about 0.65 x 102* moleculess- ’ for this fall event.
2.2.3. twzpfications of atone flux caicuhtions. Comparison between the 0, flux calculations in the upper troposphere (Table l), and in the lower troposphere (Table 2) for the 2 spring events (13 and 19 May), and the fall event of 5 October, permits an estimate of the amount of stratospheric O3 that may end up in the lower traposphere below 700 mb. For example, in the case of 13 May 1978, the intrusion area at 300 mb is approximately 1.8 times larger than the estimated area at 700 mb, while the downward vertical motion is twice as large. Consequently, the computed downward flux of 0, at 300 mb is 3.6 times larger than that at 700 mb. These computations imply that, for this particular spring intrusion event, less than l/3 of the stratospheric 0, mass injected into the upper troposphere may be transported directly into the lower troposphere below 700 mb. In other words, more than 2/3 of the stratospheric 0, appears to be mixed and diluted into the troposphere above 700 mb; from there it can stiI1 impact the lower troposphere, but only by indirect circulations. After comparing the upper and lower tropospheric
1989
Stratospheric ozone in the lower troposphere--II Table 2. Downward flux of ozone predicted at 700 mb for three aircraft-observed events of stratospheric ozone intrusion into the lower troposphere
Intrusion event (1978)
Time period of observation (CS-U
Observed or estimated ozone concentration at 700 mb (ppb)
Predicted area of intrusion at 700 mb (10” m’)
13 May 19 May 5 October
14:35-19:19 13:11-17:40 19:15-22:23
8GlOO 8fklOO 7&90
7.7 6.1 8.5
0, fluxes for the other two evenu, our analysis, although very limited in scope, does not suggest a large, direct ground-level impact. In the next section we summarize the results from other published studies which bear on this question. 3. REVIEW OF EXISTING OBSERVATIONALDATA 3.1. Observations oj stratospheric ozone intrusions into the troposphere The structure of stratospheric air intrusions which transport natural ozone into the troposphere by direct circulation and the associated meteorological conditions, are described in detail by Danielsen (1968), Danielsen et al. (1970) Shapiro (1974) and Johnson and Viezee (198 1) on the basis of aircraft observations and by Mahlman (1973) on the basis of an early numerical experiment. These intrusions or tropopause folds have been observed mostly during spring (e.g. Danielsen et al., 1970; Danielsen and Mohnen, 1977; Shapiro, 1974, 1978, 1980; Johnson and Viezee, 1981). Observations during summer are documented by Danielsen (1980), Danielsen and Hipskind (1980) and during fall by Reiter and Mahlman (1965) and by Johnson and Viezee (1981). Table 3 lists published observations of relatively high 0, concentrations in the troposphere specifically associated with stratospheric/tropospheric air intrusions during spring (February, March, April and May), during summer (August) and during fall (October). The 0, concentrations that are tabulated represent the highest values at the lowest tropospheric levels reported by the referenced source. The observations cover various locations across the United States, from the Mississippi Valley westward to the eastern Pacific Ocean. To our knowledge, the only published observational evidence of the injection of stratospheric O3 into the free troposphere is provided by these United States data, those obtained across Canada by the Gametag flights of 1977 (Danielsen and Hipskind, 1980), and the early observations over England by Briggs and Roach (1963). 3.2. Published events of stratospheric ozone at ground level Table 4 lists 10 publications that document observations and analyses of high 0, concentrations at
Averaer downward motion in intrusion area (cm s-r)
Predicted downward ozone flux at 700 mb (10Z8molss-‘)
-2.0 -0.8 -0.5
2.2-2.8 0.709 o.m.7
ground level that were attributed to short-term episodes of stratospheric air intrusions. It is noteworthy that none of these observations was associated with simultaneous aircraft flights which showed the presence, structure and intensity of the intrusion aloft. Instead, the listed values were attributed to an impact from stratospheric 0, on the basis of either meteorological analyses and air-parcel trajectories or simultaneous measurements of beryllium-7 (e.g. Haagenson et a/., 1981; Viezee et al., 1982; Husain et al., 1977; Dutkiewicz and Hussain, 1979; Kelley et al., 1981). Values of reported ground-level impact range from a maximum of 415 ppb (over a lo-min period) at an elevated observatory in Germany (Atmannspacher and Hartmannsgruber, 1973) to a minimum value of 56 ppb (over a l-h period) observed by Kelley et al. (1981) at Pierre, South Dakota, during the summer. The length of the data record which was reportedly examined for unusual occurrences of high groundlevel 0, is listed in Table 4 for each case. This information suggests that the events were sporadic, and generally can be expected to occur less than 1 I’,,of the time. 3.3. Summary analysis of ground-level impact In Fig. 8, the data of Table 3 are plotted as a function of height in the troposphere, irrespective of season and geographic location. Of specific interest is the apparent decrease in observed stratospheric 0, concentration with decreasing height in the troposphere. This is shown by a power curve fit to the 29 data points of the form y = axb where x is tropospheric height (mb) and y is stratospheric 0, concentration (ppb): O,(ppb)
= 7.63 x 10’ [z(mb)] _ 1.40.
This decrease is evident in all observational data published in the literature, and apparently reflects the effects of diffusion and turbulent mixing on the descending stratospheric air. The large deviations from the mean curve are probably indicative of a wide range in intensity of the observed tropopause folding events, as well as of seasonal effects. It is evident, from the data of Table 3 and the summary analysis of Fig. 8, that none of the investigators has actually followed a stratospheric ozone intrusion down to ground level. The lowest altitude at which measurements of 0, of documented stratospheric origin have been reported is 1.9 km ASL (800 mb), where Johnson and Viezee
England (50”N, 1”W to 56”N, 1”W) Duluth/Minneapolis, Minnesota
Geographic location
370 443 373 444
550,450 450 700,600,500 600,500,400 500 650, 500 625, 500 750, 500 600
Colorado Colorado/Oklahoma
Colorado/Utah Colorado
Californ~a/Oregon Northwest Texas California (Los Angeles) Churchill, Canada
Eastern Pacific Ocean Mississippi/Alabama
Minnesota
Tennessee
Minnesota
April 1971 April 1975
April 1976 March 1976 March 1978 August 1977
August 1977 May 1978
May 1978
October 1978
October 1978
450,400,300
Colorado/Wyoming
600,450
365,315
350
April 1969
February 1967 San Francisco. California
April 1966
May 1961
Date
Tropospheric level (mb)
loo
70,95
100, 150
80 to 100 100, 190
Aircraft
Aircraft
Aircraft
Aircraft Aircraft
Aircraft Aircraft Aircraft Aircraft
Aircraft
300 80, 180 80 90, 100, 150 63, 100, 150
Aircraft
Aircraft Aircraft
Aircraft
Balloon
Balloon
Aircraft
Measurement technique -
320
98 225
120, 180, 300
SO, 150
300, 145
120
Stratospheric 0, concentration (ppb) Source
Lovili and Miller, (1968) Danielsen et al. (1970) Shapiro (1974) Danielsen and Mohnen (1977) Danielsen and Mohnen (1977) Danielsen and Mohnen (1977) Shapiro (1978) Shapiro (1978) Shapiro (1980) Danielsen and Hipskind ( 1980) Danielsen (1980) Johnson and Viezee (1981) Johnson and Viezee (1981) Johnson and Viezee (1981) Viezee et al. (1979)
(1968)
Briggs and Roach (1963) Kroening et al.
Table 3. Published observations of stratospheric ozone in the troposphere during extrusion events
15 March
8
6 7
19 March 24.25, 28 and 1 July 4 March July 1978
1978
1977 June 1977 1978
11, 12 July 1975
9
4 10
19 November 1972 6 March 1974 S/9 January 1975
1971
1964
26 February
3 March
Date
3 4 5
1
Case No.
location
episodes
Kisatchie Louisiana
National
Forest,
Denver, Colorado Pierre, South Dakota
Quincy, Florida (near Tallahassee) Observatory Hohenpeissenberg (1OOOm MSL), SW of Munich, Germany Santa Rosa, California Harwell, Oxon. U.K. Zugspitze Mountain near GarmischPartenkirchen, Germany (3000m MSL) Whiteface Mountain, New York (1500m MSL) Sibton, Suffolk, U.K. Whiteface Mountain, New York
Geographic
Table 4. Published
0,
82 $56 < 46 lo(r105
10&l 10 < 41
< 31
415 250 200-230 llG115 16(X193
10@300
(ppb)
Ground-level 0, concentration
of stratospheric
to ground
lh lh 24 h av. 2h
2h 24 h av.
24 h av.
10 min 50 min Ih 2h 4h
3h
Duration of observed event
transport
1971
1973
Spring
1978
1975-1978 July-September
1978
45 y discontinuous June and July 1977
July 1975
November 1972 45 y discontinuous Aug. 1973-Feb. 1976
Dec. 197&May
July 1963-July
Length of data record examined
level
Viezee et al. (1982)
et al. (1981) (1981)
al. (1978) and Husain,
et al. (1977) Derwent et Dutkiewicz (1979) Haagenson Kelly et al.
Husain
Atmannspacher and Hartmannsgruber (1973) Lamb (1977) Derwent et al. (1978) Singh et al., (1980)
Davis and Jensen (1976)
Source
WILLIAM
1992
VIEZEE
et al.
350
400
450 2
; ” Y
500 550 600 650 700 750 2
800 850
2
03
200 CONCENTRATION
1
300 -
400
ppb
Fig. 8. Comparison between observations of high ozone concentrations in the free troposphere associated with stratospheric air intrusions (solid dots with power curve fit), and episodes of high O3 concentrations observed at ground level (numbered blocks) that are purported to be of stratospheric origin. Data sources are listed in
Tables 3 and 4.
(1981) measured 60 ppb (see Paper I). Figure 8 also includes the suspected episodes of stratospheric O3 at ground-level listed in Table 4. The numbered episodes refer to the case numbers in Table 4. (Cases 9 and 10 are not included in Fig. 8 because they involve 24-h average concentrations.) The ground-level concentrations of episode cases 4,6,7 and 8 are comparable with the concentrations actually observed in low altitude stratospheric 0, intrusions. For such concentrations of stratospheric O3 to occur at ground level, the stratospheric O3 concentrations (v/v) of 100 ppb at the 650 and 700 mb levels, would have to be transported directly to the ground over short-time periods by small-scale atmospheric circulations such as convection and turbulence, without significant additional dilution. Case 5 was observed on the Zugspitze Mountain in Germany, at an elevation of 3000 m ASL. It is plausible that the high 0, densities observed in stratospheric intrusions at 500 mb, and above, could directly impact such elevated locations. The data from Cases 1, 2 and 3 seem out of place because the ground-level concentrations are much higher than those observed in intrusions below the 500 mb level. If instrumental uncertainties are ruled out, these cases could be associated with intrusion events having low-altitude intensities that have not yet been observed; or with ground-level 0, concentrations contaminated by sources other than stratospheric. Obviously, more studies are required to better quantify the ground-level impact of stratospheric 0, intrusions. As a first approximation, however, the limited observational data summarized in Fig. 8 suggest that, except for high mountain locations, the ground-level impacts of stratospheric 0, are likely to
be no larger than 1OOppb. Measurements of higher concentrations, particularly at low-altitude locations, should be analyzed carefully for possible contamination from surface transport or local photochemistry. Acknowledgements-We wish to acknowledge the support and technical guidance received from the members of the CAPAProject Committee of the Coordinating Research Council, Inc. (Mr. Timothy C. Belian, Project Manager; Mr. Bruce S. Bailey, Committee Chairman) and from Dr. A. Paul Altshuller, Senior Science Advisor, Environmental Sciences Research Laboratory, U.S. Environmental Protection Agency. Special appreciation is due to Mr. Ronald M. Reap, Techniques Development Laboratory, National Weather Service, National Oceanic and Atmospheric Administration, who supplied the FOUS50-57 Bulletin data required to construct the predicted three-dimensional air-parcel trajectories used in our study.
REFERENCES Attmannspacher W. and Hartmannsgruber R. (1973) On extremely high values of ozone near the ground. Pure Appl. Geophys. (Pageoph), 106108,1091-1096. Briggs J. and Roach W. T. (1963) Aircraft observations near jet streams. Q. JI R. met. Sot. 89, 2255247. Danielsen E. F. (1968) Stratospheric-tropospheric exchange based on radioactivity, ozone and potential vorticity. J. atmos. Sci. 25, 502-518. Danielsen E. F., Bleck R. Shedlovsky J. Wartburg A. Haagenson P. and Pollock W. (1970) Observed distribution of radioactivity, ozone and potential vorticity associated with tropopause folding. J. geophys. Res. 75, 2353-2361. Danielsen E. F. and Mohnen V. A. (1977) Project dust storm report: ozone transport, in situ measurements and meteorological analyses of tropopause folding. J. geophys. Res. 82, 5867-5877.
Stratospheric
ozone in the lower troposphere-II
Danielsen E. F. (1980) Stratospheric source for unexpectedly large values of ozone measured over the Pacific Ocean during Gametag, August 1977. J. geophys. Res. 85, 401412. Danielsen E. F. and Hipskind R. A. (1980) Stratospheric-Tropospheric exchange at polar latitudes in simmer. J. geophys. Res. 85, 393-400: Davis D. R. and Jensen R. E. (1976) Low level ozone and weather systems. Proc. APCA Specialty Conference on Ozone/Oxidants interactions with the Total Environment, Dallas, Texas, March 1976, pp. 242-251. Derwent R. G., Eggleton A. E. J., Williams M. L. and Bell C. A. (1978) Elevated ozone levels from natural sources. Atmospheric Environment 12, 2173-2177. Dutkiewicz V. A. and Husain L. (1979) Determination of stratospheric ozone at ground level using ‘Be/ozone ratios. Geophys. Res. Lett. 6, 171-174. Haagenson P. L., Shapiro M. A., Middleton P. and Laird A. R. (1981) A case study relating high ground-level ozone to enhanced photochemistry and isentropic transport from the stratosphere. J. geophys. Res. 86, 5231-5237. Husain L., Coffey P. E., Meyers R. E. and Cederwall R. T. (1977) Ozone transport from stratosphere to troposphere. Geophys. Res. Lett. 4, 363-370. Johnson W. B. and Viezee W. (1981) Stratospheric ozone in the lower troposphere-I. Presentation and interpretation of aircraft measurements. Atmospheric Environment 15, 1309-1323. Kelly N. A., Wolf G. T. and Ferman M. A. (1982) Background pollutant measurements in air masses affecting the eastern United States-I. Air masses arriving from the northwest. Atmospheric Environment 16, 1077-1088. Kroening J. L., Rosen, J. M. and Mantis H. T. (1968) On the transport of material across the tropopause. J. geophys. Rex 73, 5463-5465. Lamb R. G. (1977) A case study of stratospheric ozone affecting ground-level oxidant concentrations. J. appl. Met. 16, 780-794. Lovill J. E. and Miller A. (1968) The vertical distribution of ozone over the San Francisco Bay area. J. geophys. Res. 73, 5073-5079. Mahlman J. D. (1973) On the maintenance of the polar front
1993
jet stream. J. atmos. Sci. 30, 544556. Reap R. M. (1972) An operational three-dimensional trajectory model. J. appl. Met. 11, 1193-1202. Reiter E. R. and Mahlman J. D. (1965) Heavy radioactive fallout over the southern United States, November 1962. J. geophys. Res. 70, 45014519. Shapiro M. A. (1974) A multiple structured frontal zone-jet stream system as revealed by meteorologically instrumented aircraft. Mon. Weath. Reu. 102, 244253. Shapiro M. A. (1978) Further evidence of the mesoscale and turbulent structure of upper level jet stream-frontal zone systems. Mon. Weath. Rev. 106, 110&1111. Shapiro M. A. (1980) Turbulent mixing within tropopause folds as a mechanism for the exchange of chemical constituents between the stratosphere and troposphere. J. atmos. Sci. 37, 99+1004. Singh H. B., Ludwig F. L. and Johnson W. B. (1978) Tropospheric ozone: concentrations and variabilities in clean remote atmospheres. Atmospheric Encironmenr 12, 2185-2196. Singh H. B., Viezee W. Johnson W. B. and Ludwig F. L. (1980) The impact of stratospheric ozone on tropospheric air quality. J. Air Pollut. Control Ass. 30, 1009%1017. Viezee W., Johnson W. B. and Singh H. B. (1979) Airborne measurements of stratospheric ozone intrusions into the troposphere over the United States. Final Report, SRI Proj. 6690, CRC-APRAC Project No. CAPA- 5-76 (l-77), Coordinating Research Council, Inc. 219 Perimeter Center Parkway, Atlanta, GA 30364. Viezee W., Singh H. B. and Shigeishi H. (1982) The impact of stratospheric ozone on tropospheric air quality ~ Implications from an analysis of existing field data. Final Report, SRI Project 1140, CRC Contract No. CAPA-15-76 (l-80), Coordinating Research Council, 219 Perimeter Center Parkway, Atlanta, GA 30364. Wolff G. T.. Ferman M. A. and Monson P. R. (19791 The distributidn of beryllium-7 within high-pressure‘systems in the eastern United States. Geophys. Res. Lett. 6, 637-639. Wu Mao-Fou (1973) Observation and analysis of gaseous trace constituents in the stratosphere. Department of Transportation, DOT-TST-74-7 (Report No. ERT P-93), Washington, D.C. 20590.