Envlron. Sci. Technol. 1904, 18, 749-756
Airborne Lidar Observations of Long-Range Transport in the Free Troposphere Scott T. Shlpley,*ot Edward V. Browell,t Davld S. McDougal,+ Brlan L. Orndorff,t and Phlllp Haagensons
Atmospheric Sciences Division, NASA Langley Research Center, Hampton, Virginia 23665,Old Domlnlon University, Norfolk, Virginia 23508,and National Center for Atmospheric Research, Boulder. Colorado 80302 Airborne lidar measurements of ozone and aerosols in the lower troposphere show the presence of pollutant layers above the mixed layer. Two case studies are analyzed to identify probable source regions and mechanisms for material injection into the free troposphere above local mixed layers. An elevated haze/oxidant layer observed over South Carolina on Aug 2,1980, was found to originate in cumulus convection over Georgia on Aug 1,1980. An extensive haze/oxidant layer observed over southeastern Virginia on July 31,1981, is shown to have been in contact with the New England mixed layer on July 30,1981. This transported air mass is estimated to contribute approximately 30% of the ozone maximum measured at the surface in the Norfolk, VA, area on July 31, 1981. Such elevated “reservoir” layers are transported over long ranges and are not detected by sensors which are confined to the surface. Introduction Airborne lidar measurements of the mixed layer and lower free troposphere were performed during the summer months of 1980 and 1981 by using the NASA ultraviolet differential absorption lidar (UV-DIAL) system. A detailed description of the UV-DIAL system is given in ref 1. When operated on board the NASA Wallops Electra aircraft, the UV-DiAL system produces vertical profiles of ozone concentration with a vertical resolution -210 m and a horizontal resolution 1 500 m. Multiwavelength information on the vertical distribution of aerosol backscattering is simultaneously provided with 15-m vertical and horizontal resolution. Large-scale vertical cross sections of ozone and aerosols are obtained during long-range flights over meso- and synoptic-scale features. The temporal variation of ozone and aerosol spatial distributions is obtained by repeated flights over the region of interest. This paper examines a portion of these lidar measurements for information on long-range transport in the lower free troposphere. These lidar measurements show the vertical distribution of aerosol and ozone layers downwind of their regions of origin. Given the potential temperature as a function of altitude, an isentropic trajectory analysis can be used to trace these elevated layers backward in time to their most recent coincidence with cloud or mixed layer convection. This data analysis technique is well-suited to the interpretation of lidar survey information where elevated layers are encountered only once. Stronger and more direct evidence for long-range transport will be available when airborne lidar measurements are obtained on two or more consecutive days of operation over a targeted air mass. The NASA UV-DIAL system was used to measure Ozone and aerosol profiles inside and above the mixed layer during long-range flights over the Eastern United States as part of the EPA Persistent Elevated Pollution Episode/Northeast Regional Oxidant Study (PEPE/NEROS) ‘Atmospheric Sciences Division, NASA Langley Research Center. Old Dominion University. National Center for Atmospheric Research.
1980 summer field experiment (2). Subsequent field experiments were conducted during the summer of 1981 to obtain ozone and aerosol measurements on spatial and temporal scales not previously investigated. The 1981 experiments included diurnal observations of mixed layer dynamics over the lower Chesapeake Bay area, a case study for vertical transport of ozone from the mixed layer to the free troposphere by clouds, and uplooking DIAL ozone measurements in the upper troposphere and lower stratosphere. By use of data obtained during these experiments, this paper presents two case studies of elevated haze layers observed in the lower free troposphere above local mixed layers. Analysis Technique During ozone and aerosol measurement missions, the airborne UV-DIAL system is configured for multiple wavelength lidar return signals at 600, 300, and 286 nm with variable repetition rates (1,5, or 10 Hz). The 600-nm return signal is used to construct gray-scale depictions of aerosol spatial distributions. In processing the aerosol lidar return, the background signal level is subtracted from the lidar-plus-background signal, and the geometrical range squared signal dependence is eliminated. The resulting lidar backscatter profile is then indicative of the distribution of aerosols along the lidar line of sight. The backscatter signal level is converted into a 16-level grayscale display line where stronger scattering is indicated by a higher brightness on a monitor or a darker pixel on a printed version of the display. Sequential gray-scale lines are used to construct a real-time picture of the aerosol vertical distribution under the aircraft flight path. The two UV wavelength channels are used to calculate the vertical profile of ozone concentration by the DIAL technique (I). The DIAL technique determines the average gas concentration over some selected range interval by analyzing the difference in lidar backscatter signals for laser wavelengths tuned on and off a molecular absorption line of the gas under investigation. For each DIAL return, the background signal level is integrated over a 5-ps interval after the ground return. This average background is subtracted from each return signal profile, and these signals are then smoothed by using a running average over a range interval corresponding to the chosen range cell size. The gas concentration is then evaluated by using the smoothed lidar returns over a specified range cell size, usually 210 m. Ozone mixing ratios are determined by dividing each range cell concentration by the corresponding standard atmospheric number density at that altitude. A correction factor of 6.7 ppb is subtracted from the ozone mixing ratio to compensate for Rayleigh extinction differences between 286 and 300 nm. Each DIAL pulse pair produces a mixing ratio profile. Any number of DIAL measurements can be averaged together to reduce the profile statistics at the expense of increased horizontal range for the measurement. The standard deviation for the resulting averaged profile is computed a t range intervals equivalent to the range cell size and displayed on the mixing ratio profile. The DIAL measurements pres-
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ented in this paper were averaged over 100 individual profiles at a 5-Hz repetition rate, corresponding to a horizontal averaging interval c 2 km. Comparisons of lidar-derived and in situ chemiluminescent ozone profiles demonstrate agreement to within *lo% in and above the mixed layer (1). These comparisons were made in the absence of strong vertical gradients of aerosol scattering. Consideration must be given to the systematic influence of differential aerosol backscatter on the DIAL ozone measurement when the volume backscatter coefficient varies with range. This influence is significant in the vicinity of strong vertical gradients of aerosol concentration. It is typically on the order of a 10 ppb overestimate at the top of the continental mixed layer. An estimate of this influence can be made by using aerosol backscatter information at the DIAL off-line wavelength (300 nm). Such estimates reduce the ozone uncertainty introduced by aerosol gradients to levels less than 5 ppb under most circumstances (3).
Case I: Aug 2, 1980, Southeastern United States Approximately 42 h of W-DIAL measurements of ozone and aerosols were acquired during 14 long-range flights over the Eastern United States in cooperation with the EPA PEPE/NEROS 1980 summer field program. By cooperative agreement with the EPA, the 600-nm lidar data have been archived on digital magnetic tape along with derived values for the average mixed layer height at 5-min intervals and for the vertical distribution of ozone at l-min (6 km) intervals (4). This section discusses those data obtained over the Southeastern United States on Aug 2, 1980. As shown in Figure 1, an area of intense haze was observed in satellite imagery over most of South Carolina and eastern Georgia on the morning of Aug 2,1980. Airborne UV-DIAL data were acquired from 0800 to 1200 EDT (eastern daylight time) on Aug 2, during a long-range flight over the southeastern seaboard. The aircraft flight plan is shown in Figure 2. Lidar-derived aerosol cross sections are included in Figure 1 to show the vertical distribution of aerosol material at selected locations. These gray-scale insets show mixed layer heights 450 m AGL (above ground level) over both land and ocean surfaces. The complete lidar data set (4) shows that most of the haze visible in satellite imagery is confined to an elevated (in height) layer above both the continental mixed layer and a lower layer of cumulus cloud remnants. The top of this elevated haze layer extends above the highest lidar signal altitude 3100 m MSL (mean sea level). The base of this layer can be seen near 2500 m MSL in the upper left-hand inset of Figure 1. An 850-mbar synoptic chart at 1200 UT (universal time) on Aug 2,1980, is shown in Figure 3. This chart shows high-pressure conditions in the region of observation with light westerly winds a t the altitude of the cumulus cloud remnants. Surface mixed layer visibilities -10 km were reported with minimal cloud development during the morning over South Carolina, while aircraft observers reported low visibility in the haze layer aloft. Satellite measurements of upwelling radiance have been previously investigated for information on surface pollution levels (5,6). However, satellite radiance measurements are more appropriately related to the total optical depth of the atmospheric column (7).Surface visibility and total optical depth measurements are directly related when most of the pollutant material is trapped in a well-mixed layer with known height. Lyons and Calby have compared GOES (Geostationary Operational Environmental Satellite) visible satellite radiances to surface visibility measurements over the Eastern United States during the pe-
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riod from July 30 to Aug 2,1980 (8). They found that the radiance patterns were well correlated to surface visibility with the exception of the elevated haze event over Georgia and South Carolina on Aug 2, 1980. This upper-level “pollution episode” demonstrates the need for vertical aerosol profiles when satellite haze patterns are analyzed for information on surface visibility or surface sulfate concentrations. The airborne UV-DIAL data indicate a haze layer near 3 km, which coincides with the 312 K potential temperature surface near 700 mbar. The NCAR isentropic reverse trajectory analysis package (9) was used to trace this elevated haze layer backward in time, given the NMC Northern Hemisphere Analysis and original rawinsonde observations with 12-h resolution. Four 48-h kinematic isentropic trajectories at 312 K are shown in Figure 2 ending at 1200 UT on Aug 2, 1980, along with the brightness analysis of the GOES East visible image at loo0 EDT provided by Lyons and Calby (8). Two radar summaries of thunderstorm activity over Georgia at 2235 and 2335 UT on Aug 1, 1980, are also shown. The 312 K isentropic trajectories indicate that the elevated haze material at 700 mbar was coincident with deep convective activity over western Georgia on the evening of Aug 1, 1980. GOES images do not reveal extensive haze over southern Georgia on Aug 1. This apparent absence of haze upwind of the thunderstorm complex indicates that surface material has been transported vertically in association with convective processes on the evening of Aug 1,1980, over western Georgia. Limited DIAL measurements reveal ozone concentrations 80 10 ppb at the base of the elevated haze layer on Aug 2,1980. Surface measurements for western Georgia on Aug 1, 1980, indicate ozone concentrations between 80 and 100 ppb. The material at 3-km altitude is also horizontally uniform over scales greater than 10 km (4). These observations link the elevated haze layer to thunderstorm interaction with the mixed layer over western Georgia on Aug 1,1980. Although the nature of this interaction is not clear, these observations suggest that mixed layer material was carried aloft by the convective cells directly, a process which has been referred to as “cloud venting” (10).As demonstrated by Wakimoto, however, it is also possible that the entire mixed layer was displaced vertically by gust fronts (11).These hypotheses are both consistent with the detailed mesoscale analysis of thunderstorm activity in this region by Lyons and Calby (8). Material that has been injected into the free troposphere above the mixed layer can be horizontally transported over long distances without significant dilution due to mixing from below (12). The interaction of the elevated haze layer with local mixed layers and clouds was reconstructed by using available surface, upper air, and radar observations, and model estimates of cloud top heights (13)with cloud top verification from infrared satellite imagery (14). A representative time-height history of mixed layer, cloud top, and cloud base heights below the calculated haze layer trajectory is shown in Figure 4. This figure shows average local values for observed cloud base height along with model estimates for cloud top height (131, mixed layer height determined by the encroachment method neglecting condensation, and the nocturnal inversion height (15).As illustrated in Figure 4, our calculations indicate that subsequent long-range transport has occurred over a lower level of fair weather cumulus convective debris. This analysis indicates that deep convection can produce pollutant-rich elevated “reservoir” layers, which are then transported over long ranges without surface interactions.
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Envlron. Sci. Technol.. Vol. 18,No. IO. 1984 751
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Fgun 4. Clwd and mlxed layer heights under the 312 K isentropic trajectay for air sampled at 1040 EDT. Lidar aerosol profile measurements (shaded areas) on Aug 2, 1980, show an elevated haze layer above lower layers of cloud flux products and an active mixed layer. me elevated layer was coincident with thunderstorm activity (open box just before 0000 UT). on Aug 1. 1980.
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Flgun 2. Folty-elght-hm isentropic back tra@%mles at 312 K for f o r b c a b -he haze area,endhgat 1200 UT on Aug 2.1980. The mmwed Mghbass W for he GOES East *le h g e a t 1MM EDT is lncludsd from ref 8. tcgeth%r with two radar summarks of predpitatkm echoes (shsded areas) obtained over westem Gaorgla at 2235 and 2335 UT on Aug 1. 1980. The contoured satellite observations are given in d w l writs (258 units MI scale) a b background. uncallbratedas to actual radbmamc vaitms. The Electra flight plan for Aug 2. 1980. shows aircrafl positions at 5min ihtervab. These tra@ctor!es indlcate a causal relationship between me elevated haze over South Carolina and thunderstorm activity over Georgia.
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Flgun 5. Atborne DIAL f l w track and coneletive Sensor lxations fa dlunal meawemmtsover h e bwer Chesapeake Bay area on Juk 30-31, 1981. DIAL ozone and aerosol backscatter profiles were collected in and above mixed layer on flight legs AB, CD. EF. and OH over the 2-day period. Correlative alrborne measurements were &tamby radbsondes, tehered babaa, and an insbumemed h n a 402 aircraft at indicated locatbns. Surface ozone observations were obtained by the Virginia State Air Pollution Control Board
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npur 3. Svnopacumtys4sat 850 n h r fa 1200 won Aug 2,1980. A detalled mesoscale analysis of thunderstwm activity over the Southeastem United States for Aug 1-2. 1980, is given In ref 8.
Current research suggests that volatile organic hydrocarbons and NO, emissions may be transformed to oxidants which lead to atmospheric acidification (16). Since thunderstorm washout can involve all levels of the tropcsphere, elevated pollutant layers which have been transported over long ranges may be signifcant sources for acid 752 Environ. Sol. Technol.. Vol. IS. No. IO. 1984
Case II: July 31,1981, Lower Chesapeake Bay The NASA UV-DIAL system was operated from the NASA Wallops Electra aircraft for a second series of atmospheric measurements during the period from July 9 to Aug 9, 1981. The following discussion is confined to results on boundary layer and transport processes observed during diurnal flights over the complex coastal environment of the lower Chesapeake Bay. The airborne UVDIAL system was repetitively flown over the lower Chesapeake Bay region following the flight plan outlined in Figure 5. Lidar-derived ozone and aerosol profiles were recorded on four legs (AB, CD, EF, and GH) at 5 Hz during northerly flow on July 30, 1981, and during northeasterly to easterly flow on July 31,1981. The entire flight pass from points A to H was performed in approximately 80 min, and the Electra aircraft made four com-
plete passes on each flight. In situ measurements of ozone, NO,, SO2, winds, temperature, and dew point were measured simultaneouslyby other investigators at the locations shown in Figure 5. These measurements were obtained by aircraft spirals at B, H, W, X, and Y, tethered balloons at B and W, radiosonde observations at the Naval Air Station (NAS) and Wallops Island (WAL), and surface stations operated by the Virginia State Air Pollution Control Board (VA SAPCB). An elevated layer was observed over the lower Chesapeake Bay on the morning of July 31,1981. Data from the 600-nm lidar aerosol channel are shown in Figure 7 for leg EF during pass two (0752 to 0803 EDT) on July 31,1981. Winds were uniform with height, averaging 8 f 1m s-l out of the east up to 1600 m AGL. In situ and DIAL measurements resolved an optically thick layer over the entire experiment area from approximately 800 to 1600 m MSL with ozone levels from 50 f 5 to 120 f 10 ppb by volume. The air mass below 800 m MSL was relatively clean with surface ozone levels at 40 f 5 ppb. In situ ozone concentration measurements were obtained at the surface by using Dasibi instrumentation (Model 1003AH) and on board a Cessna 402 aircraft by using the ozone-ethylene chemiluminescent technique (Monitor Lab Model 8410). The remote DIAL and airborne chemiluminescent ozone measurements are in agreement where they overlap above =700 m AGL. The airborne chemiluminescentand surface measurements are also in agreement where they are coincident at point Y (Norfolk International Airport). An 850-mbar synoptic chart for 0800 EDT on July 31, 1981, is shown in Figure 8. An isobaric trajectory analysis at 850 mbar indicates that this elevated layer was located over the Northeastern United States 1day earlier and left the coast somewhere between New Jersey and Rhode Island. The 850-mbar trajectory shows long-range transport to Virginia above the oceanic mixed layer over distances on the order of 500 km. The top of the elevated haze layer over the Chesapeake Bay region corresponds to the 1600-m average mixed layer height estimated from radiosonde and surface station information over Connecticut on July 30, 1981. The elevated layer base height corresponds to the height of the ocean mixed layer near the Virginia coast. In situ measurements of total scattering, relative humidity, and potential temperature in the elevated layer over Norfolk, VA, are identical with those reported over Connecticut on July 30, 1981. However, surface ozone concentrations over Connecticut reached maximum values = 50 ppb a t 1400 EDT on July 30,1981. Measurements of ozone concentrations from 50 to 120 ppb in the elevated layer over Virginia on July 31,1981, indicate that the ozone concentration in that layer may have increased during transport over the Atlantic Ocean. Under this scenario, ozone precursors from stack effluents would have been released into the mixed layer over New England on July 30,1981, with subsequent ozone production occurring in the near-neutral mixed layer remnant above a lower active mixed layer later in the day or overnight. Given nonturbulent and stably stratified conditions, elevated haze layers display negligible vertical diffusion and can be transported over long ranges without significant dilution (17). Nighttime lidar observations of the Cumberland Power Plant plume show little vertical diffusion in stable air aloft, the primary cause of dispersion being organized wind shear (18). Detailed plume measurements in mixed layer remnants also show little lateral diffusion in the evening after mixed layer convection has been suppressed (19).Danielsen (20) has shown that continuous stably stratified isentropic layers persist over large hori-
zontal scales, and they are often preserved during transport over long ranges. Vertical stability in these layers is further enhanced by subsidence under high-pressure conditions. Four lidar overflights recorded the growth of the mixed layer over land as it rose up to and into the elevated haze layer on the morning of July 31, 1981. The lidar aerosol data for legs CD, EF, and GH are shown in Figure 6 for pass four (1012 to 1057 EDT) on July 31,1981. Entrainment of aerosol and ozone from the elevated layer into the relatively clean mixed layer was rapid, with aerosol material and enhanced ozone concentrations reaching the surface within 30 min of the onset of fumigation. Referring to leg EF in Figure 6, the elevated haze layer can be seen descending toward the surface downwind of point Y (between E and y),with the haze distribution approaching uniformly mixed conditions near E. Given an easterly wind averaging 8 m s-l at 1 km over point Y, air parcels travel the distance from point W to point Y in approximately 20 min. Similar fumigation rates in a coastal environment have been previously reported (21). Under conditions of easterly flow, most surface fumigation in this region occurs downwind of areas with high surface heat flux (H and Y). An in situ profile obtained at 1440 EDT over point H shows ozone uniformly mixed to 1600 m AGL with concentrations at 80 f 5 ppb. Simultaneous surface ozone measurements upwind of H near point G average 50 f 10 ppb, indicating that approximately 30 ppb of the ozone at H has been contributed by downmixing of the elevated layer. The source of high ozone during onshore northeasterly flow in the Norfolk, VA, area has been somewhat controversial. Salop and Maier (22) have shown a strong correlation between high ozone over coastal Virginia and winds from the northeast. They suggest that these high ozone levels are associated with long-range transport from New England as high-pressure centers move across the Northeastern United States. Pasceri et al. (23)contend that these high ozone levels are the result of sea breeze circulations along the Virginia coast, where a westerly return flow aloft mixes ozone-rich air into the surface flow off the ocean. Continuous tethered balloon profiles from 0654 to 1437 EDT up to 600 m AGL over Fort Story (point W in Figure 5) do not reveal a return flow aloft at any time on July 31,1981. The tethered balloon winds were uniform with height throughout the day, with speeds averaging 8 f 1m s-l out of the east. Balloon ascents at NAS taken every 2 h from 0800 to 1600 EDT show easterly winds averaging 8 f 2 m s-l up to 2400 m AGL. Given a prevailing onshore flow at 5 m s-l, Estoque has shown that the sea breeze circulation does not alter the direction of flow at any altitude (24). The transport conditions observed for northeasterly flow on July 31, 1981, over Norfolk, VA, are similar to those reported by Salop and Maier for the period July 6-Sept 30,1976. Our data, therefore, substantiate the hypothesis that significant amounts of ozone and/or ozone precursors are transported to southeastern Virginia. Summary The NASA UV-DIAL lidar system provided information on the altitude distribution of haze observed in satellite images over the Southeastern United States on Aug 2, 1980. An isentropic trajectory analysis indicates that this elevated layer was involved with thunderstorm activity over western Georgia on Aug 1, 1980. Our calculations indicate that subsequent long-range transport (>200 km) occurred over a lower level of fair weather cumulus convective debris. Long-range transport in such elevated reservoir layers will go undetected by sensors which are Envlron. Sci. Technol., Vol. 18, No. 10, 1984
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coniimed to the surface. This transport pmcess can provide long residence times without significant interaction with surface source or sink mechanisms. Since thunderstorm washout can involve all levels of the troposphere, such elevated layers may be a significant fador in acid deposition. When used in combination with appropriate meteorological and chemical instrumentation, an airborne lidar can be used to resolve the state and dynamics of the mixed layer under complex conditions such as a coastal environment. Lidar measurements obtained over the lower Chesapeake Bay area indicated transport of polluted New England air to the Norfolk, VA, area on July 31, 1981. Four successive overflights recorded the growth of the southeastern V i i n i a mixed layer as it rose up to and into the elevated haze layer. Entrainment of aerosol and m n e from the elevated layer into the relatively clean mixed layer was rapid, with aerosol material and enhanced ozone concentrations reaching the surface within 30 min of the onset of fumigation. These measurements suggest that approximately 30% of the ozone maximum measured a t the surface on July 31,1981, was imported from external sources greater than 500 km upstream. Lidar techniques can provide the altitude distribution of aerosol and ozone in the troposphere and ean therefore provide key information on the long-range transport of atmospheric pollutants. Our lidar measurements show that aerosols may be useful as tracers of transport in such elevated reservoir layers. For the two cases examined in this paper, the elevated "reservoir layers" appear to have been contained vertically and were not in contact with surface sources and sinks of aerosol or gas constituents. These elevated layers may therefore provide ideal laboratories for in situ studies of photochemical transformations. Acknowledgments
We are indebted to the following individuals for their technical support: Syed Ismail, Carolyn F. Butler, Arlen F. Carter, M. Neale Mayo, William, J. McCabe, Norman L. M c h e , and Loyd L. Overbay associated with the WDIAL group; Shirley S. Grice, Charles H. Hudgins, and Envlron. Scl. Technol.. Vol. 18, No.
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Environ. Sci. Technol. 1984, 18, 756-764
George L. Maddrea, Jr., associated with the Cessna aircraft measurements; C. Gerald Clendenin, Jr., Thomas L. Owens, Bordie D. Poole, Jr., and Otto Youngbluth, Jr., associated with the tethered balloon measurements. We thank Jason K. S. Ching, NOAA-EPA, Research Triangle Park, John Salop, Commonwealth of Virginia, State Air Pollution Control Board, Gerald M. Gregory, NASA Langley Research Center, and James E. Smith, Department of Geophysical Sciences, Old Dominion University, for their cooperation and assistance during the case studies. Registry No. Ozone, 10028-15-6.
Literature Cited (1) Browell, E. V.; Carter, A. F.; Shipley, S. T.; Allen, R. J.;
Butler, C. F.; Mayo, M. N.; Siviter, J. H., Jr.; Hall, W. M. Appl. Opt. 1983,22, 522. (2) Vaughan, W. M.; Chan, M.; Cantrell, B.; Pooler, F., Jr. Bull. Am. Meteorol. SOC.1982, 63, 258. Browell, E. V.; Ismail, S.; Shipley, S. T. (submitted for publication in Appl. Opt.). Browell, E. V.; Shipley, S. T.; Butler, C. F.; Ismail, S. “Airborne Lidar Measurements of Aerosols, Mixed Layer Heights, and Ozone During the 1980 PEPE/NEROS Summer Field Experiment”; PEPE/NEROS Archive, CAPITA Special Studies Data Center: Washineton University, St. Louis, MO, 1983. Lyons, W. A.; Dooley, J. C., Jr.; Whitby, K. T. Atmos. Environ. 1978, 12, 621. Burke, H. K. IEEE Trans. Geosci. Remote Sens. 1982, GE-20, 154. Norton, C . C.; Mosher, F. R.; Hinton, B.; Martin, D. W.; Santek, D*;Kuhlow, W*J*APPl. MeteoroL 1980, 199 633. Lyons, W. A.; Calby, R. H. 1983, Final Report on EPA Contract 68-02-3740. Haagenson, P.; Shapiro, M. A. NCAR Technical Note, 1979, NCAR/TN-l49+STR. 1
I
Ching, J. K. S., “The Role of Convective Clouds in Venting Ozone from the Mixed Layer”. Third Conference on Applications of Air Pollution Meteorology, AMs,San Antonio, TX, Jan 12-15, 1982. Wakimoto, R. M. Mon. Weather Rev. 1982, 110, 1060. Sisterson, D. L.; Shannon, J. D.; Hales, J. M. J . Appl. Meteorol. 1979,18, 1421. Austin, J. M.; Fleischer, A. J. Meteorol. 1948, 5, 240. Vukovitch, F. M.; Erlich, D. P.; Clark, T. U.S. Environmental Protection Agency, 1981, EPA 68-02-3428. Orndorff, B. L. MS Thesis, Old Dominion University, Norfolk, VA, 1983. Durham, J. L.; Whelpdale, D. M. (co-chairmen) “United States-Canada Memorandum of Intent on Transboundary Air Pollution”. Atmospheric Sciences Review Sub Group, Work Group 2, 1982, Report 2F-A. Csanady, G. T. “Turbulent Diffusion in the Environment”; Reidel: Dordrecht, The Netherlands, 1973. Uthe, E. E.; Ludwig, F. L.; Pooler, F., Jr. J. Air Pollut. Control. Assoc. 1980, 30, 889. Clarke, J. F.; Ching, J. K. S.; Godowitch, J. M. “Lagrangian and Eulerian Time Scale Relationships and Plume Dispersion from the Tennessee Plume Study”; Sixth Symposium on Turbulence and Diffusion, AMs, Boston, MA, March 22-25, 1983. Danielsen, E. F. Arch. Geophys. Bioklimatol.,Ser. A 1959, AI?
ni 1 ,
nnQ
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(21) McRae, G. J.; Shair,F. H.; Seinfeld, J. H. J. Appl. Meteorol. 1981,20, 1312. (22) Salop, J.; Maier, G. F. J . Air Pollut. Control Assoc. 1978, 28, 1217. (23) Pasceri, R.; Predale, R.; Perritt, A. J . Air Pollut. Control Assoc. 1979, 29, 639. (24) &toque, M. A. J . Atmos. Sei. 1962, 19, 244.
Received for review September 15, 1983. Revised manuscript received April 6, 1984. Accepted April 19, 1984.
Transformation and Fate of Organic Esters in Layered-Flow Systems: The Role of Trace Metal Catalysis Marianna Plastourgou and Michael R. Hoffmann*
Environmental Engineering Science, W.M. Keck Laboratories, California Institute of Technology, Pasadena, California 9 1 125
rn A simple mass transport model with chemical and biochemical reactions has been developed to predict the relative degree of degradation of organic esters in a layered-flow or density-stratified system. A numerical method was used to solve a system differential equations involved when metal-catalyzed hydrolysis, biodegradation, and adsorption were assumed to be major pathways for ester transformation. The relative importance of each pathway for the time-dependent concentration profiles of an organic ester was examined by use of dimensionless parameters given a known initial concentration profile across the depth of the water column for a catalyst, for a microbial population, for particles, and for a hypothetical ester. Both the ester and the catalyst were allowed to interdiffuse and to react. Results show that the characteristic times for metal-catalyzed ester hydrolyses alone are in the range of 6-700 days. These values depend strongly on initial concentration profiles, the magnitude of either the diffusion or dispersion coefficients, and the magnitude of the hydrolysis rate constant. Limitations of model applicability are discussed.
Introduction Current research in the fate of pollutants in aquatic environments has been focused on pathways for pollutant 756
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transformation. Important pathways that have been identified include biological degradation, photolysis, autoxidation, adsorption, and hydrolysis. A primary pathway for the transformation of organic esters in aquatic environments is hydrolysis. Hydrolysis reactions are normally sensitive to a variety of catalytic influences that include specific acid or base catalysis, metal oxide surface catalysis, general acid or base catalysis, and metal ion catalysis. Positively charged metal ions can function effectively as Lewis acids in these reactions and can be expected to catalyze reactions which are similarly catalyzed by Bronsted acids. In this paper, a model is presented that predicts mass transport effects in two-phase flow systems for a given aquatic system in which a catalyst and a substrate interdiffuse and react to yield products. Emphasis has been placed on metal-catalyzed hydrolysis of organic esters as a decomposition pathway although the mathematical formalism presented is equally applicable to biodegradation and adsorption as transformation pathways. Organic esters such as amino acid esters, carbamate esters, organophosphate esters, phthalate esters, and thiophosphate esters have physicochemical properties similar to other trace organic pollutants. In particular, they have low water solubility, they form organic microlayers or micelles, they readily adsorb to particle surfaces, and
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0 1984 American Chemical Society
