Clean Air Corridors: A Geographic and

Volume 47 March 1997 Journal of the Air & Waste Management Association 403

Green and GebhartISSN 1047-3289 J. Air & Waste Manage. Assoc. 47: 403-410

Copyright 1997 Air & Waste Management Association

TECHNICAL PAPER

Clean Air Corridors: A Geographic andMeteorologic Characterization

Mark C. GreenDesert Research Institute, Las Vegas, Nevada

Kristi A. GebhartNational Park Service, Air Resources Division, CIRA, Colorado State University, Fort Collins, Colorado

ABSTRACTMeteorological factors, pollutant emissions, and geo-graphic regions related to transport of low optical extinc-tion coefficient air to Grand Canyon National Park wereexamined. Back trajectories were generated by two mod-els, the Atmospheric Transport and Dispersion Model(ATAD) and an approach using the Nested Grid Modeloutput for a Lagrangian particle transport model (NGM/CAPITA). Meteorological information along the trajecto-ries was analyzed for its relationship to visibility at theGrand Canyon. Case studies considered days with anoma-lously clean air from the southwest and dirty air from thenorthwest. Clean air was most frequently from the northand northwest, rarely from the south. Low emissions, highventilation and washout by precipitation was associatedwith clean air. All clean days with transport from the LosAngeles area had upper-level low pressure over the regionwith high ventilation and usually abundant precipitation.The dirtiest days with transport from the northwest wereaffected by forest fires.

INTRODUCTIONVisibility at national parks and wilderness areas has been

of considerable concern since these areas were first af-forded visibility protection in the Clean Air Act Amend-ments of 1977. The Clean Air Act Amendments of 1990required the establishment of the Grand Canyon Visibil-ity Transport Commission (GCVTC), and required thecommission to issue a report to the U.S. EnvironmentalProtection Agency addressing visibility issues including“the establishment of clean air corridors, in which addi-tional restrictions on increases in emissions may be ap-propriate to protect visibility in affected Class I areas.”

Pitchford et al.1 suggested the possibility of clean aircorridors after back-trajectory analysis from the GrandCanyon revealed that six of the ten clearest days and noneof the 10 haziest days were associated with trajectoriescoming from the north and northwest. White et al.2 clas-sified back-trajectories from the Grand Canyon for 1988-1989 as coming from one of four quadrants (northwest,northeast, southwest, and southeast), or from a mixtureof quadrants. Air arriving from the northwest and north-east quadrants was found to be significantly clearer (lowerextinction coefficient) than air arriving from the south-west and southeast quadrant. White et al. and Pitchfordet al. used back trajectories calculated with the NOAA At-mospheric Transport and Dispersion Model (ATAD).3

Neither of the aforementioned papers substantiallyaddressed the meteorological factors that may have con-tributed to the cleanliness of air coming from the north.This paper considers in greater detail the effect of meteo-rological conditions along transport routes on visibilityat Grand Canyon National Park. Increased dispersion ofpollutants due to increased mixing depths or wind speedsmay be expected to be related to good visibility condi-tions. Washout of pollutants by rain or snow prior to ar-rival at a site may also be expected to contribute to goodvisibility. It has been speculated4 that clean air masses aredue to large-scale subsidence that precludes contact withpollutant emissions from surface sources. These hypoth-eses are tested in this paper. The relationship between the

IMPLICATIONSThis analysis describes meteorological factors governingvisible quality on clear days at Grand Canyon National Park.The effects of pollutant emissions along air trajectories enroute to the Grand Canyon are also considered. Clear airat the Grand Canyon comes mainly from the northwestdue to enhanced dispersion and washout by precipitationfrom the northwest and the low pollutant emission densityin areas northwest of the Grand Canyon. The analysissupports the concept of “clean air corridors”. Althoughthese areas have generally favorable meteorological con-ditions for supplying clear air to the Grand Canyon, goodvisibility days in the park are sensitive to the level of emis-sions in this region.

404 Journal of the Air & Waste Management Association Volume 47 March 1997

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extinction coefficient and emissions that air trajectoriespassed over also is investigated.

METHODOLOGYBack trajectories from Grand Canyon National Park (HopiPoint: longitude 112 degrees, 4 minutes west; latitude 36degrees, 4 minutes north, elevation 2164 m) generatedby ATAD and the National Meteorological Center’s NestedGrid Model5 coupled with the CAPITA Monte Carlo par-ticle dispersion model6,7 were used. For this analysis, itwas desirable to have several years of back trajectories tocompare with visibility data. Although the ATAD modelused here is simple, it has performed well compared tomuch more sophisticated models.8 The Meteorology Sub-committee of the Grand Canyon Visibility Transport Com-mission has evaluated the ability to predict wind speedand direction of the ATAD and the RAMS model (devel-oped at Colorado State University). Although there aredifferences between the two models, overall performancewas similar for both. Because it is computationally effi-cient, ATAD can be economically applied to many yearsof data. This helps to avoid the possible problems of in-ter-annual meteorological variability as well as to increasethe statistical power of the analysis by increasing the num-ber of cases included.

When considering ATAD results averaged over a rea-sonable number of trajectories, as used in this paper, ATADobtains an acceptable level of skill. Figure 1 shows aver-age methylchloroform concentrations (ppt) at SpiritMountain, NV, versus the percentage of trajectories pass-ing over the grid cell containing Los Angeles by synopticweather pattern (as determined by Cover9). Average con-centrations of methylchloroform, an endemic tracer ofthe Los Angeles Basin, are closely related to frequency of

trajectories predicted by ATAD to pass over the grid cellcontaining Los Angeles (r2 = 0.78).

In brief, the ATAD model is a Lagrangian particlemodel with a single variable depth transport layer, thedepth of which is determined by atmospheric stabilityusing interpolation of measured vertical temperature pro-files. Average transport layer winds are interpolated spa-tially and temporally from nearby radiosonde stations.Complex terrain is included only by its effect on the ra-diosonde observations. Trajectories are constrained to re-main within the transport layer depth; no vertical mo-tion is accounted for. Back trajectories are started every 6hours, with locations determined every 3 hours. Thisanalysis linearly interpolated the 3-hour positions to 1-hour positions. Climatological transport frequencies weredetermined for 1979-1992; comparisons to extinction(transmissometer) measurements were done for 1988-1992.

The Nested Grid Model (NGM), a numerical meteo-rological model, was designed to forecast the evolution,movement, and decay of synoptic scale weather systemsfor weather forecasting purposes. It is run by the NationalMeteorological Center. The model has a grid spacing of90 km and outputted fields of meteorological variablesevery hour for the first 12 hours. Complex terrain is con-sidered, but only crudely, due to the coarseness of thegrid spacing. For this analysis, a subset of the outputarchived by NOAA’s Air Resources Laboratory, using ev-ery other horizontal grid point and every 2 hours at tenvertical levels, was used. The CAPITA (Center for Air Pol-lution Impact and Trend Analysis) Monte Carlo modelused the NGM wind fields to generate back trajectoriesfor 10 particles released once every 2 hours, with posi-tions calculated every 2 hours. All particles determinedto be in the mixed layer were randomly assigned a heightwithin the mixed layer. Particles above the mixed layerwere transported vertically by the NGM vertical velocity.Particles at different heights have different horizontaltransport due to vertical wind shear. At each 2 hour inter-val, particle latitude, longitude, height above ground, rela-tive humidity, temperature, mixing height, and precipi-tation data were extracted. This information was suppliedby investigators from CAPITA, Washington University.This combined modeling approach, applied in 1992, willbe referred to as the NGM/CAPITA model.

For this analysis, trajectories were evaluated to deter-mine whether or not they passed over grid cells of 2 de-grees latitude by 2 degrees longitude. The frequency oftrajectories (particles) passing over grid cells were deter-mined for all conditions, and by time of year. Clean vis-ibility conditions are those in the lowest 20th percentileof the extinction coefficient as recommended by theAerosol and Visibility Subcommittee of the GCVTC. The

Figure 1. Spirit Mountain average methylchloroform concentrations(ppt) versus percentage of trajectories passing over Los Angeles gridcell, by synoptic weather pattern (as determined by Cover, et al. 1989).


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extinction coefficient was measured with a transmissom-eter system located on the southern rim of the GrandCanyon.10 Meteorological conditions occurring while thetrajectories passed over grid cells were extracted or de-rived from the data and summarized.

Parameters were calculated for each trajectory, venti-lation and emission. The ventilation parameter was theaverage of hourly values of mixed layer depth multipliedby wind speed (from ATAD). The emissions parameter wasgenerated by summing hourly time weighted emissionsfrom each grid cell over the trajectory. The emissions datawas a sum of SOX, NOX, NH3, organic and elemental car-bon, PM2.5, and reactive organic gases. These species maybe expected to affect visibility directly or by reaction withother compounds. A plain sum of the emissions of eachspecies was used to keep the analysis simple; it is acknowl-edged that the sum is not expected to accurately repre-sent the relative importance of each compound. Emis-sions were weighted inversely by distance, inversely bytime, or unweighted. The inverse time weighting was usedbecause it gave the best results (as determined by regres-sion analysis). A weighting factor of 1 was used for back-trajectory times up to 12 hours, then was 12/(time inhours) for 13-120 hours; after 5 days, emissions would becounted at only 10%. The weighting is a simple methodto account for increased dispersion, deposition, and tra-jectory uncertainty with increasing time.

Trajectories and synoptic scale weather conditions forselected days were also investigated. This included anoma-lous cases with clean air at the Grand Canyon previouslypassing over southern California and dirty air passing overusually clean areas to the north.

RESULTSThe results are presented for ATAD for 1988-1992 and for1992 for NGM. Comparisons of ATAD for the 1988-1992period with 1992 showed minor differences between 1992and the 5-year period. In 1992, transport from the south-west was more frequent than in the 1988-1992 period,while transport from the northwest was less frequent in1992. However, the differences were small.

Variations in Supplying Clean Air by Geo-graphic Region and Time of Year

Figure 2 shows the percentage of trajectories arriving atGrand Canyon that passed over each 2 degrees latitudeby 2 degrees longitude grid cell, ATAD, during the 1988-1992 period. The contour interval for all frequency mapsis 5%. Trajectories were most frequent from the north-northwest quadrant and the southwest and considerablyless common from the east. The percentage of particlesarriving at the Grand Canyon that passed over each 2 x 2cell, NGM/CAPITA, in 1992, is shown in Figure 3. Results

are similar to those from ATAD, except trajectories arriv-ing from the southwest showed higher frequency overinterior California, whereas ATAD trajectories were off-shore from the California coast. This is expected to resultfrom a bias in the NGM model.11 The bias results from theNGM model incorrectly shifting the effect of the north-westerly winds due to the Pacific High into the desertsouthwest in summer; in actuality, these winds stop nearthe coast and become mainly southwesterly. The ATADmodel does not have this bias because more input datais used in the southwestern United States for the ATADanalysis.

Because the ATAD model results are available for alonger time period and ATAD does not have the summer-time bias of the NGM, the remaining figures, except forprecipitation, present the ATAD results. Figure 4 showsthe percent of trajectories arriving with clean air at the

Figure 2. Percentage of trajectories arriving at the Grand Canyonthat passed over each 2 degrees latitude by 2 degrees longitude gridcell, ATAD, 1988-1992. Contour interval for all frequency maps is 5%.

Figure 3. Percentage of particles arriving at Grand Canyon thatpassed over 2 x 2 cell, NGM/CAPITA, 1992.


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Grand Canyon that passed over each grid cell during 1988-1992. Clean air most frequently arrives from a sector cen-tered in the north-northwest region. The NGM gave simi-lar results, except with a slight shift toward the north.

Trajectories from the northwest were most likely tobe associated with clean air, while trajectories from thesouth were rarely associated with clean air (see Figure 5).A strong gradient occurs from north to south across cen-tral California in the frequency of supplying clean air. TheNGM probability of supplying clean air is similar to theATAD results, except the maximum frequency of cleanair is from the north instead of north-northwest, and thegradient in central California is weaker.

Figures 6 and 7 illustrate seasonal differences in trans-port to the Grand Canyon. The greatest frequency of tra-jectories from the northwest occurs from November 16-

13, based on average calculations for the period 1979-1992(Figure 6) and the highest number of days with clean airat the Grand Canyon (see Figure 8). The second half ofJune has the greatest frequency of transport from thesouthwest (Figure 7), and among the lowest frequenciesof clean air at Grand Canyon (see Figure 8). Figure 8 showsthe number of clean air days at Grand Canyon for theperiod 1988-1992 by one-half month periods. Clean airdays are most frequent in late autumn through early springand are rare in summer. Note the rapid decrease in cleanair days from early to late April. The relatively high fre-quency of clean air days in late August was due to highventilation from unusually early Pacific storms in the re-gion during 1990.

Effects of Wind Speed and VentilationAverage wind speeds were higher to the north and lowerto the south, especially the southwest (see Figure 9). This

Figure 5. Percentage of trajectories passing over 2 x 2 grid cell thatarrived at the Grand Canyon during clean air periods, ATAD 1988-1992.

Figure 4. Percentage of trajectories arriving with clean air at theGrand Canyon that passed over each 2 x 2 grid cell, ATAD 1988-1992. Clean air defined by Aerosol and Visibility Subcommittee of theGCVTC is the lowest 20th percentile of extinction.

Figure 7. Percentage of trajectories arriving at the Grand Canyonpassing over 2 x 2 grid cell, ATAD June 16-30, 1979-1992.

Figure 6. Percentage of trajectories arriving at the Grand Canyonpassing over 2 x 2 grid cell, ATAD November 16-30, 1979-1992.


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indicates greater dispersion for trajectories along the axisfrom the north; that greater wind speed appears to be ameteorological factor contributing to the clean air fre-quency of coming from the north. Like ATAD, NGM/CAPITA wind speeds were higher from the north and lowerfrom the south. NGM/CAPITA wind speeds were higherthan ATAD from the north because NGM/CAPITA pre-dicts air transport that occurs higher in the atmosphere,where wind speeds are stronger.

Mean transport layer depths are generally about 2000meters, but decrease considerably over California, particu-larly offshore (see Figure 10). The exceptionally low val-ues off the southern California coast are primarily due tothe presence of inversions over the cool ocean surface insummer. Ventilation, defined here as the transport layerdepth multiplied by the mean transport layer wind speed,is shown in Figure 11. Areas with high ventilation to thenorth are likely to supply clean air to the Grand Canyon

(see Figure 5), while areas of low ventilation, notablysouthern California, are not likely to supply clean air tothe Grand Canyon.

Effects of Vertical MotionThe NGM/CAPITA model was used to test the hypothesisthat clean periods were the result of large scale subsid-ence causing air arriving at the Grand Canyon to havelittle contact with emissions. The percentage of particleswithin the mixed layer when passing over each grid cellwas determined for all visibility conditions, clean condi-tions, and dirty conditions. If the hypothesis is correct,then it would be expected that for all conditions, areas tothe northwest that frequently supply clean air would havefew particles within the mixed layer, while areas to thesouthwest, which usually provide dirty air, would havemany particles in the mixed layer. Also, trajectories for

Figure 8. Number of clean air days at the Grand Canyon, 1988-1992, by one-half month period.

Figure 9. Average wind speed over cell (m s-1) en route to the GrandCanyon, ATAD 1988-1992.

Figure 11. Mean ventilation (transport layer depth multiplied by windspeed, in 1000’s m2 s-1), over grid cell en route to the Grand Canyon,ATAD 1988-1992.

Figure 10. Mean transport layer depth, in meters, over cell en routeto the Grand Canyon, ATAD 1988-1992.


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clean periods would have a lower percentage of particleswithin the mixed layer than trajectories for dirty periods.

The analysis did not show substantial differences inthe percentage of particles in the mixed layer as a func-tion of transport direction or visibility condition. In fact,with the exception of the grid cells closest to Grand Can-yon, only 10-15% of the particles were predicted to bewithin the mixed layer for the range of visibility condi-tions and transport directions. The low percentage of par-ticles in the mixed layer is expected to be due to the unre-alistically low mixing depths predicted by the NGM. Thesedepths are much lower than ATAD mixed layer depthsand average mixing depths reported by Holtzworth.12

Thus, the analysis did not support the hypothesis thatclean air periods were clean due to large scale subsidence;however, due to apparent problems with the model, itcannot be conclusively determined that the hypothesis isinvalid.

Effects of PrecipitationNext, the relationship between precipitation (predictedby the NGM/CAPITA model) and probability of supply-ing clean air to the Grand Canyon was considered. Areasto the northwest, which frequently supply clean air tothe Grand Canyon, have higher precipitation rates thanareas to the southwest, which infrequently supply cleanair to the Grand Canyon (see Figure 12). To reduce theconfounding effects of other variables such as emissions,average precipitation rates predicted by the model forclean days and dirty days 80th-100th percentiles of extinc-tion were calculated. For transport from most areas, pre-cipitation was substantially greater for clean days thanfor dirty days.

An additional analysis13 used gridded precipitationmeasurements and the ATAD back trajectories from theGrand Canyon to assess the relationship between pre-cipitation along the back trajectories and the extinction

Figure 12. Average predicted precipitation rate (mm hr-1x100) overcell en route to the Grand Canyon, NGM/CAPITA 1992.

coefficient. Extinction coefficients from each quadrant(NW, SW, NE, and SE) were lower for back trajectories withwidespread precipitation than for those with scattered orno precipitation. In addition, the extinction coefficientassociated with back trajectories from the northwest waslower when the precipitation occurred within the previ-ous 1 to 3 days when the precipitation occurred 4 or 5days back. Even among the relatively low emissions areato the northwest of the Grand Canyon, emissions ad-versely affected visibility unless washed out by precipita-tion. Clean air conditions associated with transport fromthe southwest quadrant (high in emissions) almost neveroccurred without widespread precipitation.

Case Studies of Anomalous ConditionsIn order to more thoroughly understand the relationshipsbetween meteorology and visibility, case studies were donefor two anomalous patterns: very clean days with trajec-tories passing over the grid cell containing the Los Ange-les Basin; and very dirty air passing over each of two typi-cally clean areas. During the period 1988-1992, 13 veryclean days (10th percentile bext or less) had trajectories pass-ing over the Los Angeles area. All 13 days had an upperlevel low pressure over the area (high ventilation) andmost had abundant precipitation (washout). As an ex-ample of these conditions, back trajectories for March 24-25, 1992, are shown in Figure 13. levels of bext for thesedays were at the 1.10 and 2.27 percentile levels, respec-tively. About 1 inch of rain was recorded at Los AngelesInternational Airport during this period.

There were eight very dirty days (bext 90th percentileor greater) from each of two grid cells that typicallysupply clean air to the Grand Canyon (one in westcentral Oregon, the other near the border of Utah, Ne-vada, and Idaho). Several of these days appeared to beaffected by forest fires, others were nearly stagnant, and

Figure 13. Back trajectories for March 24-25, 1992, ATAD.


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the remainder had transport from high emission areas(central and southern California). For example, back tra-jectories for September 13-16, 1988, are shown in Figure14. These were dirty days, with bext percentile levels of99.66, 99.59, 99.31, and 98.35, respectively, passing overareas often supplying clean air to the Grand Canyon.During the summer of 1988, there were many forest firesin the western United States, including the Yellowstonefire. Aerosol data at the Grand Canyon for this periodshowed high levels of organic carbon, supporting indica-tions that the poor visibility conditions resulted from for-est fires.

Relative Effects of Emissions and VentilationSmoothed emission density contours are shown in Figure15. Numbers (in metric tons per day) represent the sum

of SOX, NH3, PM2.5, elemental and organic carbon, plusreactive organic compound emission totals for 2 degreeslatitude by 2 degrees longitude grid cells. Biogenic emis-sions are not included. Transport over areas with low emis-sions to the north and northwest are often associated withlow extinction at the Grand Canyon (see Figure 5), whiletransport over areas with high emissions to the south-west and southeast are rarely associated with clean air atthe Grand Canyon. Thus, areas to the northwest have bothmeteorological i.e., good dispersion and washout by pre-cipitation) and emissions conditions that would contrib-ute to clean air; areas to the southwest have meteorologi-cal and emissions conditions that would contribute todirty air. A question arises: What is the relative impor-tance of meteorology and emissions upon visibility? Asdescribed in the methodology section, ventilation andemissions parameters were calculated for each trajectoryto test the relative importance of emissions and ventila-tion upon the extinction coefficient.

Figure 16 shows a smoothed contour plot of bext bypercentiles of extinction and ventilation. Throughout therange of ventilation, increasing extinction is associatedwith increasing emissions, although sensitivity of extinc-tion to emissions is less at high ventilation. Ventilationshows little relationship to extinction until it reaches the70th percentile, where ventilation becomes increasinglyrelated to low extinction. For most of the domain, emis-sions show a greater relationship to extinction than ven-tilation.

CONCLUSIONSATAD and NGM results show that most of the cleanair arriving at the Grand Canyon has trajectories fromthe north to northwest. The frequency of these trajec-tories, and clean air at the Grand Canyon, is greatest

Figure 14. Back trajectories for September 13-16, 1992, ATAD.

Figure 15. Emission density contours (smoothed). Numbers (in metrictons per day) represent the sum of SOX, NH3, PM2.5, elemental andorganic carbon, and reactive organic compound emission totals for 2degrees latitude by 2 degrees longitude grid cells. Biogenic emissionsare not included.

Figure 16. Contour plot of bext (Mm-1) by percentiles of ventilationand emission.


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from October 1 to April 15 and is rare in the summermonths. The likelihood of the presence of clean air at theGrand Canyon is greater for trajectories from the northand lower for trajectories from the south, with a strongnorth-south gradient, particularly across central Califor-

nia. Meteorological factors associated with clean air werehigh ventilation and precipitation along the trajectorybefore reaching the Grand Canyon. No support was foundfor the hypothesis that clean air from the north was pri-marily due to subsidence of drifting air that had little con-

tact with emissionsClean air days with trajectories passing over the grid

cell containing the Los Angeles Basin were associated withhigh ventilation and usually heavy precipitation. Dirtyair days in usually clean areas were associated with forest

fires or stagnation conditions.Transport routes over low emission areas were often

clean, while clean air conditions rarely occurred withtransport over high emission areas. Throughout the rangeof ventilation, increasing extinction was associated with

increasing emissions; ventilation was important onlyabove the 70th percentile of ventilation.

ATAD proved to be useful for determining statisticsof back trajectories over several years, and appeared todemonstrate an adequate level of skill as compared with

RAMS and predicting endemic tracer concentrations.NGM appeared to have greater bias in the wind directionand apparent underestimation of the mixing depths,which renders the CAPITA particle dispersion results assuspect.

The concept of a clean air corridor appears to be ap-plicable for protecting high visibility periods at GrandCanyon National Park. While areas to the north are likelyto provide clean air to the Grand Canyon, much of theclean air comes from a relatively narrow sector centered

on the north-northwest region. The analysis indicates thatwhile the areas supplying clean air have generally favor-able meteorology for good visibility, visibility measuredat the Grand Canyon is sensitive to the level of emissionsencountered along the trajectory. Thus, increases in emis-

sions in areas currently providing clean air have the po-tential of degrading the high visibility periods.

DISCLAIMERThe assumptions, findings, conclusions, judgments, andviews presented herein are those of the authors and shouldnot be interpreted as necessarily representing official Na-tional Park Service policies.

REFERENCES1. Pitchford, A.; Pitchford, M.; Malm, W.; Flocchini, R.; Cahill, T.;

Walther, E. “Regional analysis of factors affecting visual air quality,”Atmos. Environ. 1981, 15, 2043.

2. White, W.H.; Macias, E.S.; Kahl, J.D.; Samson, P.J.; Molenar, J.V.; Malm,W.C. “On the potential of regional-scale emissions zoning as an airquality management tool for the Grand Canyon,” Atmos. Environ.1994, 28, 1035.

3. Heffter, J.L. Air Resources Laboratories Atmospheric Transport andDispersion Model (ARL-ATAD), Technical Memorandum ERL ARL-81; NOAA, Rockville, 1980.

4. Ostapuk, P.M.; Fosdick, E. “Regional analysis of factors affecting vi-sual air quality, discussion,” Atmos. Environ. 1982, 16, 2369.

5. Hoke, J.E.; Phillips, N.A.; DiMego, G.J.; Tucillo, J.J.; Sela, J.G. “Theregional analysis and forecast system of the National MeteorologicalCenter,” Weather and Forecasting 1989, 4, 323.

6. Patterson, D.E.; Husar, R.B.; Wilson, W.E.; Smith, L.F. “Monte Carlosimulation of daily regional sulfur distribution: Comparison withSURE sulfate data and visual range observations during August 1977,”J. Appl. Meteor. 1981, 20, 404.

7. Schichtel, B.; Husar, R. “Progress Report #3, Regional Modeling Sup-port for the Grand Canyon Visibility Transport Commission”, Na-tional Park Service, Fort Collins, CO, 1992.

8. Green, M.C.; Ashbaugh, L.; Farber, R.B. “Evaluation of Meteorologi-cal Fields Used in GCVTC Analyses,” report prepared for the Meteo-rology Subcommittee, Grand Canyon Visibility Transport Commis-sion, August 1995.

9. Cover, D.E.; Mitchell, D.S.; Zeldin, M.D.; Farber, R.B. “A computer-aided meteorological classification system for the desert southwest,”in Transactions: Visibility and Fine Particle; Matthai, C.V., Ed.; Air &Waste Management Association: Pittsburgh, PA, 1990, pp. 489-496.

10. Air Resource Specialists, Inc., National Park Service Visibility Moni-toring and Data Analysis Program, Summary of Revised Transmis-someter-Based Visibility Data: Winter 1987 through Spring 1993 Sea-sons, Air Resource Specialists, Inc.: Fort Collins, CO, August 1993.

11. Douglas, M.W. “The summertime low-level jet over the Gulf of Cali-fornia: Mean structure and synoptic variation,” in Preprints, 20th Con-ference on Hurricanes and Tropical Meteorology; American Meteorologi-cal Society: Boston, MA, 1993; pp. 504-507.

12. Holzworth, G.C. Mixing Heights, Wind Speeds, and Potential for Ur-ban Air Pollution throughout the Contiguous United States, U.S. Envi-ronmental Protection Agency: Research Triangle Park, NC, 1972;Publication AP-101.

13. Hoffer, T.E.; Green, M.C. “Precipitation Effects on Visibility Measure-ments,” report prepared for the Meteorology Subcommittee, GrandCanyon Visibility Transport Commission, August 1995.

About the AuthorsM.C. Green (corresponding author) is an assistant researchprofessor at Desert Research Institute, 755 E. Flamingo, LasVegas, NV 89119; fax: (702) 895-0496. Kristi Gebhart is aresearch physical scientist at Air Resources Division, Na-tional Park Service, CIRA, Colorado State University, FortCollins, CO 80523.

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Clean Air Corridors: A Geographic and