URBAN 2000 Multi-scale Tracer Experiment

Location and Date:

Salt Lake City, UT in October 2000.


Scientists, funded by the U. S. Department of Energy’s (DOE) Chemical and Biological National Security Program (CBNP) of the National Nuclear Security Administration, conducted a comprehensive field tracer study in an urban environment. That study has become known as URBAN 2000. Scientists wanted to see how the tracer sulfur hexafluoride (SF6), simulating a chemical or biological agent, would transport and disperse in a downtown area and then into a surburban area. Since 2000, its applicability to homeland security


Aerial view of Salt Lake City. Photo by Don Green Photography.
has been readily apparent. The study was also extended beyond the suburban scale by including DOE's region-wide Vertical Transport and Mixing (VTMX) tracer and meteorological study.


  • Conduct field studies for evaluating transport and dispersion components of atmospheric models;

  • Concurrently resolve interacting scales of motion from building to regional scales;

  • Analyze data to understand governing physical processes.


The Field Research Division released the atmospheric tracer sulfur hexafluoride SF 6 during seven nocturnal Intensive Operating Periods (IOPs) and sampled the tracer using continuous real-time tracer gas analyzers (TGAs) and bag samplers. Tracer releases included both point releases and line releases. Six TGAs were deployed both as mobile and stationary units. Stationary bag samplers called programmable integrating gas samplers (PIGS) were positioned at 100 locations in a downtown urban sampling grid and on 1 km, 2 km, 4 km, and 6 km arcs. Bag samplers were also placed on three building tops in the downtown area and on 2 and 4 km arcs upwind of the release site.

Results and Conclusions:

Tracer concentrations up to 245,000 parts per trillion by volume (pptv) were measured by the bag samplers with 4% of the samples having concentrations over 10,000 pptv. Most bag sampler tracer concentrations (35%) were below the method limit of detection (MLOD) of 14 pptv. An additional 17% of bag sample tracer concentrations ranged between the MLOD and the method limit of quantitation (MLOQ), which was 45 pptv. All other bag sample tracer concentrations ranged between 45 and 10,000 pptv. Significant tracer concentrations were measured at building tops in all IOPs. Building-top tracer concentrations appeared to be both cyclic (of same value) and periodic. On many occasions, the tracer was observed by the mobile (TGAs) to be hugging the foot of the mountains along the NW 6 km arc, indicating very distinct topographic forcing of the tracer. Generally, the SF6 tracer plume moved in a generally north-west direction from the downwind release site, as evidenced by both the bag sample and mobile real-time analyzer tracer concentrations. However, light winds observed during IOPs 2, 4, and 7 resulted in tracer concentrations being observed at 2 and 4 km arcs thought to be upwind of the release site. The tracer material during these IOPs did not rapidly disperse between tracer release periods, and significant quantities of the tracer remained to add to the concentrations from the subsequent releases. A mesoscale recirculation pattern back toward the downtown area was observed after sunrise during IOPs 5 and 7 and perhaps weakly during IOPs 2 and 4.


SF6 release site, release mechanism, and associated pictures


  • Clawson, K. L. 2000. URBAN 2000 Post Deployment Briefing. Quarterly CBNP Worksthop, Livermore, CA, 14 Nov 2000. [View / download PDF ]

  • Clawson, K. L., et al. 2001. URBAN 2000 one year later: A NOAA/FRD perspective. VTMX/URBAN Workshop, Salt Lake City, UT, 11-13 Sep 2001. [View / download PDF ]

  • Clawson, K. L., et al. 2002. URBAN 2000 SF6 Atmospheric Tracer Results from the Suburban Experiment Domain. 4th Symposium on the Urban Environment, Sponsored by the American Meteorological Society, Norfolk, VA, 20-24 May 2002. Paper P12.4, 2 pp. [View / download PDF]


  • Allwine, K. J., J. H. Shinn, G. E. Streit, K. L. Clawson, and M. Brown. 2002. An overview of URBAN 2000: A multi-scale field study of dispersion through an urban environment. Bull. Amer. Meteor. Soc., 83, 521-536. [View / download PDF ]

  • Clawson, K. L., R.G. Carter, D.J. Lacroix, T.K. Grimmett, J.D. Rich, N.F. Hukari, R. Eckman. B.R. Reese, J.F. French, R.C. Johnson, T.L. Crawford. 2004. URBAN 2000 SF6 Atmospheric Tracer Field Tests. NOAA Technical Memorandum OAR ARL-253. 155 pp. [View / download PDF]

  • Clawson, K. L., and G. H. Crescenti. 2002. Meteorological measurements during the URBAN 2000/VTMX field study. NOAA Technical Memorandum OAR ARL-243. 54 pp. [View / download PDF]

  • Fast, J.D., K.J. Allwine, R.N. Dietz, K.L. Clawson, and J.C. Torcolini. Dispersion of perfluorocarbon tracers within the Salt Lake Valley during VTMX 2000. Journal of Applied Meteorology and Climatology, 45(6):793-812.



Roger Carter
1750 Foote Dr.
Idaho Falls, ID 83402
email: Roger.Carter@noaa.gov

Last update: January 30, 2006

Modified: March 1, 2011
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