Kear, Tom P., Tony Held, Daniel P. Chang, J. J. Carroll (1996) During Worst Case Meteorology Are Buoyancy Effects Important?. Institute of Transportation Studies, University of California, Davis, Presentation Series UCD-ITS-RP-96-22
Observations of carbon monoxide (CO) adjacent to high density CO emission sources often fail to exhibit the intensity of CO concentrations predicted by current models used for conformity determination. Meteorological conditions associated with CO episodes are commonly very light wind or calm, stable conditions. Buoyancy of the emissions has been hypothesized as one possible reason for the discrepancy under those meteorological conditions. A preliminary experiment to determine the height to which CO would rise was conducted adjacent to Interstate 80 near Sacramento during February 1996. Sampling lines were carried by two tethered balloons, on either side of Interstate 80, and a 20 meter tall meteorological tower placed above the median strip on an unused overpass. The balloon sampling heights, the sampling locations and tower configuration are shown in the first three figures, respectively. Simultaneous five-minute average "bag" samples and a few continuous analyzer measurements were made. Corresponding traffic volumes during the measurements are shown in the fourth figure. The results of the experiment suggest that CO is carried to levels as high as 20 to 30 meters above the roadway, possibly greater, under strongly stable, near calm, near parallel wind situations. Two examples of CO concentration profiles are shown in the fifth and sixth figure. A model such as CAL3QHC or CALINE4 would not fully account for the extent of the vertical dispersion or the buoyant rise of the plume. Plans for further numerical modeling and experimental studies of the phenomenon are also presented.