Special meteorological and atmospheric tracer studies were conducted during a three-week period in July and August of 1982 in the Brush Creek Valley of northwestern Colorado. The experiments were conducted by the U.S. Department of Energy's Pacific Northwest Laboratory (PNL) as part of the U.S. Environmental Protection Agency-sponsored Green River Ambient Model Assessment (GRAMA) program. The objective of the field experiments was to obtain data to evaluate a model called VALMET, developed at PNL to predict dispersion of air pollutants released from an elevated stack located within a deep mountain valley in the post-sunrise temperature inversion breakup period. Three tracer experiments were conducted in the valley during the two-week period. In these experiments, sulfur hexafluoride (SF6) was released from a height of approximately 100 m, beginning before sunrise and continuing until the nocturnal down-valley winds reversed several hours after sunrise. Dispersion of the sulfur hexafluoride after release was evaluated by measuring SF6 concentrations in ambient air samples taken from sampling devices operated within the valley up to about 8 km down valley from the source. An instrumented research aircraft was also used to measure concentrations in and above the valley.