The NGS Visibility Study was conducted by the SRP, the operators of NGS, from January 10 through March 31, 1990. Its purpose was to address visibility impairment in Grand Canyon National Park during the winter months and the levels of improvement that might be achieved if SO2 emissions from NGS were reduced. The study was performed to provide input to the rulemaking process of the EPA regarding NGS SO2 controls. Perfluorocarbon tracers (PFT) were released from the three stacks of NGS. Surface and upper air meteorology, particle and gaseous components, and tracer were measured at many sites. The study concluded that the NGS plume was not present at Hopi Point for most of the days. The tracer data quality from this experiment was insufficient for quantitative source apportionment and the results emphasized the need for better tracer measurements in future studies.