This study was carried out to establish a visibility and air quality climatology for the Grand Canyon to help evaluate the extent and conditions under which the degradation of visibility occurs in the region, and to investigate long-term relationships between airmass transport and observations at the sampling sites. Data from a multi-year monitoring program called SCENES was organized according to several sampling and analysis protocols, and then sorted according to the levels of haziness, humidity and a tracer for urban emissions. The aerosol composition was studied as a function of season, location, time of the day and the air-quality parameters above. A method was then devised to investigate the potential of routine back trajectory analyses to locate sources of contaminant in air at the Grand Canyon. The distribution of back trajectory segment endpoints on a latitude-longitude grid was examined as a function of the contaminant's concentration, and the cells associated with high receptor's observations were identified as candidate emissions sources. Screening of chance associations due to statistical fluctuations was performed, and results were evaluated for real and hypothetical tracers with known distributions. SCENES data was used to investigate the preferential association of several variables with regions around the Grand Canyon.

Chemical mass balance calculations indicate that aerosols in the region exhibit substantial seasonal variation in composition and concentration, but little systematic diurnal variation. Ambient humidity affects the composition of the aerosol, but not its total concentration. Although fine mass correlated strongly with light scattering, as expected, little association between chemical composition and light scattering was seen. Results of the back trajectory analysis showed that the method was able to resolve the direction of potential sources, but with poor radial precision. In general, lowest levels of fine-mass and sulfur concentrations, as well as of light scattering by particles, are associated with northerly flow. Northern airmasses generally have higher speeds than the southern airmasses, and they are transported over regions with fewer emissions. The airmasses with the highest fine-mass concentrations and light scattering values usually come from the south, and they are consistently associated with both southern California and central and southern Arizona.