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Daily particulate concentrations in the Desert South-west from 1984 to 1988 are analysed using a synoptic climatology based upon tropospheric thermal, moisture, and flow conditions. Using a combination of principal components analysis and cluster analysis, 13 distinct synoptic types (clusters) are identified that correspond to seasonal variations in the atmosphere’s general circulation. These include summer clusters, which reflect monsoonal and non-monsoonal conditions, winter clusters defined by the positions of the polar and subtropical jets and shortwave ridges and troughs, and clusters that predominate in the transition seasons.

Particulate concentrations vary significantly by synoptic type. Three clusters have poor air quality. ’summer monsoon’ days have high humidity and cloud cover, conditions that encourage aerosol growth. ’Continental high’ days have a slow-moving anticyclone centred over the Great Basin that traps particulates within the boundary layer. ’Rockies ridge’ days are associated with strong south-westerly flow and the advection of regional haze from southern California into the Desert South-west. In all of these clusters, high humidity or cloud cover enhance particulate growth. Good air quality in the summer occurs on monsoonal ’break’ days when relative humidity and cloud cover decrease. In the winter, good air quality occurs when Arctic or continental polar air masses are present, or when either the polar or subtropical jets are strong and positioned over the South-west. In some of these latter situations, widespread precipitation removes particulates through wet deposition.

These results suggest that the Navajo Generating Station (NGS), a large coal-fired power plant on the Arizona-Utah border, does not contribute significantly to air quality degradation in the Grand Canyon National Park, Arizona. Most of the days with high particulate levels are associated with transport from southern California or unstable summer days with high humidity and contributions from many point and regional sources. There may be some contribution from the NGS on ’continental high’ days, but when this cluster occurs on consecutive days, air quality improves rather than declines. This research provides a useful framework for future mesoscale modelling studies of air quality in south-western USA.