AN ASSESSMENT OF AIR-QUALITY VARIATIONS IN THE SOUTH-WESTERN USA USING AN UPPER AIR SYNOPTIC CLIMATOLOGY

Daily particulate concentrations in the Desert South-west from 1984 to 1988 are analysed using a synoptic climatology based upon tropospheri c thermal, moisture, and flow conditions. Using a combination of princ ipal components analysis and cluster analysis, 13 distinct synoptic ty pes (clusters) are identified that correspond to seasonal variations i n 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 shortw ave ridges and troughs, and clusters that predominate in the transitio n seasons. Particulate concentrations vary significantly by synoptic t ype. Three clusters have poor air quality. 'Summer monsoon' days have high humidity and cloud cover, conditions that encourage aerosol growt h. 'Continental high' days have a slow-moving anticyclone centred over the Great Basin that traps particulates within the boundary layer. 'R ockies ridge' days are associated with strong south-westerly flow and the advection of regional haze from southern California into the Deser t South-west. In all of these clusters, high humidity or cloud cover e nhance particulate growth. Good air quality in the summer occurs on mo nsoonal 'break' days when relative humidity and cloud cover decrease. In the winter, good air quality occurs when Arctic or continental pola r air masses are present, or when either the polar or subtropical jets are strong and positioned over the South-west. In some of these latte r situations, widespread precipitation removes particulates through we t deposition. These results suggest that the Navajo Generating Station (NGS), a large coal-fired power plant on the Arizona-Utah border, doe s 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 unst able 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 da ys, air quality improves rather than declines. This research provides a useful framework for future mesoscale modelling studies of air quali ty in south-western USA.