Regional scale transport over complex terrain, a case study: tracing the Sacramento plume in the Sierra Nevada of California

Aircraft measurements of the Sacramento pollutant plume were obtained over the western Sierra Nevada on 7 days in the summers of 1995 and 1996. These measurements, as well as those documenting the meteorological setting and evolution of surface conditions, are summarized. High concentrations of residual ozone above the surface layer in the early morning hours appear to be necessary to produce afternoon concentrations in excess of 125 ppbv. The moderately polluted days were characterized by low residual pollutant concentrations aloft in the early morning and moderate to weak vertical stratification during the day. The cleanest days had good ventilation with deep layers of near neutral stratification and moderate to strong winds. Horizontal spatial variability of ozone concentrations within the mixed layer was often significant at scales 5 km. The maximum ozone concentrations were frequently observed in the afternoon, 40–80 km downwind of the city, but these decreased to about one-half those values at distances 120 km downwind. The measured oxides of nitrogen were generally just above detection limits ( 1 ppbv), except in the surface layer in the early morning hours.