Boundary-layer evolution over Phoenix, Arizona and the premature mixing of pollutants in the early morning

The 2001 Phoenix Sunrise campaign was a field measurement program to investigate the early morning chemical and meteorological processes associated with the development of ozone pollution in Phoenix, Arizona. As part of that study, atmospheric structure was measured using wind profiling radars, sodars, and radiosondes at several locations in the Phoenix metropolitan area. Chemical measurements made by other investigators showed that vertical mixing of pollutants began prior to sunrise on a number of occasions. This was surprising, since we expected sustained mixing to occur only after sunrise and the onset of solar heating. We have used the meteorological measurements to identify a density current that commonly arrives in downtown Phoenix in the hour or two before sunrise when conditions are undisturbed. Both winds and cold advection associated with this feature act to destabilize the lower atmosphere, and the resulting mixing continues through the morning transition to convective conditions. Because photochemical production of ozone is non-linearly dependent on the concentrations of precursor species, this early mixing will need to be properly represented in combined meteorological and chemical models if they are to be fully successful in simulating ozone concentrations.