Stratospheric ozone in the lower troposphere —I. Presentation and interpretation of aircraft measurements

Abstract

The United States has instituted a number of air pollution regulations to control ozone, including an air quality standard for oxidants of 120 ppb for one hour. Accordingly, there is considerable interest in determining the magnitude of the natural (i.e. nonanthropogenic) component of ozone concentration near the ground, much of which is generally believed to come from the stratosphere. Toward this end, an extensive program of aircraft measurements of tropospheric ozone originating from the stratosphere was carried out over the Central U.S. in spring and fall 1978. On 10 of these flights, the vertical structure of stratospheric ozone intrusions was well mapped by aircraft penetrations at several altitudes extending between 2 and 8 km above sea level (ASL) in the southern portions of tropospheric low-pressure troughs. The field measurements show that stratospheric ozone intrusions into the troposphere occur more frequently than earlier studies had indicated. Ozone intrusions were found in virtually every trough, regardless of intensity, within which suitable measurements were taken. A close relationship was found between: 1. (1) maximum ozone concentrations in the intrusion; and 2. (2) trough intensity as characterized by maximum wind speed at 300 mb (approximately 10 km ASL). The intrusions typically are characterized by peak ozone concentrations at higher altitudes (6–8 km ASL) in the range of 240–400 ppb, diminishing to 100–200 ppb at lower altitudes as mixing with surrounding air occurs. Measured concentrations during spring were almost twice as high as those measured during fall, but the intrusion structures were very similar during both seasons. The data show that stratospheric ozone intrusions are typically 100–300 km wide in the crosswind direction, are several hundreds of kilometers long, and can be tracked down at least as far as the top of the atmospheric boundary layer (about 2 km ASL). Possible mechanisms for downward transport within the boundary layer include normal convective mixing, organized convection associated with cloud and precipitation processes, and organized downward motion within frontal zones.