Lake Michigan Ozone Study (LMOS): Measurements from an instrumented aircraft

As part of the Lake Michigan Ozone Study, the NOAA instrumented King Air research aircraft made a series of flights over Lake Michigan during the summers of 1990 and 1991 to characterize the atmospheric conditions prevailing during times when O₃ concentrations exceeded the air quality standard. Most of the time, O₃ concentrations were within the normal range (40-70 ppbv) for the location and season, but higher concentrations were measured during the afternoon flights at several isolated locations. During three afternoon flights, high O₃ concentrations (> 120 ppbv) were observed along portions of the flight path; the highest 1-min average exceeded 160 ppbv. In two flights the highest O₃ concentrations were observed in the lower boundary layer over the eastern portion of the flight track; in one case the high concentrations were found over the western side of the lake throughout the boundary layer. The increased O₃ was accompanied by moderately increased SO₂ and NO_x (10-20 ppbv); outside the region of elevated O₃, the SO₂ and NO_x were less than 2-3 ppbv. The elevated zone concentrations were related to emissions from the urban region located near the southern and southwestern shores of Lake Michigan.