Ozone profiles over a suburban neighborhood

The development of ozone (O3) air quality models and the measurement of O3 dry deposition over built-up areas requires assumptions concerning the vertical distribution of O3. However, few [O3] profiles have been measured over urban and suburban areas. A 150 m thick layer of air over a suburban neighborhood in Lafayette, Indiana, U.S.A. was profiled for potential temperature, moisture mixing ratio, and oxidant concentrations during 38 soundings made between mid-day and early evening in late summer and early fall 1994. The residential neighborhood had 17% of the area covered by trees, with a mean vegetation canopy height of 9 m. Although some [O3] and oxidant concentration ([oxidant]) profiles showed a monotonic decrease or increase in concentration with height, many profiles (63%) exhibited a distinct local maximum in the profile. The local [oxidant] maxima were typically observed near the top of a relatively stable layer of air, and occurred more frequently when the local surface winds were from the agricultural rural regions to the north of the neighborhood and the air had resided within the Lafayette–Indianapolis region since the prior day. The magnitude of the local [oxidant] maximum was directly related to the 155 m [oxidant] during the mid-day and early-afternoon profiles. The presence of local maxima in many of the [O3] profiles indicate that one cannot safely assume that [O3] profiles monotonically decrease with decreasing height above all suburban areas. As a result, the estimation of O3 fluxes should always include profile measurements to assure the representativeness of the calculated flux at the measurement height to surface deposition. Correlations between measures of local traffic activity, above-canopy air temperatures, the magnitude of local [O3] maxima, and [O3] above the canopy indicate that a change in the dominant processes producing the maxima appeared to occur around the time of plant senescence.