Airborne measurements of boundary layer chemistry during the Southern Oxidant Study: A case study

Abstract

Measurements of chemical and meteorological variables were made at several altitudes over a surface chemistry site near Nashville, Tennessee, during the 1995 Southern Oxidants Study. The measurements were designed to reveal the effects of turbulent mixing on atmospheric chemistry. They were made under conditions of clear skies and light winds during the morning transition from nocturnal stable stratification to the afternoon convective mixed layer. Early morning ozone mixing ratios measured by the aircraft were ∼70 parts per billion (ppb), while those measured by surface instrumentation were ∼25 ppb. Corresponding to growth of the morning turbulent layer, surface ozone values steadily increased with time until they matched the 70 ppb values aloft by midmorning. The mixing ratios of isoprene at altitudes above the surface increased by several orders of magnitude with the onset of turbulence at each measurement altitude. The slope of O3 as a function of NOy for each of the flight legs was also sensitive to the presence of turbulence. Measurements from nonturbulent flight legs yielded slopes that were considerably steeper than those from measurements made in turbulence. This study shows that the concentration of ozone precursors aloft is clearly dependent on the presence of turbulence, and turbulent mixing could explain the evolution of ozone concentrations at the surface. In general, conclusions regarding pollutant concentrations must account for both chemical and local dynamic processes.