Ozone production and transport near Nashville, Tennessee: Results from the1994 study at New Hendersonville

During the summer of 1994 the photochemical production of ozone and the rel ation of this production to the photochemical precursors were studied at a suburban ground site near Nashville, Tennessee. The study, which was carrie d out as part of the Southern Oxidant Study (SOS), investigated the ozone p roduced by urban outflow during one period of high photochemical activity a round July 1, 1994. Estimates of the instantaneous rate of in situ ozone pr oduction, P(O-3), are inferred from deviations of the photostationary state (PSS) for clear-sky conditions. The biggest contributor to the large error s in P(O-3) are the systematic errors in the derived, nor measured, j(NO2) levels. Ozone entrainment from aloft has been quantified by simple subtract ion of calculated production and loss terms from the observed rate of ozone change. The uncertainty of this derived transport term was estimated to a factor of 2 at best and 2 orders of magnitude at worst. Entrainment provide s a substantial contribution to the observed increase of [O-3] (similar to 20 ppbv h(-1)) in the mornings between 0700 and 1000 Central Standard Time (CST being 1 hour behind LT) when advection can be neglected. An average en trainment velocity of 1.5-2 cm s(-1) agreed within 30 to 50% with a result found from another, completely independent study. P(O-3) dominates the obse rved [O-3] increase from 1000 CST until early afternoon, when entrainment w eakens and even turns into an effective ozone loss term due to cloud ventin g processes. The July 1 case clearly demonstrates that the morning ozone en trainment occurs on a more regional scale covering the entire study area, w hereas the midday ozone exceedance was spatially more confined to the area covered by the advective outflow of the chemically processed Nashville urba n plume. The data show that the entrained [O-3]-rich air aloft is the remna nt of the previous day's local ozone buildup and subsequent nocturnal advec tive redistribution.