Ozone production and transport near Nashville, Tennessee: Results from the 1994 study at New Hendersonville

During the summer of 1994 the photochemical production of ozone and the relation of this production to the photochemical precursors were studied at a suburban ground site near Nashville, Tennessee. The study, which was carried out as part of the Southern Oxidant Study (SOS), investigated the ozone produced by urban outflow during one period of high photochemical activity around July 1, 1994. Estimates of the instantaneous rate of in situ ozone production, P(O-3), are inferred from deviations of the photostationary state (PSS) for clear-sky conditions. The biggest contributor to the large errors 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 subtraction 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 provides 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 entrainment 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 observed [O-3] increase from 1000 CST until early afternoon, when entrainment weakens and even turns into an effective ozone loss term due to cloud venting processes. The July 1 case clearly demonstrates that the morning ozone entrainment occurs on a more regional scale covering the entire study area, whereas the midday ozone exceedance was spatially more confined to the area covered by the advective outflow of the chemically processed Nashville urban plume. The data show that the entrained [O-3]-rich air aloft is the remnant of the previous day's local ozone buildup and subsequent nocturnal advective redistribution.