Boundary layer vertical exchange processes and the mass budget of ozone: Observations and model results

An Eulerian chemical model is used to assess the relative importance of a v ariety of processes associated with producing high surface ozone episodes d uring selected periods of the North American Research Strategy for Troposph eric Ozone (NARSTO) 1995 field campaign over the northeastern United States . A comparison of the observed and predicted hourly surface ozone mixing ra tios showed that the model qualitatively reproduced the observed ozone tren ds over the northeastern United States. The model had a mean hourly bias of similar to 15 ppb with a wide range of skill compared to individual observ ing stations clustered around one node. The simulated mass budget tendency terms are compared for days with low ozone values immediately followed by d ays with high values. The later days showed observed and simulated ozone mi xing ratios aloft to be of order twice that found on preceding days, althou gh the associated chemical mix appeared to have relatively little potential for the subsequent generation of "new" ozone. Under conditions of shallow mixing over urban regions, simulated surface ozone production rates were ne gative (a net loss) throughout much of the day with convective mixing bring ing newly produced ozone from aloft to the surface. It is noted that surfac e ozone levels appeared to be relatively insensitive to mixing layer growth rates.