Correlation between downward transport of ozone and Surface Ozone Over the Eastern U.S. During the Summer of 1991

Vertical meteorological processes, such as subsidence and stratospheric intrusions, can transport significant concentrations of ozone down from the lower stratosphere and upper troposphere into the mid to lower troposphere. Previous studies have found a strong correlation between ozone and potential vorticity (PV) in the mid to upper troposphere such that the ozone concentration equals PV times a constant. PV has higher values at levels above the tropopause; therefore, large PV values within the troposphere can be used as an indicator of stratospheric air at tropospheric levels.

In this study, we examine the relationship between downward transport of ozone and surface ozone concentrations over a one month period in the summer of 1991 using PV analyses, back trajectories, and hourly surface ozone data. A harmonic analysis technique is used to determine the intra-day, diurnal, and multi-day components of ozone variations over the eastern U.S. The diurnal signal can be attributed to local production of ozone, while the multi-day component is due to changes in the "background" concentrations of ozone. Meteorological conditions during the period, including three-dimensional PV fields and back trajectories, are obtained from the National Center for Environmental Prediction’s (NCEP) global reanalysis fields. A dynamical mesoscale model and a photochemical model is also used with a 60 km grid spacing to simulate the meteorological processes in more detail and to determine the relative contribution of anthropogenic and natural sources of near-surface ozone. We found that high background ozone concentrations are often correlated with high PV in the mid troposphere and transport of air parcels from the mid to lower troposphere, suggesting that the magnitude of the background ozone concentrations are sometimes influenced by stratosphere-troposphere exchange of ozone. Back trajectories during these periods usually originate in the mid troposphere over central and northern Canada. A stronger correlation between higher background ozone concentrations and downward transport are found at northern sites during this summer period, as expected, because mixing processes associated with stratospheric intrusions of ozone are usually located near the jetstream.