Intercontinental transport of ozone and its precursors in a three-dimensional global CTM

The coupling of chemistry with atmospheric transport processes provides a m echanism for local and regional pollution from heavily populated continenta l regions to influence tropospheric composition at hemispheric and global s cales. In this study we use the FRSGC/UCI 3-D chemical transport model to q uantify the impact of ozone precursors from anthropogenic sources in the Un ited States, Europe, and East Asia on regional and global ozone budgets and to identify the key controlling processes. We find that the East Asian reg ion has the greatest potential to affect tropospheric ozone due, principall y, to efficient vertical transport but that Europe experiences the greatest intercontinental effects due to rapid, short-distance transport from North America. In addition to significant boundary layer ozone production in eac h region, we find that 25-40% of the total net regional production occurs a bove 730 hPa in the free troposphere and that on a hemispheric scale, 70-85 % of ozone from anthropogenic sources in the upper troposphere, above 400 h Pa, is due to in situ chemistry rather than direct vertical transport. Incr eased surface ozone concentrations over remote continents are largest in sp ring and autumn at northern midlatitudes; while this seasonality is driven by horizontal transport in the free troposphere followed by subsidence, bou ndary layer and upper tropospheric chemical production make a substantial c ontribution. Although the effects are greatest in periodic episodes when me teorological conditions are favourable, there is significant enhancement in background ozone concentrations. We suggest that increasing emissions will significantly impact the oxidizing capacity of the troposphere by leading to greater polarization between ozone production and destruction environmen t.