A synoptic climatology of surface-level ozone in Eastern Wisconsin, USA

Areas in Eastern Wisconsin (EWI), USA, experience exceedances of the federal health standard for ozone each summer. Recent work from the Ozone Transport Assessment Group has provided support for the association between slowly migrating anticyclones over the Eastern United States and widespread amounts of high ozone throughout much of the region. Case studies have yielded additional evidence of this connection, and, further, have shown that synoptic-scale transport in association with this anticyclone can be an important factor in elevating ozone levels in EWI. A case study approach however is Limited since only a small number of events can be analyzed. To address this deficiency, our investigation employed a synoptic climatology, which is capable of linking patterns of atmospheric circulation with patterns of tropospheric ozone over an extended period of time. To classify the atmospheric circulation a correlation-based scheme was applied to 30 yr of gridded sea-level-pressure data. Our synoptic climatology showed that the highest ozone levels occurred when an anticyclone was present to the east of the Lake Michigan Air Quality Region (LMAQR); the same type of anticyclone identified by others as being responsible for the transport of regional-scale ozone and precursors to the LMAQR. Our synoptic climatology, therefore, supports the contention that the synoptic-scale transport of ozone from the Southern and Eastern US plays an important role in episodes of elevated ozone in EWI. Moreover, our results suggest that synoptic-scale systems (i.e. anticyclones situated over the Eastern US), which operate at much greater space and time scales than sub-regional scale circulations, are likely to transport sufficient ozone into Eastern WI to often exceed the new 8 h, 80 ppb health standard-unless steps are taken to reduce regional-scale emissions.