a Synoptic Climatology of Ozone Concentrations in the Forests of Pennsylvania.

Ozone (O_3) pollution is adversely affecting forest health in Pennsylvania. Concentrations of O_3 are strongly influenced by weather conditions within synoptic-scale atmospheric circulation patterns, and this thesis examines the synoptic climatology of O_3 in the forests of Pennsylvania. A conceptual framework for studying rural-O _3 climatology is derived from a review of the pertinent literature. The broad O_3 -climate relationships in the region are identified using a ten-year baseline O_3 climatology for Pittsburgh (the closest long-term data to the forest). Recently collected O_3 data from three forest sites are analyzed with respect to the baseline climatology for the 1988, 1989 and 1990 growing seasons. A climatology of related air-mass trajectories locates major trajectory paths and specific regions of influence that impact O _3 concentrations in the forests. Results indicate that high O_3 <=vels develop under anticyclonic systems, when conditions are typically warm, with high intensities of ultraviolet radiation and low wind speeds, promoting photochemical activity. Air-mass stagnation leads to localized episodes in emissions source areas, while transport of a polluted air mass is required for a rural episode. Areas surrounding the Ohio River Valley form the major precursor source-region impacting the Pennsylvania forests. Air masses stagnating over this area eventually experience transport from southwest to northeast into Pennsylvania, accumulating precursors and O_3. Low O_3 <=vels occur in cyclonic conditions that are typically cold, cloudy and windy with precipitation, or in strong northwesterly transport conditions following a cold-front. Both situations offer limited opportunities for O_3 formation. There is a strong seasonal cycle of O _3 in all weather patterns, with maximum O_3 concentrations in summer coinciding with the maximum in photochemical activity. Background wintertime O_3-maxima may be associated with tropopause folding and a stratospheric source. Interannual O_3 variability is very weakly related to variations in weather-pattern frequencies. Year-to -year changes are more strongly related to other sources of variation. These include anomalous large-scale circulation changes, such as occurred in 1988, as well as variations in anthropogenic precursor emissions.