Mesoscale Meteorological Structure of a High-Ozone Episode during the 1995 NARSTO-Northeast Study

Abstract

Observations and numerical model fields were analyzed to study the meteorological structures contributing to high concentrations of lower-tropospheric ozone over the northeastern United States on 14?15 July 1995. It was found that the episode is characteristic of high-ozone events associated with the Bermuda high, having light winds, high temperatures, few clouds, and sparse rain over the entire region. The specific distribution of ozone at the peak of the episode on 14 July is of particular interest, however, since only the area from the urban corridor to the Atlantic Coast experienced ozone exceedances of the National Ambient Air Quality Standard. The analyses showed that an Appalachian lee trough (APLT) played a vital role in this pattern. Mesoscale structures associated with the APLT that affected ozone formation and distribution included 1) south-southwesterly winds east of the trough, which favored accumulation of emissions in an airstream that passed directly along the urban corridor; 2) west to northwesterly winds behind the APLT, which led to lower accumulation of emissions in that sector; 3) mixing depth contrasts across the APLT, which favored less dilution of primary and secondary pollutants to the east of the trough; and 4) low-level convergence and upward vertical velocities at the APLT, which led to the development of an elevated mixed layer over the planetary boundary layer on the east side of the trough, where pollutants could be trapped and transported for long distances by a low-level jet.