A Graphical Explanation of a Regional Ozone Episode

June, 1997

Bret A. Schichtel and Rudolf B. Husar

Center for Air Pollution Impact and Trend Analysis (CAPITA)
Washington University, St Louis, Missouri


Elevated ozone most frequently occurs near large metropolitan areas like the Washington-New York corridor, Atlanta and Chicago. However, about five times each summer, regional episodes develop when accumulated ozone blankets large, multi-state areas of the Eastern U.S with unhealthy levels of ozone. The accumulated regional ozone can be transported from one state to another contributing to exceedances of the ozone standard at the receptor sites. One such episode occurred in July of 1995 and this animation is an anatomy of that regional episode.

This animation was used to illustrate regional scale ozone transport in the OTAG video new release that was developed by Wolcoff for informing the general public about OTAG and its findings.

Scene 1: Ozone accumulation

Air quality data from over 600 monitoring sites indicates that the daytime ozone concentrations increased throughout the Ohio River Valley reaching over 80-100 parts per billion, and persisted in the region for seven days.

Scene 2: Airmass stagnation over high NOX emission areas

The ozone accumulation during the episode is caused by airmass stagnation over multi-state areas of high emissions. The rapid movement of the airmass (arrows) during the first day and a half indicates swift transport of a "relatively clean" airmass from Canada towards the industrial Midwestern states. The red shading on the map indicates that this region contains high NOx emissions densities. Upon arrival to the Midwest, the clean Canadian airmass meanders over several states for about five days accumulating ozone precursors as well as ozone.

Scene 3: Ozone accumulation and transport

In this scene the airmass histories are superimposed on observed ozone concentration patterns. The rise in the ozone concentrations over the industrial Midwest coincides with the airmass meandering over the high NOX emission region. Near the end of the episode, Midwestern concentrations drop, as the polluted airmass is swiftly transported eastward, carrying high ozone concentrations toward the Northeastern seaboard where regional ozone levels peak a day later.

Hence, large scale ozone transport across state boundaries occurs when airmasses stagnate over multi-state areas of high emissions creating high concentration ozone "blobs" which are subsequently transported to downwind states.

The Episode Animation File

The ozone episode animation file can be downloaded by clicking on this link:
O3Epi95.zip (3.4 megs) (7.2 megs unzipped)

The animation is in AVI format. An AVI player can be downloaded from the CAPITA website.