Ozone as a Function of Local Wind Speed and Direction: Evidence of Local and Regional Transport Rudolf B. Husar and Wandrille P. Renard CAPITA, Center for Air Pollution Impact and Trend Analysis Washington University St. Louis, Missouri

Background Atmospheric conditions can exert a powerful influence on the distribution of pollutant concentrations in space and time. Low wind speeds lead to the buildup of high local pollutant concentrations. Strong ventilation with high wind speeds prevents the local build- up near the sources, but contributes to long-range transport and regional ozone particularly during directionally persistent wind conditions. In this work, the dependence of ozone concentration on transport is analyzed by classifying the existing ozone concentration data into wind direction and wind speed bins, followed by concentration averaging in each bin.

Schematic illustration of a simple one-dimensional model

Concentration as a function of wind speed at different local source strengths

Average ozone concentration at low ( <3 m/s) wind speed. a) 0-90 degrees. b) degrees c) degrees d) degrees

Average ozone concentration at intermediate (3-6 m/s) wind speed. a) 0-90 degrees. b) degrees c) degrees d) degrees

Average ozone concentration at high( >6 m/s) wind speed. a) 0-90 degrees. b) degrees c) degrees d) degrees

Average ozone concentration at all wind speeds. a) 0-90 degrees. b) degrees c) degrees d) degrees

Dependence of ozone concentration on wind speed and direction at different metropolitan areas

Relative decline of ozone concentrations with wind speed at different metropolitan areas. Strong decline indicates the influence of local sources, while weak decline suggests transported ozone

Ozone concentration roses for selected metropolitan areas

Summary and Conclusions Given a ten year ( ) record, the dependence of ozone on wind direction and wind speed has been extracted. In southern urban areas, where episodes are caused by local stagnation, ozone levels decline rapidly with increasing wind speed. In northern cities, more influenced by transport, ozone levels depend less on wind speeds. Tropospheric O 3 background levels are found at nearly all of the borders of the OTAG domain, with the exception of the Windsor-Quebec corridor. The wind directional analysis indicates that high ozone concentrations in excess of the tropospheric background originate from within the OTAG region. The high ozone concentration regions roughly coincide with the pattern of anthropogenic precursor emissions as modified by atmospheric dispersion. Consequently, it can be inferred that most of the excess ozone concentrations within the domain result from anthropogenic emissions within the domain and thus controllable by measures within the OTAG domain