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The Role of Weather in Air Pollution

Weather plays a large role in the transport of pollution.  Winds carry ozone, fine particle pollution and acid rain to areas beyond our urban centers and industrial corridors where the source pollution is emitted.  Mountainous areas can receive more pollution at high elevation than in adjacent valleys due to winds moving faster, less impeded aloft.

One way scientists can track where pollution comes from is using “back trajectories” of wind fields.  Using wind direction data from National Weather Service sites flow paths can be generated going backwards in time to show where an air mass came from.  The images below, created from the VIEWS back trajectory tool, shows different back trajectories, as dotted lines, to Mount Washington on days when the air pollution varied. 

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Weather at the smoke stacks and tailpipes

The atmospheric conditions that pollution is directly emitted into influences how that pollution is transformed.  For example, sulfur pollution is predominately emitted from smoke stacks as sulfur dioxide.  If it is emitted into a cloudy atmosphere with lots of moisture, and ammonia gas is present, it will quickly be converted to ammonium sulfate, a particulate.  If the atmosphere is cloudy but no ammonia gas is present the sulfur will convert sulfuric acid. Sulfur dioxide emitted into a sky with little moisture will largely stay as sulfur dioxide gas.  See an animation of these processes.

 

Weather also influences ozone formation.  Ozone pollution forms when nitrogen oxide emissions, from automobiles and power plants, react with volatile organic compounds in the presence of heat and sunlight. 

 

Ozone pollution is usually worse on days with greater than 90 *F temperatures and little wind to mix and move it.  Check out Smog City where you can change the environmental conditions and emission sources to see how it changes ozone pollution levles.