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Emission Sources of Winter PM 2.5

Update: January 15, 2015

This is the most current updated emission data. It differs slightly than the previous 2008 inventory because it accounts for cleaner vehicles.

Utah's Unique PM2.5 Chemistry

In order to identify the sources that contribute to the formation of PM2.5 in Utah, the Division of Air Quality (DAQ) first had to define emissions inventories. Emissions inventories are databases that list, by source, the amount of air pollutants discharged into the atmosphere of a specific geographical area during a given time period. DAQ scientists worked with industry and transportation planning organizations to gather industry and mobile emissions data. For other pollution sources, scientists used calculated emission sets from EPA. The collective emissions data was used to understand what types of precursor emissions were emitted into the atmosphere and in what quantities.

The chart shown is based on the Division of Air Quality (DAQ) 2008 emissions inventory for the four urbanized Wasatch Front counties: Salt Lake, Davis, Utah, and Weber. These are average winter weekday emissions.

imageThe inventory measured emissions that lead to the formation of PM2.5: direct PM2.5 , nitrogen oxides (NOx), volatile organic compounds (VOCs) and sulfur oxides (SOx).

Sources that emit PM2.5 include fuel combustion from vehicles, wood burning, and industrial processes as well as vapor releases from industrial sources, paints, solvents, and coatings.

During an inversion, anywhere from 60 percent to 85 percent of all PM2.5 found on DAQ's monitoring filters is created by secondary particulate formation. Secondary particulate formation happens when precursor emissions, usually the gases NOx and SOx and VOC, react in the atmosphere and combine to create PM2.5. The remaining portion of PM2.5 is considered primary, because it is directly emitted as a particle and enters the atmosphere as soot from roadways or tailpipe emissions.  

A Quick Chemistry Lesson

Description: pm-reaction
Chart Courtesy of the Bay Area Air Quality Management District

The type of PM2.5 particles that make up the majority of our air pollution problem are created through chemical reactions in the atmosphere. Precursor emissions that contribute to this secondary formation of fine particulates include nitrogen oxides (NOx), volatile organic compounds (VOCs), sulfur dioxide (SO2) and ammonia (NH3). VOCs are highly reactive in the atmosphere, breaking apart and combining with other gaseous chemicals to form nitrates. These nitrates react with ammonia to form ammonium nitrate, creating these fine particles.

Ammonia is so pervasive in our atmosphere that no realistic control measure exists for it. Extensive air emissions modeling for the State Implementation Plan indicates that reducing VOC emissions provides the best approach to reducing PM2.5 levels during winter inversions in Utah in the near future. Over the next five-plus years, as emissions from automobiles are reduced with a newer fleet of cars, the reduction of both VOC and NOx emissions from all sources will be the most appropriate control strategy.


Utah Department of Environmental Quality

Office of Planning and Public Affairs

Phone: (801) 536-4000 | E-mail