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The Cache Valley is unique in its air quality problems. Despite a small amount of heavy industry there are significant levels of PM2.5 during the wintertime. Rather than being a product of heavy industry emissions, the Cache Valley's wintertime PM2.5 is a complex byproduct of many sources including home heating, industry, agriculture, and vehicles. This complicated mix is unique to the Cache Valley and has provided scientists with an intricate puzzle to piece together.
One pollutant that has been found in the Cache Valley during wintertime events is ozone, a pollutant that typically occurs on sunny summer days. This discovery has added more complexity to the reactions that are occurring in the air in the Cache Valley and suggests that the pollution situation is the Cache Valley is somewhat different than that experienced by areas along the Wasatch Front.
Another pollutant that seems particularly specific to the Cache Valley is ammonium nitrate, a pollutant that was found in abundance on our collection filters during 2004's poor air quality events. Ammonium nitrate is a particle that is formed through complex chemical reactions in the air. The reactions involve ammonia and nitrogen oxide gases that combine to form a particle. The rate at which particles form and the particles life span is increased when the weather is very cold and foggy, conditions that often occur under Cache Valley's wintertime inversions.
Ammonia comes mostly from agricultural activities (cow manure and chicken manure) Ammonia builds up in the ground level inversion layer of the atmosphere during inversions due to lack of vertical mixing. 
It reacts directly with gaseous nitric acid to form ammonium nitrate particulate. This reaction is promoted by cold temperatures and high relative humidity. Nitric acid is formed by complex chemistry in the atmosphere from nitrogen oxides which come mostly from the exhaust pipes of older and poorly maintained automobiles and trucks. Volatile organic compounds (VOCs) are likely also involved in the formation of nitric acid in the atmosphere because they supply radical oxygen species (ozone and hydroxy radicals) that are needed for the formation of nitric acid from nitrogen oxides.
NH3 (gas) + HNO3 (gas) <-->NH4NO3 (particle)
Ammonia + Nitric Acid <-----> Ammonium Nitrate
Similar reactions occur for sulfur compounds, however, the amount of sulfur compounds available to react is significantly less than nitrogen compounds in the Cache Valley.
The most common manner in which surface inversions form is through the cooling of the air near the ground at night. Once the sun goes down, the ground loses heat very quickly, and this cools the air that is in contact with the ground. However, since air is a very poor conductor of heat, the air just above the surface remains warm. Conditions that favor the development of a strong surface inversion are calm winds, clear skies, and long nights. Calm winds prevent warmer air above the surface from mixing down to the ground, and clear skies increase the rate of cooling at the Earth's surface. Long nights allow for the cooling of the ground to continue over a longer period of time, resulting in a greater temperature decrease at the surface. Since the nights in the wintertime are much longer than nights during the summertime, surface inversions are stronger and more common during the winter months. A strong inversion implies a substantial temperature difference exists between the cool surface air and the warmer air aloft.
During the daylight hours, surface inversions normally weaken and disappear as the sun warms the Earth's surface. However, under certain meteorological conditions, such as a strong high pressure over the area, these inversions can persist for many days. In addition, valley topography, low wintertime sun-angle and snow covered ground can enhance the formation of inversions.
Surface temperature inversions play a major role in air quality, especially during the winter when these inversions are the strongest. The warm air above cooler air acts like a lid, suppressing vertical mixing and trapping the cooler air at the surface. As pollutants from vehicles, fireplaces, and industry are emitted into the air, the inversion traps these pollutants near the ground, leading to poor air quality. The strength and duration of the inversion will control air pollution levels near the ground. A strong inversion will confine pollutants to a shallow vertical layer, leading to elevated PM2.5 concentration levels, while a weak inversion will lead to lower PM2.5 concentration levels.