The air we breathe is a complex mixture of particles and gases. It was not until recently that researchers examined what happens to particulate matter and gases after they are emitted.
PM2.5 is directly emitted into the atmosphere from combustion sources and includes fly ash from power plants, carbon black from cars and trucks, and soot from fireplaces and woodstoves. The category PM2.5 also consists of fine particles of dust and metal from construction sites and metallurgical operations.  .
The majority of PM 2.5 in Utah is created when sulfates, nitrates, and carbon combine with other gasses in the air and condense into fine particles that find their way deep into your lungs. This is called secondary PM 2.5 because it is created after it is emitted from a tailpipe or smokestack. 
From 2002 to 2008 we have reduced PM2.5 emissions by 30% despite the population increases. .
The majority of Utah’s PM2.5 is called secondary aerosol, meaning that it is not emitted directly as a particle, but is produced when gasses such as SO2 and NOx, react with other gasses in the atmosphere, such as ammonia, to become tiny particles. Scientists have discovered that as small particulates hover in the air further chemical reactions occure. It's believe that as much as 70% of the PM2.5 particulates in the air are not emitted from an original source but created by a chemical reaction. Essentially, during the inversion the chemicals in the air combine and increase the number of harmful particulates over time. 
Wintertime temperature inversions not only provide ideal conditions for the creation of secondary aerosols, they also act to trap air in valleys long enough for concentrations of PM2.5 to build up to levels that can be unhealthy.