• Heavy traffic in cities is a large cause of smog.Los Angeles Smog image by CanonXTiGuy from Fotolia.com
  
  Air pollution hangs over cities like Los Angeles and Mexico City as a heavy, dark fog. However, unlike normal air pollution, photochemical smog has adverse effects on humans and the earth. Caused primarily by emissions from cars, this smog forms from a reaction of multifarious chemicals and particles with ozone to create harmful air pollution.
  
  

Chemicals in Photochemical Smog

• According to the Asia-Pacific Economic Corporation (APEC), "The major component of photochemical smog is produced as a secondary chemical reaction in the air, not by a direct source like cars."
  
  Nitrogen oxide, hydrocarbon and ultraviolet rays combine to create makeup of photochemical smog. When nitrogen oxide releases into the atmosphere, it absorbs ultraviolet rays from the sun and transforms into nitric oxide (NO) and atomic oxygen (O) through the process of photodegradation. Due to the high reaction state of atomic oxygen (O), it forms ozone as it mixes with oxygen (O2). With the addition of hydrocarbons and other volatile organic compounds, the amount of ozone increases and accumulates, forming into harmful compounds such as aldehyde, acrolein and peroxyacetyl nitrate (PAN) that comprise photochemical smog.
  
  Ozone's high oxidizing properties make it particularly harmful to the earth. It can irritate the eyes, throat and respiratory system, as well as affect agriculture and other plant life.
  
  

Urban Causes

• While high traffic rates and the condensed presence of factories in cities make them prime generators for photochemical smog, pollution comes from other sources, too. Wind serves to transport pollutants released into the air to other regions of the earth. However, the largest production of nitrogen oxide comes from car and truck engines. Materials such as paint, pesticides and gasoline also give off volatile organic compounds.
  
  Other sources of pollution in urban areas that produce carbon monoxide, carbon dioxide, sulfur dioxide and other particles include man-made products such as stoves, refrigerators, incinerators and simple things such as aerosol cans and fumes from other materials.
  
  

Other Causes

• In addition to concentrated activities in urban areas, two other factors exist that cause photochemical smog in areas other than urban ones. Ozone already present in the stratosphere can fall onto the earth's surface, raising the levels of ozone, which then reacts with other chemicals to cause the smog to form. "The other factor," APEC writes, "is the wide-area ozone transport by tropospheric ozone production." This ozone transfer comes in from the ocean as well as from land and creates a global-scale problem with photochemical smog.
  
  Many natural processes also release chemicals that mix with the ozone to form photochemical smog. Volcanoes, wildfires, even the digestive systems of cattle produce volatile organic compounds. Nevertheless, while these natural processes affect air quality, they do not have as heavy an effect as vehicles and factories in urban areas.