With the air quality control strategies currently in place, EPA estimates that the pollution from motor vehicles will decrease over the next 10 years unless the number of vehicles on the road increases significantly. In colder months, domestic wood combustion is the major source of winter smog. Heaters not meeting this standard contribute more than twice as much particle pollution per kilogram of wood burnt, and open fires contribute three times as much, compared with modern heaters that meet the standard. Photochemical Smog Photochemical smog forms as a result of sunlight acting on various pollutants - mainly oxides of nitrogen and volatile organic compounds. This smog is made up of a mixture of ozone and other organic compounds, and nitrogen dioxide.
Photochemical smog is more likely to build up and hang like a blanket in the air on still days.
As well as the emission sources and pollutants, it is important to consider the prevailing winds and weather conditions of a city, and the duration of sunlight hours, in determining pollution patterns and effects. Melbourne is fortunate that its prevailing winds and weather systems move quickly, tending to disperse pollution. However, under certain light wind conditions, the air and the pollutants it contains is recirculated, creating more smog. Smog can have a brownish tinge due to the presence of particles together with nitrogen dioxide. Later, the colour may change to white due to the presence of sulfate and nitrate aerosols.