About half the air pollution in Boston comes from cars, pickup trucks, heavy duty trucks, and busses.    The dirtiest 10 percent of these vehicles is responsible for about half the pollution from all vehicles.     The remaining, non-vehicle air pollution comes somewhat equally from off-road diesel engines (e.g., construction equipment, generators, trains, and ships), power plants, other industrial facilities, and commercial and residential "area sources", such as solvent and fuel vapors, automobile refinishers, home heating, and lawn and garden equipment.   All these pollution sources contribute to the formation of ground-level ozone , fine particulate matter , and air toxics. Every three years, an inventory is taken of volatile organic compunds (VOCs) and nitrogen oxides (NOx).   These pollutants react to form ozone during the summer months.   The results of the latest inventory are shown in the pie charts below.

As stated above, the pollution sources contributing to ozone are largely the same as those contributing to fine particulate matter.   But unlike ozone, there is no inventory of fine particulate matter which identifies the largets and smallest emitters.   This is because healthbased standards for fine particulate matter were put into place only recently.   However, a one year experiment was conducted at Kenmore Square in 1995 which identified the principal chemical components in Boston's fine particle mixture.  These components tell us a little bit about where the fine particulate pollution is coming from, as shown in the chart below.

Organic carbon comes mostly from cars, trucks, fuel and solvent evaporation, industrial processes, home heating, and other activities that release hydrocarbons. Most of the elemental carbon is believed to come from diesel engines, including trucks, buses, marine vessels, and construnction equipment. Sulfates come mostly from coal-fired power plants. Nitrates come from motor vehicles, power plants, and other activities involving combustion of fossil fuels. Crustal material is mostly natural, and trace elements are mostly metals that come from motor vehicles and industrial processes.