Some Characteristics of Major Pollutants
Type	Toxicity	Visibility	Sources	Comments
Particulates (also called "aerosols")	Some are toxic, e.g., arsenic, asbestos, H2SO4, oils, PCBs, pesticides, heavy metals such as Fe, Cu, Ni, Pb	High	Natural (dust, smoke, pollen), Industrial processes, Autos	Most, even non-toxic ones, cause respiratory problems.  Settlings times:  d ~ 10 μm, ~ 1 day; d ~ 1 μm, several weeks.  Removed from atmosphere by rain and snow.  With d £ 10 μm, too small to be filtered out by bodily defense mechanisms.
Carbon Monoxide, CO	Highly toxic	Colorless, Odorless	Autos, incomplete combustion of fuels	Attracted to and binds with hemoglobin.  Down 40% since early 1970s.  Quickly removed from atmosphere by soil microorganisms
Sulfur Dioxide, SO2	Respiratory irritant	Colorless	Burning of fossil fuels containing S.  Power plants, smelters, refineries, paper mills, volcanos, ocean sprays	Damaging to certain plants.  Acts as source of secondary pollutants SO3 and H2SO4.
Volatile organic compounds (VOCs)	Some are toxic.  Some are carcinogenic, e.g., benzene.  Some harmless ones interact with nitrogen oxides in sunlight to produce toxic secondary pollutants.	Various	Industrial processes and transportation	Most are hydrocarbons.  Thousands exist.  Methane, CH4, a significant greenhouse gas, is the most common.  Includes formaldehyde and chlorofluorocarbons.
Nitrogen oxides, NOx	Induce heart and lung problems.  lower resistance to respiratory infections.	NO2 is reddish-brown	Automobiles, power plants, waste disposal systems	Produced in small quantities by bacterial action, but have 10 to 100 greater concentrations in urban areas than in rural areas.  NO2 dissolves in rain to produce HNO3 and interacts with O2 in sunlight to produce O3.
(Carbon dioxide), CO2	Technically, not a pollutant, since it causes no direct harm to animals or plants.  However increasing atmospheric CO2 is believed to be the main cause of global warming.	Invisible and odorless	Burning of fuels, volcanos	Atmospheric concentration has increased by ~20% in the last half century.  CO2 is a major greenhouse gas.