Page 30 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

1.2 Air Pollution and Greenhouse Gases 3

particles and the gas in which they are suspended. Particulate matter can be both
primary pollutants and secondary pollutants that are sent directly into the atmo-
sphere in the form of windblown dust and soil, sea salt spray, pollen, and spores.
Other examples of PM are smoke, fumes, and haze.
For particulate matter where particle diameters are smaller than x micrometers, it
is defined as PMx. Commonly used terms are PM 10 and PM 2.5 . Sometimes, par-
ticulate matter and aerosol is exchangeable. Monodisperse aerosols, in which all
particles have the same size, can be produced in laboratory for use as test aerosols.
In practice, engineers deal with polydisperse aerosols (i.e. suspended particles are in
a wide range of sizes), and statistical measures should be used to characterize
particle sizes. Aerosol Technology by William Hinds (2006) is one of the reference
books for this subject.
Air pollutants other than PM present primarily as gases. Volatile organic com-
pounds (VOCs) are chemicals that contain carbon and/or hydrogen and evaporate
easily. VOCs are the main air emissions from the oil and gas industry, as well as
indoor consumer products and construction materials, such as new fabrics, wood,
and paints. VOCs have been found to be a major contributing factor to ground-level
ozone, a common air pollutant, and a proven public health hazard. Sulfur dioxide
(SO 2 ) and nitric oxides (NO x ) are two major gaseous air pollutants generated
through combustion processes. Carbon monoxide (CO) and hydrocarbon (HC) are
generated from incomplete combustion and are converted into CO 2 through a
complete combustion process.
Secondary air pollutants are those formed through complex physical and/or
chemical reactions, e.g., coagulation and condensation or photochemical reactions,
respectively, e.g.:

SO 2 þ NH 3 þ VOCs ! Aerosol ð1:1Þ

Atmospheric physics and chemistry are so complicated that they are still not well
understood. Most gaseous air emissions such as NO x ,SO 2 , HC, VOCs, and
ammonia (NH 3 ) are converted into PM. Readers are recommended to read Atmo-
spheric Chemistry and Physics by Seinfeld and Pandis [20] for in-depth knowledge
on this subject.
Air pollution is an evolving subject and inevitable, as the demand for energy
increases. Air pollution really flourished with the Industrial Revolution and con-
tinues to grow with the human appetite for comfort and speed.
At first, the study of air pollution focused on recurring episodes of high levels of
air pollution in areas surrounding industrial facilities, such as coal burning power
plants and chemical refineries. These pollution episodes were accompanied by acute
human sickness and the exacerbation of chronic illness. After the mid-twentieth
century, when industrialized nations’ economies recovered rapidly from World War
II, many urban regions without heavy industrial facilities also began to experience
high levels of photochemical smog and nitrogen oxides.