ABSTRACT
Earth’s dry air consists of about 78% nitrogen, 21% oxygen, and 1% other noble and trace gases. Variable trace constituents include water vapor and a variety of chemical compounds such as carbon dioxide, methane, ozone, carbon monoxide, nitrogen species, volatile organic compounds (VOCs), ammonia, and particulate matter (PM) that contribute to air pollution. Some of the pollutants are directly emitted from anthropogenic activities such as industrial and agricultural practices and traffic or through natural occurrences such as forest fires, wind-driven suspensions of surface materials, and volcanic eruptions. Air also carries water droplets (clouds) and small particles such as dust, smoke, soot, organic matter, and sea salts. Once released in the atmosphere, these gases and particles go through chemical reactions and physical processes, forming other pollutants, are transported and diffused by air motions, and are removed from the atmosphere via dry deposition and wet scavenging by precipitation. Among these substances, ground-level ozone and fine PM (particulate matter of aerodynamic diameter less than 2.5 μm, PM2.5) are of concern because of their direct harmful effects on human health. Ozone, a major constituent of smog, is not emitted directly, but is formed in a complex series of gasphase reactions involving oxides of nitrogen and VOCs in the
presence of sunlight. When inhaled, ground-level ozone damages lung tissue, worsens asthma, and increases mortality rate. It can also damage the leaves of plants and trees. Fine PM is produced from combustion processes (burning of fossil fuels, residential fireplaces, agricultural burning, and both prescribed and natural fires), volcanic emissions, windblown dust, and as a result of chemical reactions in the atmosphere. Exposure to fine PM is harmful to most people but much more damaging to children and people with respiratory and cardiovascular diseases.
