Chapter 12: Air Pollution • A brief history of air pollution • Types and sources of air pollutants • Factors that affect air pollution • Air pollution and the urban environment • Acid deposition
A Brief History of Air Pollution • disastrous London smog event of 1952 smog: smoke and fog; 5 days, nearly 4000 deaths; Clean Air Act in 1956 • Los Angeles: photochemical smog forms in sunny weather and irritates the eyes • U.S. Clean Air Act, 1970, 1990 set federal emission standards for states to implement and enforce
Types and Sources of Air Pollutants Air pollutants are airborne substances (either solids, liquids, or gases) that occur in concentrations high enough to threaten the health of people and animals, to harm vegetation and structures, or to toxify a given environment. They come from natural sources and human activities: Natural: dust, volcano, forest fire, ocean waves, … Human: fixed sources (power plants, homes, …) mobile sources (cars, ships, …)
Principal Air Pollutants • primary and secondary pollutants • particulate matter: a group of solid particles and liquid droplets that are small enough to remain suspended in the air • PM10, PM2.5: <10 or 2.5 micrometer in diameter • Carbon monoxide: colorless, odorless, poisonous • Globally, a large percentage of air pollution sources are natural. Within localized areas, however, human-caused sources are often the largest contributors.
Principal Air Pollutants • Volatile organic compounds (VOCs): represent a class of organic compounds that are mainly hydrocarbons – individual organic compounds composed of hydrogen and carbon • nitrogen oxides: nitrogen dioxide (NO2) and nitric oxide (NO), together called NOx • Along with ozone, VOCs and NOx are major components of photochemical smog. • Photochemical smog is a problem on most major cities of the world.
Ozone in the Troposphere • Photochemical smog: in the presence of sunlight • Ozone: unpleasant odor, irritates eyes and hurt human health, reduce crop yield
Ozone in the Stratosphere • relationship to ultraviolet radiation • chlorine compounds chlorofluorocarbons (CFCs); a single chlorine removes as many as 100,000 ozone molecules • Montreal Protocol • When scientists first measured extremely low ozone values in the Antarctic stratosphere, they thought the instruments were malfunctioning.
Ozone hole in 2006; mainly due to changes in polar stratospheric temperatures Figure 1, p. 337
Air Pollution: Trends and Patterns • Air Quality Index (AQI): includes the pollutants CO, SO2, NO2, particulate matter, and O3 • Secondary air pollutants (e.g., O3) are particularly difficult to control, because they are not emitted directly into the atmosphere.
Factors affecting air pollution The role of the wind • dilution • turbulence • mixing • “Dilution is the solution to pollution” - in the 1950s this motto led to the construction of tall smokestacks for large factories. Pollution was released higher in the atmosphere where winds were stronger. Air quality improved locally but suffered downwind.
The Role of Stability and Inversions • temperature lapse rates • inversions • mixing depth • The mixing layer can often be easily seen from an airplane.
The Role of Topography • cold air drainage • air blockage by mountain ranges
Severe Air Pollution Potential • Sources (clustered close together) • high pressure (for inversion and weak wind) • Inversions • Stagnation (unable to disperse pollutants) • A valley (for accumulation of pollutants) • Some locations, like Los Angeles and Mexico City, have an unfortunate combination of surrounding topography, frequent inversions, abundant emissions and plentiful sunlight - perfect conditions for photochemical smog.
Air Pollution and the Urban Environment • urban heat island • country breeze
Acid Deposition • pH • wet deposition • dry deposition • acid fog: SO2 and NOx • acid rain effects Precipitation pH values