Chapter 12: Air Pollution

1. 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

2. A Brief History of Air Pollution • disastrous London smog event in December 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

3. 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. Q1: What are the natural sources of air pollutants? A: dust, volcano, forest fire, ocean waves, vegetation, … Q2: What are the anthropogenic sources? A: fixed sources (power plants, homes, …) mobile sources (cars, ships, …)

4. Principal Air Pollutants • Primary pollutants: from direct emission (next slide) secondary pollutants: from chemical reaction • 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 PM2.5 is especially dangerous to health (e.g., lung) • Globally, a large percentage of air pollution sources are natural. • Within localized areas, however, human-caused sources are often the largest contributors.

5. Q3: Is CO2 a pollutant? a) yes; b) no Q4: What do air pollutants include? a) gases; b) liquids; c) solids; d) all of them Over U.S.

6. Principal Air Pollutants • Carbon monoxide (CO): colorless, odorless, poisonous; primarily from incomplete combustion of fuels (e.g., cars) That is why we need to have CO detector at home! • Sulfur dioxide (SO2): colorless; primarily from the burning of fossil fuels (such as coal and oil) and from volcano and ocean spray • Volatile organic compounds (VOCs): represent a class of organic compounds that are mainly hydrocarbons – individual organic compounds composed of hydrogen and carbon, such as methane; primarily from industrial processes, transportation, and vegetation • Nitrogen oxides: nitrogen dioxide (NO2) and nitric oxide (NO), together called NOx; primarily from motor vehicles, power plants, and waste disposal systems. • Dust: PM10 and PM2.5; from dust storm and agricultural and industrial activities

7. Q5: What do automobiles emit? A: CO, NOx, VOCs, particulate matter, … Q6: Do automobiles emit O3? a) yes; b) no Q7: What do power plants (using coal) emit? A: SOx, NOx, ash Q8: What do vegetation emit? A: VOCs, pollen Q9: Can the dust from Asia travel across the Pacific to reach Tucson? a) yes; b) no

8. Ozone in the Troposphere • Photochemical smog: smog in the presence of sunlight • Ozone: unpleasant odor, irritates eyes and hurt human health, reduce crop yield (tropospheric) O3 forms as a secondary pollutant from a complex series of chemical reactions involving NOx and VOCs in the presence of sunlight. Q10: does Tucson have O3 problem? a) yes, b) no Q11: does Tucson have particulate matter problem? a) yes, b) no

9. Ozone in the Stratosphere • Stratosphere O3 absorbs ultra-violet radiation to reduce human skin cancer; its concentration is highest at ~25 km altitude • chlorine compounds such as chlorofluorocarbons (CFCs) are widely used as refrigerants, propellants (e.g., in hairspray) and solvents; these gases are safe in the troposphere but destroy O3 in the stratosphere: a single chlorine removes as many as 100,000 ozone molecules • Montreal Protocol in 1987

10. When scientists first measured extremely low ozone values in the Antarctic stratosphere, they thought the instruments were malfunctioning. Ozone hole in 2006 Q12: why are there still ozone holes when chlorine is decreasing? A: there are still CFCs in the stratospphere; mainly due to changes in polar stratospheric temperatures

11. Air Pollution: Trends and Patterns • Air Quality Index (AQI): includes the pollutants CO, SO2, NO2, particulate matter, and O3; on any day the pollutant measuring the highest value is the one used in the index • Secondary air pollutants (e.g., O3) are particularly difficult to control, because they are not emitted directly into the atmosphere. • Higher emission standards along with cleaner fuels (such as natural gas) have made the air over our large cities cleaner than it was years ago

12. Q13: why was the lead emission reduced to zero in 1980s? a) federal regulation; b) new technology; c) voluntary industrial decision Q14: Why did the emission peak for CO, SO2, VOCs occur in early 1970s? Q15: What are the main air quality concerns in Tucson? a) PM2.5 and O3 b) CO and SO2 c) SO2 and O3 d) CO and PM2.5

13. Results based on AQI in 2003 Fig. 12-11, p. 338 Q16: Why is air in California not as good as rest of the U.S.? A: higher anthropogenic emission; dust; weather pattern; topography

14. Factors affecting air pollution Role of the wind: dilution by advection and turbulent 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.

15. The Role of Stability and Inversions Q17: if air temperature decreases with height, what is the air stability? a) stable, b) unstable, c) undecided Q18: if air temperature increases with height, what is the air stability? a) stable, b) unstable, c) undecided • Inversions: temperature increases with height (i.e., very stable) • mixing layer: from surface to inversion base where air is unstable and hence well mixed Q19: is the layer below inversion in the figure a mixing layer? a) yes, b) no

16. The Role of Topography • Nighttime drainage flow • air blockage by mountain ranges Los Angeles

17. Severe Air Pollution Potential • Sources (clustered close together) • high pressure (for inversion and weak wind) • Inversions • Stagnation (weak wind; 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.

18. Air Pollution and the Urban Environment • urban heat island • country breeze

19. Acid Deposition • pH: measure of the acidity or basicity of a solution • Acid deposition: wet and dry deposition • wet deposition: removal of pollutants by precipitation • dry deposition: by gravity • acid rain or fog: SO2 and NOx Q20: why is natural rain somewhat acidic? Because of a) CO2 dissolution b) SO2 dissolution c) NOx dissolution

20. Acid Rain • acid rain effects: damaging forest; eroding the foundations of structures; polluting lakes • Control of acid rain is an international issue due to long-distance transport of pollutants Precipitation pH values in 2005