Air Pollution: Chapter 12

1. Air Pollution: Chapter 12

2. What’s in the air? O3, NO, NO2, CO, CO2, SO2, HCHO And other things like particulates and Black Carbon!

3. Types and Sources of Pollutants • primary and secondary pollutants • Gases and particulate matter • Gases: Carbon monoxide, Sulfur Dioxide, Nitric Oxide, Nitrogen Dioxide • Particulates are classified by size (PM10, PM2.5—diameter size in microns 2.5 μm)

4. Pollutants can be classified as primary pollutants and secondary pollutants.

5. Primary Pollutants Carbon Monoxide (CO): is a tasteless, colorless and odorless gas. Sources of CO include: fossil-fuel combustion (Incomplete combustion in autos, trucks and airplanes, some industrial process), biomass burning, and photolysis reactions. Concentrations of CO in urban air are typically 2-10 ppmv. On freeways and in traffic tunnels values rise to more than 100 ppmv.

6. Primary Pollutants Sulfur Dioxide (SO2): is a colorless gas that exhibits a taste at levels of 0.3 ppmv and a strong odor at levels greater than 0.5 ppmv. Sources of SO2 include: coal-fired power plants, automobile tailpipes, and volcanoes. Background concentrations in the troposphere range from 10 pptv to 1 ppbv. In polluted air, 1-30 ppbv.

7. Primary Pollutants Nitrogen oxides are gases that form when some of the nitrogen in the air reacts with oxygen during the high-temperature combustion of fuel. Nitric Oxide (NO): is a colorless gas and a free radical. Precursor to tropospheric ozone. Sources include: fossil-fuel combustion/autos and planes, and biomass burning. Nitrogen Dioxide (NO2): is a brown gas with a strong odor. Absorbs short wavelengths. Intermediary between NO and O3 production.

8. Primary Pollutants Background concentrations range from 20 to 50 pptv. In urban areas, concentrations are 0.1-0.25 ppmv. In midmorning NO2 is more prevalent than during midday because sunlight breaks down most NO2 past midmorning. NOx = NO + NO2

9. Primary Pollutants Volatile organic compounds (VOCs) represent a class of organic compounds that are mostly hydrocarbons-individual organic compounds composed of hydrogen and carbon. Methane (naturally occurring) is the most abundant. Others include: benzene, formaldehyde (HCHO)

10. Secondary Pollutants Secondary pollutants form chemically in the air. Ozone is a secondary pollutant in the troposphere. Ozone (O3): is a relatively colorless gas. Ozone exhibits an odor at concentrations of 0.02 ppmv or greater. Ozone has a sweet smell (Copy machine). Ozone is not emitted. Background concentrations in troposphere are 20 to 40 ppbv. In urban air, concentrations range from 0.01 ppmv at night to 0.5 ppm during afternoons in the most polluted cities. Typical mixing ratios are 80-100 ppb in urban air. In 2008, EPA lowered the 8 hr primary standard from 84 ppbv to 75 ppbv.

11. NO + O3 NO2 + O2 Chemistry of Photochemical Smog Photochemical smog involves reactions among NOx and VOCs in the presence of sunlight. On a typical day, ozone forms following emission of NO and VOCs. In polluted air, ozone production occurs along the following lines. Sunlight dissociates NO2 into NO and atomic oxygen: NO2 + solar radiation NO + O The atomic oxygen combines with molecular oxygen (in presence of a third molecule, M) to form ozone, as O + O2 + M O3 + M The ozone is then destroyed by combining with nitric oxide, thus

12. NO + O3 NO2 + O2 Chemistry of Photochemical Smog If sunlight is present, the newly formed NO2 will break down into NO and atomic oxygen. The atomic oxygen then combines with molecular oxygen to form ozone again. Large concentrations of O3 can form in polluted air only if some of the NO reacts with other gases without removing ozone in the process.

13. Chemistry of Photochemical Smog

14. EPA Standards National Ambient Air Quality Standards (NAAQS) • 75 ppb O3 for an 8-hour averaging period • 120 ppb O3 for a 1-hour averaging period

15. The number of unhealthy days during 2003 Fig. 12-11, p. 338

16. The days exceeding 1-hr standard in LA

17. Air Quality Index (AQI)

18. Particulate Matter Particulate Matter (PM) represents a group of solid particles and liquid droplets that are small enough to remain suspended in the air. Collectively known as aerosols, this grouping includes solid particles that may irritate people but are usually not poisonous. These include: soot, dust, smoke, and pollen. Some more dangerous substances include asbestos fibers and arsenic. Particulates are classified by size (PM10, PM2.5—diameter size in microns 2.5 μm)

19. Black Carbon Black Carbon is a primary aerosol component of Diesel Particulate Matter, a known toxin and regulated pollutant by several regulatory agencies, including the California Air Resources Board (CARB). Diesel Particulate Matter is known to cause adverse health effects in people who are exposed, including premature hospitalization, asthma attacks, bronchitis, other respiratory and cardiovascular symptoms, and premature death. Black Carbon is the second leading cause of Global Warming. Black Carbon is emitted as a primary pollutant to the atmosphere through a variety of incomplete combustion of sources and fuels; BC concentration cannot be modeled or predicted, it must be measured. Black Carbon is NOT adequately characterized through PM-2.5 mass only measurements, chemical speciation is necessary. (Magee Scientific)

20. Black Carbon Aerosol black carbon (“BC”) is a particulate pollutant species emitted from the combustion of any carbonaceous fuel. The name “Aethalometer” is derived from the classical Greek verb ‘aethaloun’, (αεφαλουν) meaning ‘to blacken with soot’. It is this optically-absorbing material that the ‘standard’ Aethalometer measures. The only known source of aerosol black carbon in the atmosphere is the combustion of carbonaceous fuels. There are no known significant biological, geological or meteorological sources.

21. Factors that Affect Air Pollution

22. 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.

23. Atmospheric stability determines where pollutants get transported

24. The Role of Stability and Inversions • temperature lapse rates • inversions • mixing depth • The mixing layer can often be easily seen from an airplane.

25. The Role of Atmospheric Stability

26. The Role of Topography • cold air drainage • air blockage by mountain ranges

27. Severe Air Pollution Potential • sources • high pressure • inversions • stagnation • 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.

28. A strong subsidence inversion along the coast of Californiav

29. A strong subsidence inversion along the coast of California

31. Summary of Chapter 12 • Primary and secondary pollutants -gases and particulate matter • Photochemical smog formation • Controls on air pollution: - wind, stability, topography