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Chemical Aspects of Air Pollution: Overview, Pollutants, and Sources

This article provides an overview of the chemical aspects of air pollution, including the basic pollutants such as ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen oxides. It discusses the discovery of ozone in smog and the sources of pollutants, including combustion and evaporation.

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Chemical Aspects of Air Pollution: Overview, Pollutants, and Sources

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  1. Section 7Chemical Aspects of Air Pollution Overview of Basic Pollutants Ozone Particulate Matter Carbon Monoxide Sulfur Dioxide Nitrogen Oxides

  2. Section 7 – Chemical Aspects of Air Pollution

  3. Photochemical Smog Air pollution formed by sunlight catalyzing chemical reactions of emitted compounds Los Angeles, California • Early pollution due to London-type smog. 1905-1912, L.A. City Council adopts regulation controlling smoke • Early 1900’s, automobile use increases. 1939-1943 visibility decreases significantly. • Plume of pollution engulfs downtown (26 July 1943). 1943: L.A. County Board of Supervisors bans emission of dense smoke and creates office called Director of Air Pollution Control • 1945. L.A. Health Officer suggests pollution due to locomotives, diesel trucks, backyard incinerators, lumber mills, dumps, cars. • 1946. L.A. Times hires air pollution expert to find methods to ameliorate pollution. Section 7 – Chemical Aspects of Air Pollution

  4. Los Angeles, California (December 3, 1909) Library of Congress Prints and Photographs Division, Washington, D. C. Section 7 – Chemical Aspects of Air Pollution

  5. Discovery of Ozone in Smog • 1948: Arie Haagen-Smit (1900-1977), biochemistry professor at Caltech, begins to study plants damaged by smog. • 1950: Finds that plants sealed in a chamber and exposed to ozone exhibit similar damage as did plants in smog • Also finds that ozone caused eye irritation, damage to materials, respiratory problems. • Other researchers find that rubber cracks within minutes when exposed to high ozone. • 1952: Haagen-Smit finds that ozone forms when oxides of nitrogen and reactive organic gases are exposed to sunlight. Postulates that ozone and precursors are main constituents of L.A. smog. • Oil companies and business leaders argue that ozone in L.A. originates from stratosphere. • Measurements of low ozone over Catalina Island disprove this. Section 7 – Chemical Aspects of Air Pollution

  6. Basic Pollutants (1 of 3) Categories of pollutants • Primary – emitted directly from a source • Secondary – formed in the atmosphere from a reaction of primary pollutants • Precursors – primary pollutants (gases) that participate in the formation of secondary pollutants Pollutants originate from • Combustion of fossil fuels and organic matter • Evaporation of petroleum products or compounds used in commercial products, services, and manufacturing • Natural production of smoke from fires, dust from strong winds, and emissions from the biosphere and geosphere Section 7 – Chemical Aspects of Air Pollution

  7. Basic Pollutants (2 of 3) Pollutant Abbreviation Type Carbon Monoxide CO Primary Sulfur Dioxide SO2 Primary Ozone O3 Secondary Nitrogen Dioxide NO2 Secondary Hydrocarbon Compounds (also called VOCs – volatile organic compounds ) HC Primary & Secondary Particulate Matter PM Primary & Secondary Section 7 – Chemical Aspects of Air Pollution

  8. Basic Pollutants (3 of 3) Section 7 – Chemical Aspects of Air Pollution

  9. Basic Pollutants – Toxics (1 of 2) • Air toxics (hazardous air pollutants) are known or suspected to cause cancer or other serious health effects. • EPA’s 188 hazardous air pollutants include • Benzene (motor fuel, oil refineries, chemical processes) • Perchlorethylene (dry cleaning, degreasing) • Chloroform (solvent in adhesive and pesticides, by-product of chlorination processes) • BTEX, Dioxins, PAHs, Metals (Hg, Cr) National air toxics emissions sources in 1996U.S. Environmental Protection Agency, 1998 Section 7 – Chemical Aspects of Air Pollution

  10. Basic Pollutants – Toxics (2 of 2) • Differences between toxics and criteria pollutants • Health criteria are different • No AQI-like standards for toxics • Cancer/non-cancer benchmarks (long-term exposures) • Short-term exposure limits for some • A challenge to monitor • Usually not available in real-time • Example: Dioxin requires 28 days of sampling to acquire measurable amounts in ambient air • Often localized near source Section 7 – Chemical Aspects of Air Pollution

  11. Basic Pollutants – Sources(1 of 4) • Combustion • Evaporation • Natural Production Section 7 – Chemical Aspects of Air Pollution

  12. Basic Pollutants – Sources(2 of 4) Combustion • Complete combustion Fuel  water and carbon dioxide (CO2) • Incomplete combustion Fuel  water, CO2, and other pollutants Pollutants are both gases and particles Section 7 – Chemical Aspects of Air Pollution

  13. Basic Pollutants – Sources (3 of 4) Evaporation • Thousands of chemical compounds • Liquids evaporating or gases being released • Some harmful by themselves, some react to produce other pollutants • Many items you can smell are evaporative pollutants • Gasoline – benzene (sweet odor, toxic, carcinogenic) • Bleach – chlorine (toxic, greenhouse gas) • Trees – pinenes, limonene (ozone- and particulate matter forming) • Paint – volatile organic compounds (ozone- and particulate matter forming) • Baking bread, fermenting wine and beer – VOCs and ethanol (ozone-forming) Section 7 – Chemical Aspects of Air Pollution

  14. Basic Pollutants – Sources (4 of 4) Natural Production • Fires (combustion) produce gases and particles • Winds “pick up” dust, dirt, sand and create particles of various sizes • Biosphere emits gases from trees, plants, soil, ocean, animals, microbes • Volcanoes and oil seeps produce particles and gases Section 7 – Chemical Aspects of Air Pollution

  15. Ozone • Colorless gas • Composed of three oxygen atoms • Oxygen molecule (O2)—needed to sustain life • Ozone (O3) —the extra oxygen atom makes ozone very reactive • Secondary pollutant that forms from precursor gases • Nitric oxide – combustion product • Volatile organic compounds (VOCs) – evaporative and combustion products Section 7 – Chemical Aspects of Air Pollution

  16. Solar radiation and chemistry • The reaction that produces ozone in the atmosphere: O + O2 + M  O3 + M • Difference between stratospheric and tropospheric ozone generation is in the source of atomic O • For solar radiation with a wavelength of less than 242 nm: O2 + hv O + O Section 7 – Chemical Aspects of Air Pollution

  17. Solar radiation and chemistry • Photochemical production of O3 in troposphere tied to NOx (NO + NO2) • For wavelengths less than 424 nm: NO2 + hv NO + O • But NO will react with O3 NO + O3  NO2 • Cycling has no net effect on ozone Section 7 – Chemical Aspects of Air Pollution

  18. 30o equinox midday Solar spectrum Tropospheric Ozone Photolysis Troposphere ozone photolysis takes place in a narrow UV window (300-320 nm), NO2 broadly below 428 Section 7 – Chemical Aspects of Air Pollution

  19. Nitrogen Oxides • Nitrogen oxides, or NOx, is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. • Nitrogen dioxide is most visually prominent (it is the yellow-brown color in smog) • The primary man-made sources of NOx are motor vehicles; electric utilities; and other industrial, commercial, and residential sources that burn fuels • Affects the respiratory system • Involved in other pollutant chemistry • One of the main ingredients in the formation of ground-level ozone • Reacts to form nitrate particles, acid aerosols, and NO2, which also cause respiratory problems • Contributes to the formation of acid rain (deposition) Section 7 – Chemical Aspects of Air Pollution

  20. Must make NO2 • To make significant amounts of ozone must have a way to make NO2 without consuming ozone • Presence of peroxy radicals, from the oxidation of hydrocarbons, disturbs O3-NO-NO2 cycle NO + HO2·  NO2 + OH· NO + RO2·  NO2 + RO· • leads to net production of ozone Section 7 – Chemical Aspects of Air Pollution

  21. The Hydroxyl Radical • produced from ozone photolysis • for radiation with wavelengths less than 320 nm: O3 + hv O(1D) + O2 followed by O(1D) + M  O(3P) + M (+O2O3) (~90%) O(1D) + H2O  2 OH· (~10%) • OH initiates the atmospheric oxidation of a wide range of compounds in the atmosphere • referred to as ‘detergent of the atmosphere’ • typical concentrations near the surface ~106 - 107cm-3 • very reactive, effectively recycled Section 7 – Chemical Aspects of Air Pollution

  22. THE OH RADICAL: MAIN TROPOSPHERIC OXIDANT • Primary source: • O3 + hn  O2 + O(1D) (1) • O(1D) + M  O + M (2) • O(1D) + H2O  2OH (3) • Sink: oxidation of reduced species –leads to HO2(RO2) production • CO + OH  CO2 + H • CH4 + OH  CH3 + H2O • HCFC + OH • Global Mean [OH] = 1.0x106 molecules cm-3 Major OH sinks Section 7 – Chemical Aspects of Air Pollution

  23. Oxidation of CO - production of ozone CO + OH·  CO2 + H· H· + O2 + M  HO2· + M NO + HO2·  NO2 + OH· NO2 + hv  NO + O O + O2 + M  O3 CO + 2 O2 + hv CO2 + O3 Section 7 – Chemical Aspects of Air Pollution

  24. Carbon Monoxide • Odorless, colorless gas • Caused by incomplete combustion of fuel • Most of it comes from motor vehicles • Reduces the transport of oxygen through the bloodstream • Affects mental functions and visual acuity, even at low levels Section 7 – Chemical Aspects of Air Pollution

  25. What breaks the cycle? • Cycle terminated by OH· + NO2 HNO3 HO2· + HO2·  H2O2 • Both HNO3 and H2O2 will photolyze or react with OH to, in effect, reverse these pathways • but reactions are slow (lifetime of several days) • both are very soluble - though H2O2 less-so • washout by precipitation • dry deposition • in PBL they are effectively a loss • situation is more complicated in the upper troposphere • no dry deposition, limited wet removal Section 7 – Chemical Aspects of Air Pollution

  26. Meteorology Chemistry Emissions Ozone Chemistry Summary of ozone chemistry • NO2 + Sunlight  NO + O Production • O+ O2  O3 Production • NO + O3  NO2 + O2 Destruction • VOC + OH  RO2 + H2O Production of NO2 without the • RO2 + NO  NO2 + RO Destruction of O3 RO=Reactive Organic compound such as VOC • Key processes • Ample sunlight (ultraviolet) • High concentrations of precursors (VOC, NO, NO2) • Weak horizontal dispersion • Weak vertical mixing • Warm air Section 7 – Chemical Aspects of Air Pollution

  27. Day and Night Chemistry Section 7 – Chemical Aspects of Air Pollution

  28. Meteorology Chemistry Emissions Ozone Precursor Emissions (1 of 2) • Man-made sources • Oxides of nitrogen (NOx) through combustion • VOCs through combustion and numerous other sources • Natural sources (biogenic) • VOCs from trees/vegetation • NOx from soils (Midwest fertilizer) • Concentration depends on • Source location, density, and strength • Meteorology Section 7 – Chemical Aspects of Air Pollution

  29. Stratosphere 0.2 Lightning 5.8 Soils 5.1 Fossil Fuel 23.1 Biomass Burning 5.2 Biofuel 2.2 Aircraft 0.5 NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE Section 7 – Chemical Aspects of Air Pollution

  30. An example of gridded NOx emissions Section 7 – Chemical Aspects of Air Pollution

  31. Mapping of Tropospheric NO2 From the GOME satellite instrument (July 1996) Section 7 – Chemical Aspects of Air Pollution

  32. GOME Can Provide Info on Daily Info Section 7 – Chemical Aspects of Air Pollution

  33. DJF JJA Lightning Flashes Seen from Space 2000 data Section 7 – Chemical Aspects of Air Pollution

  34. Global Budget of CO Section 7 – Chemical Aspects of Air Pollution

  35. Satellite Observations of Biomass Fires (1997) Section 7 – Chemical Aspects of Air Pollution

  36. Daily Los Angeles Emission (1987) Gas Emission (tons/day) Percent of total Carbon monoxide 9796 69.3 Nitric oxide 754 Nitrogen dioxide 129 Nitrous acid 6.5 Total NOx+HONO 889.5 6.3 Sulfur dioxide 109 Sulfur trioxide 4.5 Total SOx 113.5 0.8 Alkanes 1399 Alkenes 313 Aldehydes 108 Ketones 29 Alcohols 33 Aromatics 500 Hemiterpenes 47 Total ROGs 2429 27.2 Methane 904 6.4 Total emission 14,132 100 Section 7 – Chemical Aspects of Air Pollution

  37. Percent Emission by Source-LA Source Category CO(g) NOx(g) SOx(g) ROG Stationary 2 24 38 50 Mobile 98 76 62 50 Total 100 100 100 100 Table 4.2 Section 7 – Chemical Aspects of Air Pollution

  38. Most Important Gases in Smog in Terms of Ozone Reactivity and Abundance 1. m- and p-Xylene 2. Ethene 3. Acetaldehyde 4. Toluene 5. Formaldehyde 6. i-Pentane 7. Propene 8. o-Xylene 9. Butane 10. Methylcyclopentane Table 4.4 Section 7 – Chemical Aspects of Air Pollution

  39. Lifetimes of ROGs Against Chemical Loss in Urban Air ROG Species Phot. OH HO2 O NO3 O3 n-Butane --- 22 h 1000 y 18 y 29 d 650 y trans-2-butene --- 52 m 4 y 6.3 d 4 m 17 m Acetylene --- 3 d --- 2.5 y --- 200 d Formaldehyde 7 h 6 h 1.8 h 2.5 y 2 d 3200 y Acetone 23 d 9.6 d --- --- --- --- Ethanol --- 19 h --- --- --- --- Toluene --- 9 h --- 6 y 33 d 200 d Isoprene --- 34 m --- 4 d 5 m 4.6 h Table 4.3 Section 7 – Chemical Aspects of Air Pollution

  40. Summary Section 7 – Chemical Aspects of Air Pollution

  41. Meteorology Chemistry Emissions Ozone Meteorology – Key Processes • Dispersion (horizontal mixing) • Vertical mixing • Sunlight • Transport • Weather pattern • Geography • Diurnal • Season Section 7 – Chemical Aspects of Air Pollution

  42. Wind speed (WS) Concentration  S/WS S Vertical mixing (VM) S Concentration  S/VM Courtesy of New Jersey Department of Environmental Protection Ozone Precursor Emissions (2 of 2) • Key processes • Source location, density, and strength • Dispersion (horizontal mixing) - wind speed • Vertical mixing - inversion Section 7 – Chemical Aspects of Air Pollution

  43. Daily Variation Section 7 – Chemical Aspects of Air Pollution

  44. Volume mixing ratio (ppmv) Volume mixing ratio (ppmv) Figure 4.10 Source/Receptor Regions in Los Angeles Urban center Sub-urban Section 7 – Chemical Aspects of Air Pollution

  45. Ozone Isopleth Plot 0.32 0.08 0.24 NOx (ppmv) 0.16 Contours are ozone (ppmv) Figure 4.9 Section 7 – Chemical Aspects of Air Pollution

  46. Section 7 – Chemical Aspects of Air Pollution Slide courtesy of D. Jacob

  47. EU/USA Section 7 – Chemical Aspects of Air Pollution

  48. Section 7 – Chemical Aspects of Air Pollution

  49. Particulate Matter(1 of 3) • Complex mixture of solid and liquid particles • Composed of many different compounds • Both a primary and secondary pollutant • Sizes vary tremendously • Forms in many ways • Clean-air levels are < 5 µg/m3 * • Background concentrations can be higher due to dust and smoke • Concentrations range from 0 to 500+ µg/m3 * • Health concerns • Can aggravate heart diseases • Associated with cardiac arrhythmias and heart attacks • Can aggravate lung diseases such as asthma and bronchitis • Can increase susceptibility to respiratory infection Ultra-fine fly-ash or carbon soot *24-hour average Section 7 – Chemical Aspects of Air Pollution

  50. Particulate Matter(2 of 3) Particles come in different shapes and sizes Particle sizes • Ultra-fine particles (<0.1 μm) • Fine particles (0.1 to 2.5 μm) • Coarse particles (2.5 to 10 μm) PM10 Crustal material Carbon chain agglomerates Section 7 – Chemical Aspects of Air Pollution

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