Transcript METO 621
AOSC 200 LESSON 25 Air Pollution William Shakespeare 1564-1616, from his play ‘Hamlet’ Air Pollution Air Pollution History of Air Pollution • Air pollution is not a new problem • In England, wood for burning became scarce, and the populace resorted to burning coal which had a high sulfur content. The by-products were soot (carbon particles) and sulfur dioxide. • John Evelyn in 1661 wrote about the notorious London pea-soup fog. These occur in the fall when the Thames is warm but the ground is cold. The natural fog this produces is enhanced by the extra soot particles, and the sulfur dioxide reacts in the water droplets to produce sulfuric acid. London Killer Smog SMOG • Word coined by Dr. Harold Des Veaux, a London physician in 1903. • SMOKE + FOG = SMOG • He meant London smog – sulfurous fumes from coal burning + large water droplets formed around smoke particles (soot) • 1952 – Killer smog – 4000 deaths. Another episode in 1956 led to 1000 deaths. • Similar events have also occurred in the US. • Large industrial cities such as St.Louis and Pittsburg also suffered from ‘London’ smog, as the use of coal increased. PHOTOCHEMICAL SMOG • In 1940 vegetable crop damage began to be seen in the Los Angeles basin. Pine trees began to lose their needles. • Haagen-Smit and colleagues at the University of California, Riverside studied this effect using smog chambers - large plastic tents into which pollutants could be injected and their reactions investigated. • They showed that the effect was due to ozone in the atmosphere. • The ozone was produced by a series of reactions involving the oxides of nitrogen and organic compounds (e.g. gasoline), both of which are emitted by automobiles. • It is this form of smog that gives the pollution seen in the Baltimore/Washington corridor. Sources and Types of Air Pollutants • can be grouped into two categories: primary and secondary. • Primary pollutants are emitted directly from identifiable sources. They pollute the air immediately upon being emitted. • Secondary pollutants are produced in the atmosphere when certain chemical reactions take place among primary pollutants. • Sources. Two types of sources are identified fixed sources and mobile sources. Composition of the Earth’s Troposphere H2 O2 CH4 N2 N2O PM CO O3 ←SO2, NO2, CFC’s, etc Ar CO2 Inert gases Denver, Colorado on a clear day Denver, Colorado when particulate matter reduced visibility Fine Particles or Particulate Matter (PM) • PM is made up of suspended particles of either solid or liquid pollutants. • PM is grouped by size: under 10 microns is called PM10, under 2.5 microns is called PM2.5. • PM causes increased mortality and morbidity. • Examples of PM include diesel soot, acids, dust, sulfates, nitrates, and organics. Photochemical SMOG Ozonereactswith sunlight to give an excit ed atomof oxygen: O3 h O* O2 T hen theexcit edoxygenatomreacts with water vapor: O* H 2O 2OH OH is known as theHydroxylRadical SMOG Chemistry OH can strip a HYDROGEN AT OMfrom hydrocarbons C2 H 6 (Ethane) OH C2 H 5 H 2O C2 H 5 thencombineswit h MOLECULAROXYGEN to producean OXIDANT: C2 H 5 O2 C2 H 5O2 T hisoxidant then reactswit h NIT RICOXIDE to form NIT ROGENDIOXIDE; C2 H 5O2 NO C2 H 5O NO2 T henitrogendioxide is thenbrokenapart by sunlight NO2 h NO O followedby : O O2 M O3 M Schematic of ozone production from a Volatile Organic Compound (VOC) SMOG • • • • NEEDS Hydrocarbons and nitrogen oxides Strong sunlight to start reactions Warm temperatures to maintain reactions – the higher the temperature the faster the rate. • Peak ozone will be when temperature is highest – in the afternoon. Daily Ozone Cycle Ozone production follows a daily cycle with maximum concentrations typically observed in the late afternoon. Ozone Concentration Sunrise Sunset Time of day This cycle is a signatu of the dynamic processes of atmospheric air pollution Highly Polluted Areas In highly polluted areas the concentration of nitric oxide can be large. In t his case nitric oxide can react with ozone: NO O3 NO2 O2 T his reaction removes ozone and short circuit s the ozon formation chain of reactions. In highly polluted areas the ozone concentration falls off as the level of nit ric oxide increases. Ozone vs NOx for Non Methane HydroCarbon level of =0.6 ppmc High Pollution days • The figure illustrates one of the problems in the abatement of pollution. The ozone concentration is used as the standard, and yet one can reduce the nitrogen oxides by a significant fraction and see no change, or even an increase in the ozone level. • Most of the pollution is emitted in the cities, which typically puts the atmosphere at the right of the figure. As the pollutants move away from the city center their concentration gets smaller, and the atmosphere is moved toward the left, and the ozone increases. • Hence the suburbs can see more ozone than the cities. Overall scheme for air pollution Haze Haze is a subset of PM and is primarily composed of sulfur and nitrogen compounds. Particles of a certain size can reflect or refract light, causing a reduction in visibility. This reduction in visibility is known as haze. Hazy conditions occur frequently in conjunction with severe O3 events. An Example of the Effects of Haze in the Mid-Atlantic The Great Smoky Mountains National Park A Clear Day Photos from www.epa.gov A Hazy Day VOC Emissions 18% Solvent Utilization 33% Storage & Transport 13% On-Road Vehicles Non-Road Vehicles 7% 29% All Other NOx emission inventory 5% 28% 19% Fuel Combustion - Electric Utility Fuel Combustion - Industrial Fuel Combustion - Other On-Road Vehicles 13% Non-Road Vehicles All Other 30% 5% Sulfur Dioxide emissions 2% 3% 4% 7% Fuel Combustion - Electric Utility Fuel Combustion - Industrial Fuel Combustion - Other 17% Metals Processing 67% Non-Road Vehicles All Other Acid Precipitation Robert Angus Smith (1817-1884) was a 19th-century Scottish chemist who investigated numerous environmental issues. Smith did innovative studies of air and water pollution and was one of the few at the time to realize the importance of finding solutions to the environmental problems caused by urban growth. He is most famous for his 1852 research on air pollution, in the course of which he discovered acid rain. Acid Precipitation • Rain is naturally weakly acidic because CO2 from the atmosphere dissolves in water. • Unperturbed rainwater has a pH of 5.6 • Precipitation near urban areas has a much lower pH. This rain or snow is called acid precipitation.