Types of Thermal Inversions• Radiative: Earth cools during night by radiating thermal energy into space. In morning, air near surface will be cooler than air above creating thermal inversion. More frequent, but less problematic and persistent than -• High pressure subsidence: high pressure mass of air moves towards earth. Is compressed and heated, causing thermal inversion some distance above ground.
AVERAGE COMPOSITION OF THE ATMOSPHERE Element Parts per Million Nitrogen 780,800 Oxygen 209,500 Argon 9,300 Carbon Dioxide 300 Neon 18.2 Helium 5.2 Krypton 1.1 Hydrogen 0.5 Nitrous Oxide 0.5
Primary and Secondary Pollutants• Primary Pollutants: Those emitted directly from sources (CO, SO2, VOC, CO, lead, PM)• Secondary Pollutants: Those formed in the atmosphere by chemical interactions among primary pollutants and normal atmospheric constituents. (Ozone, photochemical oxidants)
Sulfur Containing Compounds• Carbonyl sulfide (COS), carbon disulfide (CS2), dimethl sulfide ((CH3)2)S), hydrogen sulfide (H2S), sulfur dioxide (SO2)• Sources are: biological decay, combustion of fossil fuels and organic matter.• SO2 combines with water to produce sulfuric acid - major component of acid rain: 2SO2 + O2 = 2SO3: SO3 + H2O = H2SO4
Nitrogen-Containing Compounds• Nitrous oxide (N2O): colorless, odorless, “laughing gas”, emitted by natural sources, chemically inert, not considered an air pollutant.• Nitric oxide (NO) + nitrogen dioxide (NO2) = NOx: emitted by combustion at high temperatures. Together with hydrocarbons are important in formation of ozone. Lifetime about 1 day. Combines with water to form nitric acid, component of acid rain.• Released by stationary and mobile sources.
Carbon Compounds• CO - carbon monoxide: criteria pollutant, colorless, odorless gas. Toxic - binds to hemoglobin preventing uptake of oxygen. Formed by incomplete combustion mainly in cars and trucks. Lifetime in atmosphere about one month
Carbon Monoxide• Carbon monoxide is a colorless, odorless, poisonous gas formed when carbon in fuels is not burned completely.• It is a byproduct of highway vehicle exhaust, which contributes about 60 percent of all CO emissions nationwide. In cities, automobile exhaust can cause as much as 95 percent of all CO emissions. These emissions can result in high concentrations of CO, particularly in local areas with heavy traffic congestion.• Other sources of CO emissions include industrial processes and fuel combustion in sources such as boilers and incinerators.
Particulates• Particles of average diameter 10 microns or less (PM10). Tend to be easily inhaled and lodge in lungs. Larger particles not readily inhaled.• Sources are combustion process (particular diesel combustion), unpaved roads, fires.• PM2.5 particles with average diameter less than 2.5 microns. Considered even more problematic than PM10• New EPA regs. For PM2.5 now under consideration
Reactive Organic Compounds (ROG) and Ozone• Sometimes called VOC (volatile organic compounds or hydrocarbons)• Gasoline contains almost 100 hydrocarbons, such as octane, heptane, ethane. Evaporation of gasoline one source of HC into atmosphere• Natural sources account for 85% of HC in air, but more reactive ones are anthropogenic
OZONE• Secondary pollutant: O3• Formed in atmosphere by reaction of oxides of nitrogen, VOC and sunlight.• Classified as a criteria pollutant• Major component of photochemical smog