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NFCRC Tutorial: Environmental Impacts

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The earth is being stressed by the large amounts of pollutants being introduced into the atmosphere. Example are: At the urban level, photochemical oxidants are causing smog (the first recording of aerosols was made in 1943 and brought to the public attention in the Los Angeles Times). In the troposphere, acid rain is being formed and the greenhouse effect increasing (the first presentation of global warming was made in the newspapers in the mid 1980s). In the stratosphere, the ozone which is tri-atomic oxygen (O3) is being depleted (DuPont had to stop making chloro fluoro carbons or CFCs which play a role in the depletion of the O3; the Nobel prize was awarded to the discoverer of this phenomena).

Urban Level. The automobile is a major source of pollution in the urban areas and is responsible for the vast majority of the primary pollutants such as NO, hydrocarbons (HCs) and CO. This is strongly supported by data collected since 1940 and it can be seen that a decline in each of these pollutant concentrations was observed as legislatures for control were implemented. The NO is oxidized to NO2 which is a brown gas and causes the brown haze observed in the early morning period. In the south coast basin, half of the NOx is produced from automobiles. Later in the morning under the action of sunlight, NO2 decomposes generating NO and O. The atomic O combines with O2 to produce the secondary pollutant O3. The NO can react with the HCs forming PAN, aerosols etc. The automobiles (and not refineries) were first identified as the major sources of HCs in 1950, and in 1963 the first control was implemented to keep the HCs from escaping from its crankcase. The first control of the automobile exhaust was implemented in 1966 to reduce the HCs and CO emissions and finally, in 1970 the Clean Air Act was passed largely due to Edmund Muskie's efforts. This paved the path for the establishment of the EPA by Nixon. In 1971, the second exhaust control was passed to limit the NOx emissions. Further reductions (by 90%) in the HCs/CO and NOx were enacted in 1975 and 1977, respectively. In 1966, the NOx increased when the HC emissions control was implemented. In 1973, it was determined that the implementation of exhaust gas recycle to limit the NOx increased the HC emissions. Thus, the NOx and HCs emissions have shown an inverse relationship.

So far, the use of automobiles in the urban areas has been unscabed while the control strategies have been essentially exhausted. For example, currently (in 1997), the CO emissions which are essentially all from cars with control devices in the Los Angeles south coast basin are as high as 8,000 ST/D or 7260 MT/D or Mega Grams/D (CO replaces O2 in the blood and starves the body/brain from O2). Thus, measures are being instituted in the Los Angeles are such as diamond lanes, car pools, increased parking fee and encouragement of electric cars (combination gasoline/electric cars have been also announced, equipped with gas turbines or diesel engines, the fuel cell being ideal, however, because of its significantly higher efficiency).

Troposphere. The lower region of the atmosphere which is characterized by decreasing air temperatures as the altitude is increased is the troposhere. Types of pollution within the troposphere include those caused by (1) SO2 as the primary pollutant with particulates as the secondary pollutant formed from the SO2 (the well known London Killer Fog episode that occurred in the mid 20th. century being an example), and (2) NOx and organics as the primary pollutants with O3, HNO3, H2SO4, particulates and Peroxy Acetyl Nitrate (PAN) as the secondary pollutants formed by photochemical reactions from the primary pollutants (the air pollution in Los Angeles being an example of this type of pollution).

In the troposphere SOx and NOx produce their respective acids and cause the acid rain. Also the build up of CO2 and O3 in this region of the atmosphere causes the greenhouse effect (CO2 is transparent to ultra violet radiation that strike the earth's surface during the daytime but CO2 is not transparent to infra red which is reradiated by the earth's surface). Taxes are being levied on the airlines when flying over some of the European countries (e.g.. Norway) to force a reduction in these emissions, leading both Lufthansa and Swiss Air to push GE in designing jet engines with reduced NOx emissions.

A brief overview of the role played by O3 and associated species, as well as the predominant role played by the hydroxyl (OH) radical in tropospheric chemistry is presented in the following:

Photochemical Air Pollution. Under the action of sunlight (essentially in the visible region of the spectrum, noting that majority of the ultra violet radiation from the sun is blocked by the O3 present in the upper regions of earth's atmosphere), photochemical reactions occur which convert the primary pollutants such as Volatile Organics Compounds (VOCs), NO and NO2 with the ultimate formation of the secondary pollutants such as O3, PAN, HNO3, particulates (such as H2SO4, sulfates, nitrates, organics).

VOCs. VOCs are responsible for the formation of a number of secondary pollutants such as PAN and particulates and are also responsible for the formation of a major portion of the O3 where the NO concentration is low, such as in less polluted rural areas.

OH radical. The chemistry of the atmosphere is dominated by the reactive OH radical in the daytime (while the nitrate NO3 radical plays a major role in the night time), resulting in photochemical air pollution, acid rain and fogs, and the toxic organics. A major source of the OH radical is the O3 which under the action of light decomposes to generate the atomic O which in turn reacts with H2O to produce the OH radical. Photolysis of nitrous acid which is formed from NO2 also produces the OH radical.

Halogen Atoms. Chlorine atoms play a similar role in the chemistry of the stratosphere as the reactive OH radical and oxidize most organic compounds with high reaction speeds. A mechanism proposed for the production of Cl atoms involves the reaction of N2O5 with sea-salt aerosol.

Ozone Control Strategies and Health Issues. The amount of O3 formed depends on the VOC/NOx ratio of the air mass and is the greatest at smaller VOC/NOx ratios. The reactivity of each of the VOCs is also different and thus, O3 control strategies for highly polluted urban regions should be implemented in conjunction with stringent NOx control policy while taking into account the reactivity of the individual VOCs. The emissions for compressed natural gas fueled vehicles show very low concentrations of the more reactive VOCs and thus, this fuel should be an attractive alternative.

Finally, OH, NO3 and O3 play a major role in the formation and fate of airborne toxic chemicals, mutagenic polycyclic aromatic hydrocarbons and fine particulates (PM10 and PM2.5). Epidemiological studies have shown a direct link between mortality and particles less than 10 microns.

Stratosphere. The region of the atmosphere above the troposphere in which the temperature starts increasing with altitude is the stratosphere and there is interaction and mixing between the stratosphere and the troposphere. The stratosphere contains large concentration of O3 which is responsible for the increase in temperature of the stratosphere. The O3 is formed here by the Chapman cycle which is the steady state formation and destruction of O3. The atomic O formed by absorption of ultra violet radiation (<220 nm) by the O2 reacts with O2 to form O3 (the presence of a third body to take away the energy from the newly formed O3 is also required). O3 is toxic to breathe but in the upper layers of the atmosphere O3 blocks the ultra violet radiation. With the addition of destruction cycles, however, this steady state is destroyed and O3 is depleted. Perturbations in the O3 level may be caused by CFCs and possibly the proposed high speed civil transport (HSCT) plane. Ultra violet radiation can cause cataracts, skin ailments and crop damage. 90 to 95% of the O3 has been destroyed in the South Pole as measured in 1993 (it should be noted that the O3 measurement is also effected by solar intensity and seasons).

The NO introduced into this zone reacts with the O3 and thus, the protective O3 layer is depleted. Thus, the development of the HSCT which will fly in the troposphere and is NASA's highest priority (in order to maintain U.S.'s leadership role in the aircraft design area, help the balance of payments and the U.S. industry), is at stake. The plane which would be capable of carrying 300 passengers and traveling at Mach 2.4 is technically feasible but is unable to meet the NOx goals at the present time.

Alternatives. Alternatives available are: (1) Conservation - one way to enforce this is to add an energy tax (Japan uses 1/3 of the energy per capita as the U.S. because of higher energy costs in Japan), (2) more efficient energy systems such as cogeneration and fuel cells (with fuel cells the efficiency may be almost doubled resulting in a 50% reduction in fuel use, (3) these systems still produce CO2, pointing towards use of solar energy, and finally (4) evolution of an electric society which has already started with the introduction of the electric automobiles.

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