Chapter 5 air pollution- smog Photochemical Smog Air Pollution in Megacities Not just Los Angeles What’s the most common sight when you approach a large city by airplane? Urbanization and Air Quality Urbanization high concentration of people, industries and automobiles. Exhaust gases from internal combustion engines: CO, CO2, HC (hydrocarbons), and nitrogen dioxide (NO2). NO2 is the precursor of photochemical smog. In the presence of sunlight, photochemical reactions occur that produce smog. (Photochemical reactions are chemical reactions that involve light photons). Local climate can make the problem worse. Photochemical Smog The Case of Los Angeles Air Quality Three main ingredients of photochemical smog: high automobile traffic volume plenty of sunlight very stable atmosphere Eastern Pacific High – Subsidence produces inversion, resulting in an absolutely stable condition. Subsidence also produces clear condition and hence more sunlight. Topography – basin helps trapping pollutants Chemical Reactions of Smog A few important reactions: (1) NO2 + h (photon) NO + O (h represents a photon) fast (2) O + O2 + M O3 + M (M represents a neutral molecule) (3) O3 + NO NO2 + O2 (4) O + HC (hydrocarbon) S.P. (stable product) + F.R. (free radical) slow (5) O3 + HC S.P. + F.R. (6) F.R. + HC S.P. + F.R. (Being a stable product doesn’t mean it is pleasant! It can be irritating to our body.) (7) F.R. + NO F.R. + NO2 fast (8) F.R. + NO2 Stable Product (PAN-Type, Peroxyacetyl Nitrate ) (9) F.R. + F.R. Stable product Typical Daily Concentration Variation of Smog Chemicals Static Stability of the Atmosphere controls the local air quality Actual lapse rate < the dry adiabatic lapse rate → stable. Actual lapse rate = the dry adiabatic lapse rate → neutral. Actual lapse rate > the dry adiabatic lapse rate → unstable. In the following charts, the red dashed lines represent the dry adiabatic lapse rate and solid black lines represent the actual lapse rates of the air You only need to compare the “slope” of the lines to determine the stability. You can tell the air stability by watching the motion of the chimney smoke So what is the atmospheric condition here? Photochemical Smog and Weather Whereas the photochemistry produces smog, the severity of smog pollution is largely controlled by the weather conditions. In a local scale, the air stability controls the pollution as we described above. In a larger scale sense, it is the weather systems that determine the air stability. The following slides show the close relationship between the air quality and meteorological conditions. •Surface high – sinking air, dry, clear, stable air, may cause upper level inversion •Surface low – rising air, cloudy or rainy, unstable air How about this? 10/31/2006 1930UTC Air Stagnation due to High Pressure Systems Source: Williamson, 1973. Source: National Park Service During the life cycle of plants, some organic particles (such as turpanoids) are ejected. Due to the high frequency of air stagnation these particles tend to stay in the air for a long time and hence the hazy look. This is of course a natural phenomenon and not a man-made pollution, but it is another good example of the close relation between particle concentration and weather. Emission Control Because of the photochemical smog problem and its relation with automobile emissions, California has enacted a series of regulations purported to cut down the emissions. Two specific actions are of interest in this regard: (1) The installation of catalytic converter to reduce then amount of NOX emitted. (2) The designation of ‘diamond lanes’— only cars with two or more passengers can use these lanes, so as to encourage carpooling. The problem, however, will stay as long as the traffic volume is large. One really needs to develop an efficient public transportation system in a city like that. Vacuum-Insulated Automotive Catalytic Converter Converter Basics: Variable-conductance vacuum insulation and phase change material (heat storage) are used to keep catalytic converters hot for up to 24 hours. By having a hot (> 250oC) converter at the start of a trip, auto emissions can be reduced by up to 80%. NREL developed and patented this concept, and has worked with Benteler Industries of Grand Rapids, Mich., to commercialize it under a Cooperative Research and Development Agreement (CRADA).
Pages to are hidden for
"7f7da1594004ef51babd7bc1275-19944"Please download to view full document