2.5: air pollution

air and the atmosphere

• atmosphere: a sea of air that surrounds the earth
• divided into several layers, separated from each other by abrupt changes in temperature due to differences in absorption of solar energy

layers of the atmosphere

• troposphere
    * lowest layer, contains 75% of the mass of earth’s air
    * extends 10km/6mi up from sea level at the equator
    * temperature declines with altitude and then abruptly rises when entering the tropopause
      * tropopause: the boundary between the troposphere and upper layers
    * air composition
      * 78% nitrogen, 21% oxygen, 1% other eg. permanent gases and variable gases
• stratosphere
    * second layer, extends from 10-50km/6-30mi above earth’s surface
    * air is very calm
    * temperature increases with altitude until another reversal in the stratopause
      * stratopause: the boundary between the stratosphere and upper layers
    * air composition
      * similar to troposphere with two exceptions:
        * less water vapor
        * 1000x more ozone (O3)
        * ozone in troposphere = bad, in stratosphere = good

air pollution

• air pollution: the presence of chemicals in the air in quantities and durations that are harmful to the environment and human health
• some pollutants are natural
    * natural fires
    * terpenes (from forests)
    * pollen (allergen)
    * dust (from volcanos)

types of pollutants

• primary pollutants: harmful substances emitted directly in that form
    * may react with one another or with a basic air component to become secondary pollutants
• conventional pollutants + pollutant criteria
    * the EPA sets the National Ambient Air Quality Standards (NAAQS) which limits six major pollutants
      * particulate matter (PM)
        * classified based on diameter
          * PM10: 10 micrometers or smaller
          * PM2.5: 2.5 micrometers or smaller
        * comes from wildfires, industrial activities, motor vehicles, cooking, and fuel burning
        * particles can enter the lungs, and the smallest can enter the bloodstream
          * this can cause heart attacks, strokes, and negatively impact children’s nervous systems
        * associated with ~80,000 deaths in the US
      * ozone (ground level)
      * nitrogen oxides (NOx)
      * sulfur oxides (SOx)
      * carbon monoxide (CO)
      * lead

smog

• “smog” = fusion of words “smoke” and “fog”
• two types
    * industrial smog
    * photochemical smog

industrial smog

• important until the mid-1900’s in industrial cities (eg. London, Donora, Pittsburg, Chicago)
• now, rarely a problem in MDCs but still a problem in China, India, Eastern Europe
• caused by large amounts of heavy oil and coal burnt in homes for heat and in industrial work
    * generally worse in winter
• releases sulfur oxides (SOx) and particulate matter
• appears grey in color

the great smog

• took place in London in December 1952
• unusually cold → more coal burnt
• visibility fell to ~12 inches
• 100,000+ people treated for respiratory problems
• caused 3,000-4,000 extra deaths
    * extra death: a death caused by smog above the normal rates which are attributed to the abnormal smog conditions
    * mainly of cardiac and respiratory ailments
• asthma
    * disease which negatively effects lungs
    * more common among people alive/born during the great smog
• US equivalent: Donora, PA, October 1948

photochemical smog

• more pressing current problem in more developed countries
• composed of a mixture of primary and secondary pollutants
• formed when primary pollutants interact under the effect of sunlight and give origin to a mixture of 100 chemicals dominated by ozone
    * worse in summer
• primary pollutants involved and their sources
    * nitrogen oxides (NOx) mainly from vehicles
    * volatile organic compounds (VOCs) from vehicles, gas stations, dry cleaners, print shops, vegetation
• appears orange in color

ozone

• stratospheric → good
• ground level → bad
    * very irritating for the respiratory system

thermal inversions

• thermal inversion: a process which occurs when a layer of cold air temporarily trapped near the ground by a warmer, upper layer
    * pollutants do not get dispersed during thermal inversions
    * warm air is less dense than cold air
• normal conditions
    * during the day, sun warms up the air near the surface, which expands and rises, carrying low-lying pollutants higher
    * under normal conditions, there is a constant movement of air
• sometimes a layer of cold air is trapped beneath a warmer, less dense error
    * warm air prevents ascending air currents and the dispersion of pollutants
    * common in valleys and urban areas

urban heat islands

• structures eg. buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies
• urban areas become islands of higher temperature relative to outlying areas
• result in thermal inversions and more thunderstorms
• dust domes: pollutants including soot trapped in urban air through thermal inversions

case study: mexico city

• megacity with over 22 million people in the metro area
    * more people, more industry, more cars
• located at a high altitude → solar radiation is intense
• surrounded by mountains, air can be stagnant with winter thermal invasions
• deforestation and construction have exposed soil to wind erosion