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