Urban air pollution
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Urban Air Pollution. Public and Environmental Health Concerns Elevated levels of toxic compounds Regional and Global Impacts Background Chemistry and Composition Climate. Joel Thornton, Asst. Professor. ftp.atmos.washington.edu/thornton/atms501. The Urban Smog Problem. Major components:

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Urban air pollution
Urban Air Pollution

Public and Environmental Health Concerns

Elevated levels of toxic compounds

Regional and Global Impacts

Background Chemistry and Composition


Joel Thornton, Asst. Professor


The urban smog problem
The Urban Smog Problem

  • Major components:

  • “invisible”: O3, CO, SO2

  • “visible”: PM (aerosols) + some gases (NO2)

  • Ingredients to Make Smog

  • Sun (photochemistry)

  • Stagnation

  • Sources of NOx, SO2, PM and VOC

  • Smog – “Smoke” + “Fog”

  • Coined due to reduced visibility associated with pollution episodes

Houston, TX

Aug. 2000

Ozone damage
Ozone Damage

Needle damage (tip necrosis) is a common sign of ozone stress on pines.

Often observed in forests downwind of major urban areas—Sierra Nevada, New England, Mexico City, etc.

Large subset of u s population exposed
Large Subset of U.S. Population Exposed

#of people living where NAAQS are not attained


1-hr Avg O3 < 125 ppb

8-hr Avg O3 < 85 ppb

24-hr Avg PM2.5 < 60 g/m3

Annual Avg PM2.5 < 15 g/m3

Violation of o 3 naaqs by region
Violation of O3 NAAQS By Region

1hr avg > 125 ppb

8hr avg > 85 ppb

Annual average pm 2 5 in urban areas
Annual Average PM2.5 in Urban Areas

Surface o 3 and transport
Surface O3 and Transport

90th percentile O3 concentrations for summers 1991-1995 and mean 850hPa winds on days when O3 > 90th percentile

  • Stagnation enhances chemistry

  • Persistent stagnation in regions of strong subsidence: LA, Mexico City, Athens

  • Air pollution is not just an urban problem

  • Chemical production of o 3 main ingredients
    Chemical Production of O3: Main Ingredients

    Fossil Fuel Combustion and Use

    Evolution of NOx and O3 in Nashville, TN June 1999

    NOx = NO + NO2


    hv = uv-vis radiation

    Biogenic Activity

    Cycling of ho x and no x leads to net o 3
    Cycling of HOx And NOx Leads to NET O3

    1. NO + O3 NO2 + O2 k1

    2. NO2 + hv  NO + O k2

    3. O + O2 + M  O3 k3


    Often (not always)


    NO + XO2 NO2 + XO

    Net o 3 production
    Net O3 Production

    The Null Cycle







    The Net O3 Production Cycle










    • The rate limiting step in NET O3 production is the conversion of NO to NO2 by peroxy radicals. What is the rate expression for photochemical O3 formation?

    • Ronald Regan famously noted that “trees pollute too”. What did he mean by this statement?

    Biogenic voc major role in o 3 pm pollution
    Biogenic VOC: Major role in O3/PM Pollution


    Important source of peroxy radicals: enhance O3 production

    + OH  RO2

    In U.S. isoprene emissions > total anthropogenic hydrocarbon emissions

    GEIA Isoprene Emissions

    In July, Guenther, et al

    [1012 atoms C cm-2 s-1]

    Global impacts of urban air pollution
    Global Impacts of Urban Air Pollution


    Urban areas (mega-cities) are major point sources

    NO2 - OMI

    Courtesy of P. Weiss-Penzias and D. Jaffe, UWB

    Global pollution transport makes meeting own air quality standards more challenging


    • A vast majority of NOx is emitted at the surface, and most NOx is a result of human activity. Given that NOx catalyzes O3 production, it is important to consider the global impact of anthropogenic NOx emissions.

    • NOx is removed from the atmosphere primarily by the reaction NO2 + OH  HNO3, k ~ 1x10-11 cm3 molec-1 s-1 what is a typical lifetime for NOx w.r.t. this process? ([OH] ~ 1x106 molec cm3)

    • Is this lifetime sufficient to allow NOx to be transported away from an urban area to the remote troposphere?

    • Do anthropogenic emissions of NOx reach the remote troposphere at all?


    • An important biogenic hydrocarbon is isoprene. Isoprene is very reactive towards OH. Given that [Isoprene] ~ 1 ppb, and [propane] < ~ 1 ppb outside of Atlanta, would it make sense to try to regulate propane emissions to combat O3 production?


    k ~ 1x10-11 cm3 molec-1 s-1

    + OH  RO2


    k ~ 1x10-13 cm3 molec-1 s-1

    CH3CH2CH3 + OH  RO2 + H2O