air pollution- 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.
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
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.
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
(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
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