9. 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
10.
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14. Chemical Reactions of Smog
A few important reactions:
(1) NO2 + hν (photon) 🡪 NO + O (hν 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
15.
16. You can tell the air stability by watching the
motion of the chimney smoke
18. 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.
19. •Surface high – sinking air, dry, clear, stable air, may
cause upper level inversion
•Surface low – rising air, cloudy or rainy, unstable air
21. We see that there was a low pressure system out in the Pacific . The whole
California was controlled by a high pressure system. Near Southern California, the
weather was clear and the air was stable. This usually implies that air quality
would be poor.
Saturday
22. We can use the ozone level s a general air quality index. On this day, the
peak ozone level at Pasadena was nearly 200 units while it was about 80
units in West LA. High ozone level indicates poor air quality.
23. Look at the remarkable improvement of ozone levels at Pasadena and W.
LA! The above series of slides show clearly the intimate relation between
local air quality and weather.
26. 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.
Source: National Park Service
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30. 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.
31. Vacuum-Insulated Automotive Catalytic
Converter
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).
Converter Basics:
Three main ingredients of photochemical smog: high automobile traffic volume, plenty of sunlight, and very stable atmosphere.
One example of the locations that have all three necessary ingredients is the Los Angeles Basin.
Due to the lack of efficient public transportation, residents there depend on their cars. The traffic jam there is no news at all.
Los Angeles’ climate is dominated by the Eastern Pacific High – the subsidence above produces compression heating of the air, and the temperature is often higher at a few hundred feet level than at surface – an inversion condition. This is of course an absolutely stable condition.
The subsidence not only produces inversion but also clear air. It is hard to form clouds and rain due to the weak vertical motion. Thus sky is clear most of the time—a lot of sunshine!(That’s why Hollywood is a good place for making movies!)
The topography of Los Angeles – a basin also helps to trap air pollutants.
Thus there is no surprise that photochemical smog can be very severe in LA.
There are many chemical reactions involved in producing the photochemical smog. We only list a few important ones (note the chemical symbols used here are not necessary exact, some are ‘descriptive’ only):
(1) NO2 + hν 🡪 NO + O (hν represents a photon)
fast (2) O + O2 + M 🡪 O3 + M (M represents a neutral molecule)
(3) O3 + NO 🡪 NO2 + O2
(4) O + HC (hydrocarbon)🡪 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
For our purpose here, the most important reactions are (1)-(3).
As explained before, the severity of smog problem is closely related to weather condition. The inversion layer that traps air pollutants in Los Angeles is caused by the subsidence in the Eastern Pacific High.
Eastern Pacific High is a semi-permanent high pressure system that dominates the weather pattern of southern California most of the time. It is one of the ‘subtropical high pressure systems’.
If the influence of this high pressure system decreases (for example, if a low pressure system moves in), then this inversion layer may disappear and the air quality improves.
The following slides illustrate the above-said weather-air quality relation . They are taken from Los Angeles Times in 1988.
Because clouds usually form in ascending air, they often appear in low pressure regions.This often accompanied by rain, snow, or thunderstorms. In contrast, high pressure regions are often sunny and calm. In summer, this could mean clear but hot weather. In winter, this could mean extremely cold weather (as is often the case for Wisconsin).
We can use the ozone level s a general air quality index. On this day, the peak ozone level at Pasadena was nearly 200 units while it was about 80 units in West LA. High ozone level indicates poor air quality. Air quality in W. LA is generally better than that in Pasadena because it is next to the ocean and a small scale circulation –the sea breeze - helps to vent air pollutants out.
Sea breeze is a wind phenomenon caused by the difference of heating over land and ocean. During the daytime, it is usually warmer over land than over the ocean. The temperature difference causes a pressure difference that in turn causes a wind that blows from the sea towards the land. In the night, the situation is reversed and the wind blows from the land towards the ocean.
So it appears that high pressure systems tend to cause more severe local air quality problems due to their characteristic weak wind condition- called air stagnation. The frequency of air stagnation is a useful index of local air quality.
One example of air stagnation frequency distribution in the eastern half of US is shown in the figure here.
We see the highest frequency occurs in the bordering region between Tennessee and Georgia . Do you know any special phenomena relevant to the air quality there?
The Great Smokies got its name from the often hazy appearance in the mountain valleys there. Why is it so? Remember that this is a densely forested region. 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.
