PAN, or photochemical, smog
Smog is a combination of ozone, carbon monoxide, PAN, and organic molecules. PAN—
peroxyacetylnitrate—is produced in the atmosphere by the action of sunlight on the
chemicals present in urban areas. PAN has a very unpleasant odor. Smog precursor
chemicals include volatile organic compounds (VOCs, or basically hydrocarbons), carbon
monoxide, carbon dioxide, nitrogen oxides, ozone, and sulfur oxides.
Fig. E14.4.1 Smog in Denver. Source: National Renewable Energy Laboratory
Smog restricts visibility (Fig. E14.4.1) because the complex chemical reactions of the
constituents in smog with sunlight create fairly large particles. When sunlight scatters
from very small particles such as molecular oxygen or nitrogen, it scatters more in the blue
than the red, creating Earth’s beautiful blue sky and reddish sunsets. But when it scatters
from large particles, such as water droplets, the scattering is not selective, and they
Energy, Ch. 14, extension 4 PAN , or photochemical, smog
appear as white clouds. Smog consists of fairly large particles that scatters sunlight nonselectively, similar to the way clouds scatter sunlight, but not so densely.(90)
Many of the chemicals in smog are emitted by vehicles (see Fig. E14.4.2 for sources of
smog in Los Angeles; the single greatest is vehicular traffic). In fact, smog-chamber
experiments with gasoline showed that gasoline vapors (all the “aromatic fractions”) are
what contribute the smog-forming VOCs to the mix, and that one could even predict the
amount of particulates formed by knowing which vapors were present.(91) So it is the
vapors rising from refineries, from filling stations’ pump nozzles, leaking out of cars’ fuel
systems, and the unburned hydrocarbons that create the VOCs that go into forming smog.
Additionally, experiments with more sensitive equipment indicate that older equipment
may have missed detecting many VOCs,(92) indicating that the problem may be more
severe than thought.
Fig. E14.4.2 Sources of smog in the Los Angeles metropolitan area. Source: South Coast Air Quality
Management District
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
PAN smog mainly affects cities in the western United States, such as Denver (shown in
Fig. E14.4.1) and Los Angeles, where cars are virtually the only means of transportation.
Even after smog was recognized as a problem, more and more roads were built to
accommodate more and more people moving and living there. The greater number of roads
led to more traffic, which meant more smog. The inability of the cities with the worst
ozone problems to get their ozone levels down during the 1980s influenced the writing of
the 1990 Clean Air Act Amendments (see Chapter 12 and the Extension 15.4, The
Clean Air Act Amendments, California standards, and zero emission and Extension
14.5, The 1990 CAAA, NOx, particulates, and the EPA).
Tropospheric ozone can travel. A news report highlighted this dilemma. A small
California town, Grass Valley in Nevada County, is high in the mountains at an altitude of
about 750 m, but fails on clean air because of high ozone levels.(93) The ozone
presumably comes from Sacramento, about 100 km away.
Smog is formed in the presence of many different chemicals, and the rates of formation of
the combinations are much higher than they could be without the presence of these other
chemicals. The smog also forms fine particulates; about 5% of organic smog vapor is
particulate.(94) These “secondary” particulates are the most effective size (0.1 to 1 µm) to
scatter light and reduce visibility by causing haziness, as Fig. E14.4.1 showed.
Ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, particulate matter, and lead are
the criteria pollutants. The VOCs are not themselves criteria pollutants, but ozone—
produced by the interaction of nitrogen oxides, VOCs, and sunlight to warm the air—is.
Emission of criteria pollutants is regulated by the EPA.
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
VOCs have health effects. Minnesota schoolchildren were monitored for VOCs outdoors,
indoors while at school, indoors while at home, and in personal samples. It found that
home exposure was by far the most effective at determining personal exposures. Results
were similar to those of studies of adults. However, the researchers found that children
were exposed to very high levels of p-dichlorobenzene, which was not found in school or
outdoors, implying that this VOC would have been missed in assessing the children’s
exposure to VOCs.(95)
Clean air violations are severe in many places, In 2004, the EPA ordered 474 counties in
31 states to clean up smog and smog precursors. About 160 million people live in these
areas. In California alone, 36 counties made the list, the most of any state.(96)
After many years of improvements, California’s smog problem got worse again after
2001. While nowhere near the pollution levels of the 1950s, this turnabout has caused
great concern. One year may be an anomaly, but three straight years of deterioration
makes it seem like a trend. In summer 2003, there were 30% more smoggy days than in
summer of 2002, which in turn was smoggier than 2001.(97,98) California is under a 2010
deadline to reduce smog, and this smog increase makes it that much more difficult to
achieve. One consequence of failing to eliminate smoggy days could be loss of federal
money as well as imposition of fines, all of which could be in the billions of dollar
range.(97)
A University of California study suggests that the state could meet the 2010 deadline, but
only if the state mandated sales only of super-ultra-low-emissions vehicles (SULEVs)
from 2004 on, and if all older cars, those more than 15 years old, were removed from the
road.(99) However, the chances of both occurring are not high. In addition to political will,
these two steps would take money (always in short supply) for implementation.
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
This situation has led to weeping and gnashing of teeth. Some agencies, such as the South
Coast Air Quality Management District, are relatively toothless, but held accountable.At
a meeting in August 2003, a board member said, “We are drowning in ozone, and there is
no relief in sight,” according to news reports.(100) Apparently, ozone levels jump on
Saturdays and Sundays when truck traffic and industrial activity is lower, and as of this
writing, no one has figured out why.(98)
This lack of understanding has persuaded some observers that the government’s
antipollution program is taking the wrong set of tracks. They argue that instead of the
current focus on NO x and ozone, the VOCs could be the major problem.(98,101)
Ozone’s effect on living things
Ozone was chosen in the 1970 CAA as one of the criteria pollutants because of its
established record of causing injury to human health. High ozone level was one of the
pollutants found to be correlated with excess deaths in Barcelona.(102) Concentrations as
low as a few parts per billion can cause eye irritation, and concentrations as low as a few
parts per million are lethal to mice exposed to PAN smog for only 2 hours.(103) The
pollutant mix of ozone and particulates apparently works by causing constriction of the
arteries, which could make sick people sicker.(104) An experiment exposed healthy people
to 150 µg/m3 of concentrated ambient fine particles and 120 parts per billion of ozone for
two hours (similar to exposure to rush hour traffic) and measured the resulting
constriction. While this poses no threat to healthy people, “such constriction could
conceivably trigger cardiac events in those individuals who are at risk for heart
disease.”(104)
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
6
Such problems are major, but there are also more minor irritations caused by ozone. For
instance, a study by researchers from USC found that as ozone levels increased by 20
parts per billion, absences from school increased 83% because of respiratory illness.(105)
Montrealers died more on days with high ozone levels.(106) Ozone on normal days in the
Los Angeles Basin may average 30 nmol/mol (often referred to in the literature as parts
per billion, ppb), while on a smoggy day ozone levels can exceed 100 nmol/mol. Also, the
ozone levels vary whether it is day or night, illustrating how important industrial
emissions are.(107) Ozone was found to impair lung function in healthy young
volunteers.(108) It exacerbated wheezing in children who had a history of wheezing.(109)
Seventh Day Adventists studied had many more medical problems as concentrations in
their neighborhoods increased.(110) Heartbreaking stories of individuals in the San Joaquin
Valley affected by California smog are to be found in the sweeping article “Last gasp,”
published in the Fresno Bee.(111) The article lists many myths and corresponding
realities, some of which are paraphrased in Table E14.4.1.
TABLE E14.4.1
Myths and rEalities about Smog and Air Pollution in the San Joaquin Valley (SJV)
Myth
air pollution is a summer problem
Reality
the problem in summer is ozone
fall is dust
winter is soot
LA has the worst pollution in the U.S.
the SJV is worse on ozone
a few small particles in air do no harm
there is no “safe” level for PM 2.5 or PM10
the SJV is dry and good for asthma sufferers the SJV has the highest asthma rate in CA
blue skies mean clean air
ozone is colorless
most SJV pollution is from agriculture
vehicles produce 60% of SJV pollution
farming is nearly pollution-free
farm diesels and burning cause problems
farm animals make VOCs, soon > cars
smog check catches dirty vehicles
10% of cars produce 50% of the problem
old cars fail smog check, new cars pass
poor maintenance is the problem, not age
lawnmowers are too small to be a problem one lawnmower = 40 new cars’ emissions
Source: Adapted from Ref. 111
Energy, Ch. 14, extension 4 PAN , or photochemical, smog
Health damage is not restricted to people. Plants are affected by very low pollutant
concentrations (especially for ozone). About 60% of the plants on land areas near
freeways in San Bernardino have been severely or moderately damaged,(112) and damage
has been reported as far east as 120 km from the Los Angeles urban area.(113) Smog also
causes damage to romaine lettuce, alfalfa, and spinach among farm crops, as well as to
flowers.(114) Such local pollution sources as a smelter may produce enough smog and
other pollution to destroy plant life in the vicinity.(114)
Ozone concentrations of even 60 nmol/mol (60 ppb), far below regulated levels, damage
conifers. In the Black Forest in Germany, ozone concentrations have averaged 90
nmol/mol, with some readings as high as 270 nmol/mol.(113,115) The sick condition of
German and American forests are discussed in the following chapter. This was one
condition leading European research on ozone to leap ahead, giving a boost to secondary
standards to protect vegetation.(116)
Standards help us decide how to deal best with the threats ozone poses to health (see
Extension 14.5, The 1990 Clean Air Act Amendments, NOx, particulates, and the
EPA). Research in Europe has suggested that there is a need for an ozone standard aimed
at plants and the food supply in addition to one aimed at preserving health.(116) While the
U.S. secondary standard has been reduced from 120 nmol/mol to 80 nmol/mol, the more
extensively researched European standard is 60 nmol/mol (with a limit on cumulative
exposure of 3,000 nmol/mol) in order to protect crops adequately. (116)
It is interesting that enhanced carbon dioxide levels apparently reverses some of the
damage caused by ozone. Research indicates that the carbon dioxide may close plant
pores somewhat and reduce uptake of ozone.(117)
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
Forming—and eliminating—PAN smog
PAN smog is formed because of the presence of hydrocarbon radicals (radicals are
extremely reactive chemicals) in urban atmospheres, which combine with oxygen and
nitrates produced by sunlight. Let us see how peroxyacetylnitrate may be formed in an
urban area.(118)
Methane (CH4) is formed naturally in small concentrations in the atmosphere and is
found in higher concentrations near cities. It has been found that estimates of emissions in
the southwest were seroiusly underestimated.
Methane can react with free oxygen to form hydroxyl radicals (OH) and methyl radicals
(CH3), both of which are very reactive. The free oxygen probably came from the breakup
of nitrogen dioxide (NO2) by light into nitrogen monoxide (NO) plus oxygen. The methyl
radical can react with molecular oxygen (O2) to form CH 3OO, which can again react with
an oxygen molecule to form ozone (O3) and yet another radical, CH3O; or with nitrogen
dioxide (NO2) to form CH 3OONO2, peroxyacetylnitrate.
So, ozone may be fought by eliminating methane or any organic chemical that can produce
methane or methyl radicals. While the hydroxyl radical is a constituent of smog, it helps
“eat” the smog-making chemicals and eventually eliminates them. It and ozone, for similar
reasons, are part of the system cleaning the atmosphere.(119)
Interrupting the chemical reaction chain anywhere can reduce smog formation. In the Los
Angeles region, an estimated emissions reduction of VOCs by 900 tons a day and 4,000
tons a day of carbon monoxide occurred between 1988 and 2001 because of California’s
and the South Coast Air Quality Management District’s efforts.(74) As reporter Gary
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
Polakovic of the Los Angeles Times writes, the changes have occurred because
“Californians drive the world’s cleanest cars powered by the world’s cleanest fuels, use
the world’s cleanest consumer products and have some of the world’s cleanest factories
and power plants.”(74)
Wisconsin has struggled to remove its designation as a “severe non-attainment” region
under the CAAA. As of 2001, it appears that this has been accomplished.(120) Even as it
enjoys success, it is adopting tougher rules on one-hour concentrations of ozone to
prevent growth from undermining the gains already made.
The reason that Houston has emerged as the smog capital of the United States, displacing
Los Angeles,(121,122) is not so much the increase in ozone and nitrogen oxides in Houston,
but the strides California and the South Coast have made in attacking the precursors of
smog, with VOCs the target of choice.(74,123) One study suggested that 435 Houstonians
may die prematurely every year because of its air pollution.(121) Houston has had to
adopt tough new rules to limit pollutants, including lowering interstate speed limits to 55
mi/h in the counties containing and surrounding Houston and a mandated cut of emissions
by industry of a staggering 90%.(124)
But tough regulations still don’t guarantee clean air—there are seven California cities
among the U.S. smoggiest 25 cities as identified by the American Lung Association.(106)
The California achievement is all the more remarkable, though, because more than twice as
many people live in the four-county Los Angeles metropolitan region as in 1960, and
there are three times as many cars on the roads.(74) As the people keep streaming in to
California, the problems get tougher to resolve; California’s air pollution will have to be
reduced further, and the easiest measures have already been taken. Nevertheless,
California continues to work on improving air quality.
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
The future
Even as smog is being reduced in large cities, and over entire regions such as the east and
west coasts,(125) it is growing in formerly pristine spots such as Cape Cod and the Great
Smokies.(126,127) This shows that pollution can travel great distances and still be a
problem, and that regional solutions to regional problems will have to be found. For
example, it has been reported that some 30 species of plants in the Great Smoky
Mountains National Park are dying out or have been seriously damaged because of ozone
and acid rain.(126)
Future Environmental Protection Agency actions that should reduce smog and ozone by
reducing its precursors are treated in Extension E14.5, The 1990 Clean Air Act
Amendments, NOx, particulates, and the EPA. It also explores the visibility issue in
more detail.
Another ray of hope is the development of a replacement for perchloroethylene, the
solvent used in dry cleaning that releases vapor to the air. Perchloroethylene, in addition
to being a VOC implicated in air pollution, is a groundwater contaminant and may cause
cancer and so dry cleaners must treat the solvent as hazardous waste. A chemistry
professor at the University of North Carolina, Joseph DeSimone, discovered a process
using liquid carbon dioxide and special detergents that produces cleaner clothes that smell
better and suffer less damage than those cleaned in perchloroethylene.(128)
There is hope that the cause of the decline of the hydroxyl radical in the atmosphere will
be identified,(81,83) and something done to stem it. It is possible that biogenic volatile
organic carbon emission, that is, hydrocarbon vapors emitted by trees and bushes, is
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
responsible for “eating” the missing hydroxyl radical.(129) The hydroxyl radical cleans the
atmosphere, and of course, the more polluted the air is, the more demand there is for the
hydroxyl radical to clean it. The attack on dirty air may involve unanticipated benefits.
Reductions in emissions of SO2 and VOCs mean a relative increase in UV-absorbing
aerosols such as SO2, mineral dust and soot, and such particulates apparently inhibit
smog production, so there is a positive feedback loop. (58,78,84)
The aerosols from urban areas also alter the weather downwind. For instance, rainfall is
suppressed in some regions and enhanced in others due to aerosols.(130) Urban areas also
spawn thunderstorms and lightning.(131) On a larger scale, the Asian fires that create great
clouds of haze contain carbon black particles that can change rainfall patterns over
Asia.(132) The changes in cloud cover caused by aerosols lead to what is called the
“indirect aerosol effect.”(133,134) It appears that the suppression does not occur in the
same way when polluted air travels over the ocean because sea salt can provide the
nucleus of raindrops.(135)
There is some concern that the developing countries will follow the path taken by the
industrialized countries and create high pollution levels in the pursuit of economic
development, but the problem may be more acute in those countries because of their
economic dependence on agriculture. Heightened ozone pollution will certainly injure
crops there,(116) just as it has in the U.S. Additional research is needed to help those
countries adjust, either by helping them reduce ozone levels or by helping them find
strains of crops that resist injury, or both simultaneously.(116)
As indicated in Chapter 1, recent studies predict that ozone levels near Earth’s surface
will increase substantially. This comes about from a tradeoff between the decrease in
ozone (especially in the stratosphere) caused by chlorofluorocarbons, methyl chloride,
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Energy, Ch. 14, extension 4 PAN , or photochemical, smog
and carbon tetrachloride, and the increase in ozone concentrations down at ground level
due to nitrogen oxides, carbon dioxide, and methane.(78,136) It appears that we will
continue for some time to come to lose ozone where we need it and gain it where we do
not wish it to be!
12