12 Photochemical Smog and Classical Smog

Prof. K.S. Gupta

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Contents

  1. Introduction
  2. London Smog/Classical Smog
  3. Chemistry of London Smog
  4. Adverse Health Effects of Fog
  5. Photochemical Smog/Los Angeles Smog
  6. The Photochemical Smog Precursors and their Sources
  7. Spatial and Temporal Variation in Ozone Concentration
  8. Peroxyacetyl Nitrate (PAN)
  9. Meteorological and Topological Factors in Photochemical Smog formation
  10. Measurement of Pans
  11. Effects of Photochemical smog
  12. Effects of PANs
  13. References

Introduction

   The worst form of air pollution appears as smog. Primarily, smog is an urban pollution phenomenon. The word smog is formed by combining words – smoke and fog. Smog are of two types, classical or London Smog, and Photochemical Smog. Both smog comprise toxic pollutants and are reason of high mortality in areas, where such smog are common feature.

 

London Smog/Classical Smog

Historical Perspective

   Peter Brimblecombe (1987), an authority on environmental science and till recently editor of celebrated journal, Atmospheric Environment, in his book on the London smog has recounted its history. The city of London has the history of largest number of coal stoves, foul air and foggy weather. The burning of the coal in London began in thirteenth century. Consequently, the city was engulfed by foul black smoke. Mounds of coal were seen everywhere – streets, alleys or docks. Coal combustion was thought then to be the cause of phenomenon of air pollution possibly. Moses Maimonides(1135 – 1204) is credited with providing one of the earliest recorded description of air pollution. He described the air to be ‘—-stagnant, turbid, thick, misty and foggy—- because of ‘the height of buildings, the narrowness of the streets, and all that pours forth from its inhabitants and their superfluities—‘. Later in fourteenth century, the air in London became so polluted that King Edward I banned the burning of the coal, whenever English Parliament was in session. John Evelyn has described vividly the severity of the urban air pollution problem. Evelyn was engaged by King Charles II to examine the London’s air quality and its effect on public health. His report titled Fumifugium described ‘the smoke to be horrid, obstructed the Church and made the Palace to look old, fouled the clothes, corrupted the water, and the rain that precipitated was black, quality was tenacious and contaminated’. The children suffered the most. Air was so foul that nearly half of the children that were born in London died within two years. He was emphatic in concluding that the phenomenon was primarily urban and rural areas remained affected. He suggested the coal tar and other combustion products to be responsible for sweeps’ deaths. During 15th century, blackening of the city’s sky and foggy weather were a regular feature.

 

   This kind of smog was not limited to London only but was a recurrent feature of many cities of the world. Many mega-cities including from India suffer from smog. The severe smoggy pollution is a common feature in Beijing, Mexico, Shanghai, Calcutta, Mumbai, Delhi, etc.

 

   Chimneys of a zinc smelter belched so much toxic sulfur dioxide, carbon monoxide, metal dust etc that in Donora, Pennsylvania, on Oct. 26, 1948 a smog similar to London smog appeared. The smog continued for five days, resulting in ~7000 deaths and thousands suffering from respiratory diseases.

 

Beijing woke up to first ever red- alert for smog on Dec 8, 2015. The air pollution was at its worst.

 

How bad the London Smog was?

   The extent of the severity of smog can be guessed from the eye- witness account of Prof. P. V. Hobb, a well – known atmospheric chemist. He wrote that when he was a boy, ‘in London in 1950s, smog were often so thick that visibility was restricted to few feet and buses had to be guided by pedestrians’. According to Peter Brimblecombe (1987), ‘disastrous smog struck in Dec. 1952, the Sun remained unseen, days became murky shadowed nights and the residents suffered huge health toll, and 4,703 people died within a week’. It is known as Great Smog of ’52 or Big Smoke.

 

Who were the culprits, how they acted?

The culprits identified without doubt were the following.

1. The more than one million chimneys were the culprit. These burned dirty coal and belched continuously an endless stream of copious black smoke, sulfur dioxide and other pollutants.

2. The air was cool and still with dense fog. These meteorological conditions did not allow the dispersal of pollutants, which remained confined in the city. Since these were heavier, these fell down to the ground and reduced the visibility. This disastrous situation continued for about a week.

3.December 5, 1952 had a clear sky in the London city. Wind speed was low and the air near the ground was moist. These conditions were suitable for fog formation. When the moist air descended, it touched the cold ground. Water vapors cooled to dew -point and condensation occurred and dense fog formed. There was temperature inversion above 100-200 m disallowing the dispersion of pollutants. In the atmosphere, below the inversion point more than a million chimneys emitted the smoke along with sulfur dioxide, smoke particles, fluorine compounds, carbon dioxide. SO2 changed into sulfuric acid.

4. Whereas the Government failed to recognize the reason for smog, the public could see the real reason for smog formation first, and blamed the burning of the dirty coal as the reason for the grim catastrophic situation. However, the focus of Government of the day was on its grim economic reality under the garb of the arguments that ‘an enormous number of broad economic considerations have to be taken into account’. It tried to scuttle the issue by permitting the issuance of smog masks and by appointing a committee. Ultimately, in 1954, the committee came out with its recommendations and, in 1956 a new law came in to force, which mandated reduction in coal burning.

 

Chemistry of London Smog

   Brimblecombe et al. (2004) have described the chemistry of London smog. The basic features of smog formation are as follows.

Fuel combustion

   The coal contains organic components in the form of chemical compounds from carbon, hydrogen, oxygen, sulfur, and nitrogen, etc. The mineral species that occur in coal comprise carbonates, sulfides, sulfates and hydrous clay minerals. Ash is made from residues left after decomposition of silicate, carbonates and sulfides.

   Normally, the expected product of complete oxidation of hydrocarbons and carbon particles is carbon dioxide as in Eqs.( 1-2). This requires at least stoichiometric amount of oxygen.

 

   When oxygen is insufficient, toxic carbon monoxide, a partly oxidized product of hydrocarbon or carbon, forms along with CO2 as in Eqs. (3- 5).

   Under the conditions of relatively low oxygen and low temperature, the pyrolysis of fuels may occur leading to the formation of decomposition products. Subsequently, the arrangement of atoms may eventually form toxic polycyclic aromatic compounds.

 

Sulfur Oxidation

All fossil fuels inherently contain sulfur, the amounts of which may vary from about 0.2 –

7.0% in coal to about 0.3 – 0.9 % in diesel fuel and in traces in wood. The sulfur is oxidized into SO2.

   If the pyrites ores, which are sulfur based, such as those of iron, zinc, copper, etc., are associated with as impurities in coal, these will also undergo oxidation along with coal releasing SO2 in the atmosphere. They are also source of aerosols in the air.

 

Formation of sulfuric Acid, H2SO4

Interesting chemistry occurs in fog droplets. SO2 dissolves in atmospheric water and forms hydrogen sulfite under the prevailing pH conditions:

   Traces of metal ion contaminants particularly iron and manganese, which are well- known to catalyze the oxidation of dissolved sulfur dioxide by oxygen, are always present in water. Since all ingredients necessary to form H2SO4 are present, the following oxidation reaction(11) takes place.

Sulfuric acid due its high absorption capacity for water, helps the droplets to grow and consequently, the fog thickens.

In sum, the necessary ingredients and the conditions for the development of classical smog are as follows.

  1. Primary pollutants- sulfur dioxide and soot particles
  2. Secondary pollutants – H2SO4, sulfate aerosols, etc
  3. Radiation temperature inversion
  4. High relative humidity and usually foggy
  5. Low temperature (< 35oF)

Air pollution is the maximum in the early morning.

 

Adverse Health Effects of Fog

Some of the adverse effects are as follows.

  1. Respiratory diseases, breathing problems, coughing and puffing are most common effects.
  2. Those having pre-existing heart and respiratory problems are highly susceptible to adverse effects. The elderly are in general worst sufferers.
  3. The sulfur dioxide is a known irritant and the presence of soot particles has a synergistic effect in accentuating harmful smog effects.
  4. The tar, polycyclic aromatic hydrocarbons and many other inorganic and organic compounds are potentially toxic and have serious harmful effects.

How to Prevent London Smog

   The simplest solution is to ban the use of coal – combustion as a source of energy in urban areas. That is easier said than done. There must be a cost-effective alternative source of energy to replace coal. On the other hand, the burning of fossil fuels like gasoline in place of coal can create atmospheric pollution problem of photochemical smog.

 

Photochemical Smog/Los Angeles Smog

 

What is Photochemical Smog?

   Photochemical smog is a form of air pollution that is caused by the reaction of sunlight with other pollutants such as hydrocarbons and nitrogen oxides under certain meteorological conditions. Photochemical smog is characterized by haze, ozone formation, eye irritation and damage to vegetation. It may be seen as brown haze over the skylines of metros/megacities and near factories. The photos of Beijing shrouded in a veil of thick smog are common.

 

Historical Perspective

   The initial signs of photochemical smog started appearing in 1940s. During World War II there was massive wartime immigration to Los Angeles(LA). LA was the largest car market in the US. There was tremendous industrial activity and huge influx of cars. More than million cars plied in the city at that time. Diesel buses had just made their appearance in 1940. The automobiles and the industries spewed the smoke relentlessly. The city started witnessing the smaller bouts of fog. However, the first disastrous attack of big smog was on July 26, 1943. It was the time of World War II and this led LA residents to suspect the chemical warfare attack by Japanese. Later on, it was found that attackers were not outsiders but their own vehicles and factories.

 

   After the first attach, the Southern California Gas Company’s Plant, which manufactured butadiene, an ingredient in synthetic rubber, was held responsible for the source of the thick cloud. The factory was temporarily shut down but without any improvement in air pollution and smog. By 1950s, the scientists identified the cars that Californians loved to be real culprit. The fumes that these cars spewed were the real reason for smog problem.

 

   Arie Haagen-Smit, a chemist at the California Institute of Technology, solved the smog-mystery. He identified ozone to be the main source of haze. The partially unburnt exhaust from automobiles and hydrocarbons in sunlight reacted to form ozone. Interestingly, the public so enamored of cars that at first it did not believe the cars to be the culprit. Ultimately, the scientific evidence became so overwhelming that they had to believe. In 50s and 60s, parts of LA witnessed thick smog up to 200 days a year. That was something shocking. Because of the brown-and-orange haze, the mountains around LA were hardly visible.

 

The Photochemical Smog Precursors and their Source

Primary Precursors

   The primary smog precursors are NOX and volatile organic compounds(VOCs). The automobile exhausts are the principle source of these precursors. At the high temperature of fossil – combustion in motor vehicles and power plants, atmospheric N2 and O2 react to form NO.

   In addition, there are natural sources of nitrogen oxides such as lightning, microbial processes in soil, and forest fires. The petrol/diesel contain hundreds of different kinds of alkanes, alkenes, and aromatic hydrocarbons, etc., which are released in the atmosphere. Evaporation of naturally occurring compounds, such as terpenes, is an important source of VOCs. The evaporation of solvents, petrol and other organics is an important manmade source of VOCs.

 

Secondary Precursors

   Secondary precursors are ozone, PAN, organics, acids and aerosols. The most notorious secondary pollutant is ozone. In presence of sunlight, the dissociation of NO2 occurs and ground state oxygen atom, O(3P), is formed. The further reactions of O(3P) generate ozone as in reactions (13-14).

   The concentration of O3 is governed by the ratio [NO2]/[NO]. The reactions(13 – 15) cannot explain the observed high [O3] in smog. To account for the high concentration of ozone, there must be some pathways to convert NO into NO2 without involving O3. The active VOCs were found to be involved in oxidizing NO in to NO2. This is achieved by the intervention of reactive hydrocarbons, out of which alkenes due to presence of double bond are most reactive

Fig . 1 The diurnal variation in the concentration of non-methane hydrocarbons, NO, NO2, aldehydes and ozone

The photodissociation of ozone generate singlet oxygen atom, O(1D), which reacts with H2O to generate hydroxyl, OH, radicals.

Equation (26) regenerates OH and the cycle restarts.

   Another pollutant carbon monoxide generated by partial oxidation of carbon and by other reactions also produces HO2 radical, which oxidizes NO.

Spatial and Temporal Variation in Ozone Concentration

   We saw how ozone is formed and how its concentration depends on [NO2]/[NO] ratio(16). We also noticed the role of VOCs in keeping this ratio high and consequently in the production of high amount of O3.The O3 concentration undergoes diurnal variation as shown in Fig. 1. The concentration of each of NO, NO2, VOCs, solar intensity and O3 are interdependent. Since early morning, as the day progresses, the traffic increases and so is the emission of primary pollutants. At the same time, the solar intensity also increases. With increase in latter, the rate of photodissociation of NO2 increases. The rates of formation of oxygen atoms and hydroxyl radicals accelerate and with the support of VOCs and its by-products, the concentration of ozone increases. In urban areas, the peak ozone concentration stays for about 1-2 h in afternoon, which is the time of most intense solar radiation. As the sun goes down, the formation of ozone stops. The remaining atmospheric ozone is then consumed by several different reactions.

Fig. 2. Formation of photochemical smog

   Besides solar intensity, two main chemical precursors on which the formation of ozone depends are NOx (NO + NO2) and VOCs. Under certain conditions, formation of ozone becomes NOx-sensitive as it is controlled almost exclusively by NOx. NOx-sensitive conditions are associated with high VOC/NOx ratios. The presence of high fraction of more reactive, usually biogenic, VOCs is associated with NOx-sensitive condition. VOC-sensitive conditions are associated with low VOC/NOx ratios.

 

   In continental areas far away from direct anthropogenic effects, ozone concentrations are generally low in the range of 20 – 40 ppb. In urban and suburban areas, ozone concentrations can be high, well over 100 ppb, In Delhi, the value lies in the range 30 – 70 ppb. In rural areas downwind of urban centers, ozone concentrations are high.

 

   Because of oxidizing nature of ozone, photochemical smog is oxidizing in nature. On the other hand, London or classical smog is reducing smog due to easy oxidation of SO2.

 

 

Peroxyacetyl Nitrate (PAN)

   The compound, CH3COO2NO2 is known as peroxyacetyl nitrate or PAN. There is a homologous series of such compounds known as acyl peroxy nitrate(APN) compounds, having the molecular formula, CxHyOO2NO2. Peroxyacetyl nitrate(PAN) is the most abundant member of the series of PANs.

 

Structure of PAN

The general chemical structure of PANs is:

On replacing R by CH3 we get peroxyacetyl nitrate.

 

Formation of PANs

   These compounds are formed in troposphere by photochemical oxidation of aldehydes and ketones in the presence of NOx. The carbonyl compounds are themselves formed by the oxidation of hydrocarbons as shown in Eqs. (29-32 ) and (33 – 36).

   It is believed to be important in the upper troposphere HOx (OH + HO2) budget. Its photolysis results in the formation of PAN, which is a marker of coupled NOx/organic chemistry and plays a key role in O3 production.

 

Secondary Aerosols

   Secondary aerosols are pollutants present in smog. Large size organic compounds present in atmosphere on oxidation yields secondary aerosols. Smog aerosols are formed by the reactions of hydrocarbons with pollutants and tropospheric ozone. Automobile exhausts emit carbonaceous soot particles comprising partly oily organics and polycyclic aromatic compounds. These are all toxic.

 

Meteorological and Topological Factors in Photochemical Smog formation

The formation of photochemical smog is driven by certain atmospheric and topological conditions such as:

  1. Densely populated area, high traffic density and congestion
  2. Sunny weather
  3. Cool and low wind speed, stagnant and stable meteorological situation
  4. .Temperature inversion

   Temperature inversion is responsible for the trapping of precursor gases. It plays a prime role in making a photochemical smog episode severe, Temperature inversion occurs when a warm layer of air overlies cool air adjacent to ground level. As a result, the atmospheric mixing and, therefore, the vertical dispersion of pollutants is poor. Due to this, pollutants are trapped near the Earth’s surface. Inversions can last from a few days to several weeks.

 

   Actually, topography of a city is responsible for the severity of a smog episode. Cities situated in valleys are highly susceptible to photochemical smog. In a city encircled by hills and mountains, the flow of air is reduced allowing the pollutant concentrations to build up. In addition, valleys are sensitive to photochemical smog because the pollutants remain trapped due to relatively low wind speed. Such areas are susceptible to strong temperature inversions, which enhance the severity of smog further.

 

   Photochemical smog was first observed in Los Angeles because the conditions for its formation existed. Los Angeles basin is surrounded by mountains from three sides and the fourth south-west side is open to ocean. The prevailing sea – winds acts as a fourth wall. Thus, basin becomes a giant chemical flask and the frequent temperature inversions acts as a lid over this flask(Spedding, 1974). The huge number of automobiles release enormous amount of primary pollutants(NOx and VOCs). Solar radiation provides the energy and with all the required conditions in place, strong smog develops.

 

   Cities like Los Angeles, New York, Sydney, Tehran, Delhi, Kolkata, Mumbai, Vancouver, etc. frequently encounter photochemical smog

   Two meteorological conditions, which alleviate the photochemical smog, are rainfall and high wind speed. The former washes out the pollutants and second blows the smog away.

 

Measurement of PANs

There are several techniques one of which, chemical ionization mass spectrometry, is mentioned briefly here. In this technique, the PAN undergoes thermal dissociation to NO2 and RC(O)O2. The latter is reacted with iodide ions, I-, to form anions RC(O)O–, which are detected by quadrupole mass spectrometer. The principal allows the identification of the individual PAN.

 

Control Measures

  1. Reduction of NOx: Use of catalytic converter fitted vehicles, lowering the combustion temperature in engines with the help of two-stage combustion, flue gas circulation, water injection and modification of burner.
  2. Reduction of VOCs: Use of cleaner burning fuels such as liquefied petroleum gas(LPG), or compressed natural gas(CNG) in automobiles. Use of renewable energy sources such as hydrogen, electricity, solar power, and even water in place of petroleum fuels
  3. Reduction in Solvent Evaporation: Attempts to reduction in solvent evaporation are necessary.
  4. Exhaust Emissions: Stringent legal limits on the emission of NOx, carbon dioxide, and sulfur dioxide, smoke.

Harmful Effects

Effects of Photochemical smog

1. Effect on Vegetation: The constituents of photochemical smog, viz., NOx, ozone, and PAN harm plants by reducing photosynthesis. Even small quantities of ozone are harmful to plants

2. Effect on Human Health: NOx contribute to damage to lungs and heart and decrease resistance to infection. VOCs cause eye- irritation, respiratory diseases; some VOCs are carcinogens. PAN causes eye-irritation and activate respiratory problems. Ozone induces coughing, wheezing, eye-irritation, asthma and other respiratory problems.

3. Effect on Materials: Ozone being an oxidant damages many materials. It cracks rubber, reduces tensile strength of the textiles, accelerates fading of dyed clothes and discolors painted surfaces.

 

Effects of PANs

   They are toxic, irritating and lachrymator (tear gas). Even small concentrations of the order offew ppb cause eye – irritation. Their high concentrations damage vegetation. PANs. Their chloro-derivatives are suspected to be mutagenic.

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References

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  13. Colin Baird (1998), Environmental Chemistry, W.H. Freeman, New York.
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  18. Daniel J. Jacob , THE OXIDIZING POWER OF THE ATMOSPHERE: Available on internet.
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  20. Farrah Quereshi, Peroxyacetyl Nitrates: Ozone in the cooling PAN; atoc.colorado.edu/~toohey/PAN.pdf

http://www.eolss.net/sample-chapters/c06/e6-13-02-08.pdf www.epa.sa.gov.au/files/8238_info_photosmog.pdf

http://www.fraqmd.org/OzoneChemistry.htm http://www.arb.ca.gov/research/apr/past/atmospheric.htm#Projects http://www.ehow.com/facts_5990313_definition-photochemical-smog.html