Major Chemical Pollutants in Photochemical Smog.
Major Chemical Pollutants in Photochemical Smog:
Sources and Environmental Effects
This seems like a simple question but it has a rather in depth answer so I’ll give you a concise answer and then some references that you can look at to find out more detail if you need it.
Photochemical smog is the atmospheric haze that is found near many large cities and it is due to the action of sunlight on the hydrocarbons coming mainly from vegetable matter and the nitrogen oxides emittedby factories and car exhausts.
Well, that is the dictionary definition but it doesn’t really say much about how or why the smog occurs.
For photochemical smog to occur there are four necessary conditions:
- nitrogen oxides (NOx)
- temperatures above about 18°C
The chemical reactions that are important are:
NO2 -----(energy form sunlight)----> NO + O (1)
O + O2 -----------------> O3 (2)
NO + O3 ------------>NO2 + O2
O3 --------(energy)---------> O2 + O (3)
O + H2O ------------------> 2OH (4)
(1) means that in the presence of sunlight nitrogen dioxide splits into nitrogen oxide and oxygen.
(2) means that oxygen spontaneously combines to form ozone, but then ozone is also decomposing back into oxygen (3).
(4) means that oxygen and water in the air are combining together to form hydroxl radicals.
However, things are not as simple as they seem. These reactions are ongoing and involve the production of many different intermediaries and by products such as aldehydes and organic nitrates.
These reactions can be used to explain how photochemical smog is formed during the morning rush hour.
Car emissions cause the levels of unburnt hydrocarbons (from the petrol) and NO to rise. The sun oxidises (converts by adding oxygen) the NO to NO2. NO2 is photolysed in sunlight which just means that the molecule breaks down to NO again. The production of NO is the first step in the production of ozone, the spare O can join up with the O2 molecules that occur naturally in the air. This means that the levels of ozone start to increase. The photolysis of the ozone leads to the production of OH. These OH molecules then react with other emissions form cars exhausts such as the unburnt hydrocarbons to form by products such as aldehydes and organic nitrates. These compounds form the irritating components found in the smog. (The yellow colour comes from the NO2).
All four of the conditions described above have to be present for smog to be produces and because of this, its occurence is quite predictable. Cars provide the NOx and the unburnt hydrocarbons so photochemical smog tends to be a city phenomenon. Sunlight and relatively high temperatures are needed so smog occurs throughout the year in places such as Los Angeles but only during the summer in places like London.
Geographical features that inhibit the dispersal of the pollutants also contribute to the prolonged presence of smog.
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injected into the atmosphere directly..... examples include:
- carbon monoxide (CO)
- odorless, colorless, poisonous gas
- created by incomplete combustion (especially bad with older cars)
- generates headaches, drowsiness, fatigue, can result in death
- oxides of nitrogen (NOx, NO)
- NO - nitric oxide
- emitted directly by autos, industry
- sulfur oxides (SOx)
- SO2 - sulfur dioxide
- produced largely through coal burning
- responsible for acid rain problem
- volatile organic compounds (VOCs)
- highly reactive organic compounds
- release through incomplete combustion and industrial sources
- particulate matter (dust, ash, salt particles)
- bad for your lungs
Certain conditions are required for the formation of photochemical smog. These conditions include:
1. A source of nitrogen oxides and volatile organic compounds. High concentrations of these two substances are associated with industrialization and transportation. Industrialization and transportation create these pollutants through fossil fuel combustion.
2. The time of day is a very important factor in the amount of photochemical smog present. The following illustrates the daily variation in the key chemical players. The diagram suggests:
- Early morning traffic increases the emissions of both nitrogen oxides and VOCs as people drive to work.
- Later in the morning, traffic dies down and the nitrogen oxides and volatile organic compounds begin to be react forming nitrogen dioxide, increasing its concentration.
- As the sunlight becomes more intense later in the day, nitrogen dioxide is broken down and its by-products form increasing concentrations of ozone.
- At the same time, some of the nitrogen dioxide can react with the volatile organic compounds to produce toxic chemicals such as PAN.
- As the sun goes down, the production of ozone is halted. The ozone that remains in the atmosphere is then consumed by several different reactions.
3. Several meteorological factors can influence the formation of photochemical smog. These conditions include:
- Precipitation can alleviate photochemical smog as the pollutants are washed out of the atmosphere with the rainfall.
- Winds can blow photochemical smog away replacing it with fresh air. However, problems may arise in distant areas that receive the pollution.
- Temperature inversions can enhance the severity of a photochemical smog episode. Normally, during the day the air near the surface is heated and as it warms it rises, carrying the pollutants with it to higher elevations. However, if a temperature inversion develops pollutants can be trapped near the Earth's surface. Temperature inversions cause the reduction of atmospheric mixing and therefore reduce the vertical dispersion of pollutants. Inversions can last from a few days to several weeks.
4. Topography is another important factor influencing how severe a smog event can become. Communities situated in valleys are more susceptible to photochemical smog because hills and mountains surrounding them tend to reduce the air flow, allowing for pollutant concentrations to rise. In addition, valleys are sensitive to photochemical smog because relatively strong temperature inversions can frequently develop in these areas.
(c). Chemistry of Photochemical Smog
The previous section suggested that the development of photochemical smog is primarily determined by an abundance of nitrogen oxides and volatile organic compounds in the atmosphere and the presence of particular environmental conditions. To begin the chemical process of photochemical smog development the following conditions must occur:
- The production of oxides of nitrogen (NOx).
- The production of volatile organic compounds (VOCs).
- Temperatures greater than 18 degrees Celsius.
If the above criteria are met, several reactions will occur producing the toxic chemical constituents of photochemical smog. The following discussion outlines the processes required for the formation of two most dominant toxic components: ozone (O3) and peroxyacetyl nitrate (PAN). Note the symbol R represents a hydrocarbon (a molecule composed of carbon, hydrogen and other atoms) which is primarily created from volatile organic compounds.
Nitrogen dioxide can be formed by one of the following reactions. Notice that the nitrogen oxide (NO) acts to remove ozone (O3) from the atmosphere and this mechanism occurs naturally in an unpolluted atmosphere.
O3 + NO »»» NO2 + O2
NO + RO2 »»» NO2 + other products
Sunlight can break down nitrogen dioxide (NO2) back into nitrogen oxide (NO).
NO2 + sunlight »»» NO + O
The atomic oxygen (O) formed in the above reaction then reacts with one of the abundant oxygen molecules (which makes up 20.94 % of the atmosphere) producing ozone (O3).
O + O2 »»» O3
Nitrogen dioxide (NO2) can also react with radicals produced from volatile organic compounds in a series of reactions to form toxic products such as peroxyacetyl nitrates (PAN).
NO2 + R »»» products such as PAN
It should be noted that ozone can be produced naturally in an unpolluted atmosphere. However, it is consumed by nitrogen oxide as illustrated in the first reaction. The introduction of volatile organic compounds results in an alternative pathway for the nitrogen oxide, still forming nitrogen dioxide but not consuming the ozone, and therefore ozone concentrations can be elevated to toxic levels.
Conventional burn coal, oil or gas to produce electricity. also burn fossil fuel in the form of petrol or diesel, products refined from oil. Coal, oil and gas are called fossil fuels because they form over millions of years through the decay, burial and compaction of rotting vegetation on land (coal), and marine organisms on the sea floor (oil and gas). Burning fossil fuels in this way releases a number of , including , , , and , such as hydrocarbons, which can all lead to .
Coal is a solid fuel formed over millions of years by the decay of land vegetation. Over time, successive layers become buried, compacted and heated, a process through which the deposits are turned into coal. Coal is widely used in the generation of electricity in power stations because it is a highly concentrated energy source. However, it is not a particularly "clean" fuel, releasing more sulphur dioxide than either oil or gas. Coal was the first fossil fuel to be exploited on a large scale during the 19th century with the beginning of the Industrial Revolution. Before the commercial introduction of electricity, coal was primarily used in industrial boilers to create steam energy to power machinery.
- form in the atmosphere through chemical and photochemical reactions from the primary pollutants
- examples include:
- sulfuric acid H2SO4
- can cause respiratory problems
- nitrogen dioxide NO2
- ozone O3
- colorless gas
- has a sweet smell
- is an oxidizing agent - lung tissue to rubber products
- irritates the eyes
The chemical reactions that result in photochemical smog occur all the time. It is only when hydrocarbon and NOx levels are heightened when they become toxic or accelerate smog conditions. These reactions allow the atmosphere to clean itself. The hydroxyl radical is the cleaning agent in the polluted atmosphere that reduces the hydrocarbon population by reacting with them. The majority of the hydrocarbons are converted into water and CO2, while the remainder is converted to PAN a toxic material. PAN is eventually destroyed but it will persist in the environment.
Conditions for photochemical smog usually occur during early morning commuter traffic when the environmentally damaging automobile emissions contribute to the increased presence of hydrocarbons and NOx in the atmosphere. Sunlight converts NO to NO2 by oxidation. NO2 is photolysed (breaks down) by sunlight back to NO and also produces O radicals. This reaction occurs more quickly as the photon density increases. The additional O radicals from this reaction can react with naturally occurring O2 to produce ozone, thus increasing ground level ozone. Ozone absorbs UV light and undergoes photolysis to produce OH radicals. Ozone concentrations are able to increase producing midday ozone peaks. At nightfall, ozone is no longer produced at the Earth's surface. The remainder of the ozone is consumed by other chemical reactions. OH radicals react with car exhaust emissions such as unburnt hydrocarbons forming aldehydes and organic nitrates. Oxidized hydrocarbons react with NO and increase NO2 concentrations by midmorning. The result of these reactions and their products yields smog. As the smog matures, visibility is reduced because the light is being scattered by aerosols.
Around midday, the sunlight becomes more intense and breaks down NOx into byproducts such as photochemical oxidants that form ozone and elevate its concentration at the surface. Also, some of the NO2 reacts with volatile organic compounds present and they produce toxic chemicals including PAN (peroxyacetyl nitrate). NO2 is responsible for the brownish colour of the smog. At the surface, ozone is formed by the reaction of hydrocarbons (also known as volatile organic compounds – VOCs) and nitrogen oxides in the presence of sunlight and warm temperatures and collects over cities. Ozone reacts with other substances to form other pollutants like acrolein, formaldehyde, PAN and others. Human activities such as burning fossil fuels have enhanced the levels of natural ozone hydrocarbons, and NOx that occur naturally in the lower atmosphere. Industry also contributes to the pollution by fossil fuel combustion, transportation of goods, generating energy, etc. Sources of NOx are from two forms nitric oxide (NO) and nitrogen dioxide (NO2). The majority of NO2 emitted is from anthropogenic sources particularly vehicles and power plants that burn fossil fuels. NO2 could react with water vapour to produce nitric acid while the remaining NO2 could convert to nitric oxide and radical oxygen atoms under the influence of UV radiation. The oxygen radical is capable of reacting with oxygen to form ozone
Oxides of Nitrogen
Nitrogen dioxide (NO2) is one of a number of important oxides of nitrogen present in the atmosphere. Nitric oxide (NO) and nitrogen dioxide (together termed NOx) are the most abundant man-made oxides of nitrogen in urban areas; these are formed in all high temperature combustion processes, although NO predominates. Nitric oxide is not generally considered to be harmful to health at the concentration found in the ambient atmosphere.
For the UK as a whole, approximately 45% of all oxide of nitrogen emission originates from motor vehicles, with most of the remainder arising from power stations and other industrial sources. Since power station and industrial emissions are usually from elevated sources, motor vehicles represent by far the lowest source of low-level NOx emission and therefore make the largest contribution (about 75% or more) to long term ground level concentrations in urban areas.
Hence, the highest NOx levels in cities are observed at kerbside locations. However, since NO2 is formed from primary emissions of NO by time-dependent oxidation processes in the atmosphere, the relative decline in NO2 concentration away from the kerbside is slower than for NO.
Several surveys using diffusion tube samplers for NO2 have been undertaken to determine the distribution of background concentrations of NO2 in UK cities. These have shown that, in general, NO concentrations are greatest in central urban areas. However, this cannot be assumed to be the case: for instance, a recent study in Sheffield identified an industrial area, close to the M1 motorway, with higher NO2 concentrations than the city centre.
Sulphur Dioxide (SO2) is formed by the oxidation of sulphur impurities in fuels during combustion processes. A very high proportion (approximately 85%) of UK SO2 emissions originates from power stations and industrial sources. As the use of coal for domestic heating has decreased, SO2 emissions and atmospheric concentrations in urban areas have decreased considerably over the last 20-30 years.
Though virtually no SO2 is emitted from petrol engine vehicles, it is emitted from diesels and, as the use of these has increased, kerbside concentrations of this pollutant are now observed to be higher than at urban background locations.
Geographically, SO2 concentrations in the UK are highest in urban areas where there is still significant use of coal for domestic heating, such as mining region in the north of England and in Northern Ireland. Modelling studies have indicated that the highest SO2 concentrations in cities usually occur in the central areas.
Carbon Monoxide in urban areas results almost entirely from vehicle emissions. The emission rate for individual vehicles depends critically on vehicle speed, being highest at very low speeds.
Since CO is a primary pollutant, its ambient concentrations closely follow emissions. In urban areas, concentrations are therefore highest at the kerbside and decrease rapidly with increasing distance from the road. No detailed investigations of the spatial distribution of CO in UK urban areas have been undertaken. However, since traffic is by far the most important source of CO, its spatial distribution will follow that of traffic: this will generally result in the highest level being observed in the city centre.