- 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.
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:
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Sunlight.
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The production of oxides of nitrogen (NOx).
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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.
Troposphere. The lower region of the atmosphere which is characterized by decreasing air temperatures as the altitude is increased is the troposhere. Types of pollution within the troposphere include those caused by (1) SO2 as the primary pollutant with particulates as the secondary pollutant formed from the SO2 (the well known London Killer Fog episode that occurred in the mid 20th. century being an example), and (2) NOx and organics as the primary pollutants with O3, HNO3, H2SO4, particulates and Peroxy Acetyl Nitrate (PAN) as the secondary pollutants formed by photochemical reactions from the primary pollutants (the air pollution in Los Angeles being an example of this type of pollution).
In the troposphere SOx and NOx produce their respective acids and cause the acid rain. Also the build up of CO2 and O3 in this region of the atmosphere causes the greenhouse effect (CO2 is transparent to ultra violet radiation that strike the earth's surface during the daytime but CO2 is not transparent to infra red which is reradiated by the earth's surface). Taxes are being levied on the airlines when flying over some of the European countries (e.g.. Norway) to force a reduction in these emissions, leading both Lufthansa and Swiss Air to push GE in designing jet engines with reduced NOx emissions.
A brief overview of the role played by O3 and associated species, as well as the predominant role played by the hydroxyl (OH) radical in tropospheric chemistry is presented in the following:
Photochemical Air Pollution. Under the action of sunlight (essentially in the visible region of the spectrum, noting that majority of the ultra violet radiation from the sun is blocked by the O3 present in the upper regions of earth's atmosphere), photochemical reactions occur which convert the primary pollutants such as Volatile Organics Compounds (VOCs), NO and NO2 with the ultimate formation of the secondary pollutants such as O3, PAN, HNO3, particulates (such as H2SO4, sulfates, nitrates, organics).
VOCs. VOCs are responsible for the formation of a number of secondary pollutants such as PAN and particulates and are also responsible for the formation of a major portion of the O3 where the NO concentration is low, such as in less polluted rural areas.
OH radical. The chemistry of the atmosphere is dominated by the reactive OH radical in the daytime (while the nitrate NO3 radical plays a major role in the night time), resulting in photochemical air pollution, acid rain and fogs, and the toxic organics. A major source of the OH radical is the O3 which under the action of light decomposes to generate the atomic O which in turn reacts with H2O to produce the OH radical. Photolysis of nitrous acid which is formed from NO2 also produces the OH radical.
