The environmental effects of lockdown
fuels for electricity generation and transport contributed significantly to NO x levels in the atmosphere. Estimated 2018 levels for NO X emissions were 165,100 tonnes and 258,7000 tonnes from electricity industries and road transport respectively. 7 NO x is also emitted in considerable amounts through lightning strikes and nitrogen fixing bacteria in the soil using atmospheric nitrogen to produce useful nitrogenous compounds. VOCs are organic compounds that evaporate easily at room temperature. Varying in type and structure, VOC pollution is produced in a wide range of ways: biomass (mainly wood) burning, incomplete combustion of fossil fuels and manmade for use in polishes, paints and aerosols. O ₃ precursors can be subject to long-range atmospheric transportation before they react and so ozone can be produced many kilometres away from the sources of NO X and VOC. That is why ozone is a global pollutant. 1.4 Basic summary of Ozone reactions in the troposphere Firstly, the main reactions causing net loss or gain of ozone in the troposphere involve the reaction of free radicals beginning with the photolysis of ozone (1) with ultraviolet radiation, to produce excited atomic oxygen andmolecular oxygen. Then the excited atom (i.e. one with its outer electron in a higher electron shell than ground state) can either collide with an inert gas in the atmosphere and go onto to reform ozone (2,3) – this path has neither net gain or loss of O 3 – or the species can react with water vapour in the air (4) to produce 2 hydroxyl radicals: these radicals play a large role in removing VOCs from the troposphere. A free radical is a species with one unpaired electron which makes it very reactive.
O ₃ + h v → O( 1 D) + O 2 O( 1 D) + M → O( 3 P) + M
λ ≤ 340 nm
(1)
(M = N
2 , O 2 )
(2)
O( 3 P) + O
2 + M → O ₃ + M
(M = air)
(3)
O( 1 D) + H
2 O → 2 OH
(4)
In regions of low NO x concentrations, less than 20 parts per trillion (ppt) NO X , 8 carbon monoxide in the atmosphere reacts with hydroxyl radicals (5), produced from the photolysis of ozone (1), to produce carbon dioxide and an electronically excited hydrogen atom (which has an unpaired electron and hence highly reactive). This hydrogen atom reacts with oxygen and collides with an inert molecule to release excess energy and form a peroxy radical, HO 2 (6).
CO + OH → CO 2 + H
(5)
H + O 2 + M → HO 2 + M
(6)
At the same time, VOCs can also react with hydroxyl radicals (7) and oxygen to produce another peroxy radical, signified by ROO (8). These peroxy radicals can react together to produce an organic compound with a hydroperoxide group as well as O 2 gas (9). These reactions form VOC degradation reactions which are the first few steps in removing them from the atmosphere (10).
VOC as RH (R indicating carbon chain) RH + OH → R (free radical) + H 2 O
(7)
R + O 2 → ROO
(8)
7 Department for Environment, F ood and Rural Affairs. ‘ Table: emissions of air pollutants in the UK, 1990-2014, by pollutant and major emissions source’ . (2020, February 15). https://www.gov.uk/government/statistical-data- sets/env01-emissions-of-air-pollutants. 8 Royal Society. ‘ Ground- level ozone in the 21st century: future trends, impacts and policy implications’ . (2008, October). https://royalsociety.org/-/media/Royal_Society_Content/policy/publications/2008/7925.pdf.
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