Photo chemical smog forms in high pressure conditions in the lower troposphere. These conditions also produce still air so there is little mixing between high altitude air. As a result of this, the smog gets trapped near the ground where wind often transports it to rural areas.
One way of forming Ozone is when an oxygen molecule reacts with an Oxygen radical:
O2 + O → O3
Oxygen radicals are formed when sunlight energy is absorbed by NO2 molecules:
hv
NO2 → NO + O
So a lot of sunlight is a favourable condition for forming photochemical smog. This explains why photochemical smog is generally a summer event in the UK.
O3 is one of the ingredients that react to produce photochemical smog but its concentration increases during the formation:
Photochemical smog formation
One O3 molecule ---→ At least two O3 Molecules
This is because of OH radicals involved in the breakdown of Hydrocarbons; they remove H atoms from molecules E.g. in the breakdown of Hydrocarbon.
RCH3 + OH → RCH2 + H2O
RCH2 + O2 → RCH2O2
RCH2O2 then reacts with NO:
RCH2O2 + NO → RCH2O + NO2
This reaction has converted NO back into NO2 which can be used in the reaction that forms O radicals that form O3.
hv
NO2 → NO + O
O2 + O → O3
The OH radicals are formed from the following reactions:
hv
O3 → O2 + O
O + H2O → 2OH
So because one ozone molecule reacts to form two OH molecules, the ozone concentration increases during the production of photochemical smog.5
At Logannet the BPEO chosen to reduce SOX emissions is the sea scrubbing process.
Flue gases are passed through sea water which naturally has a pH of about 7.5 the sulphur dioxide dissolves to form sulphite ions.
The water is aerated to oxidise these further to sulphate ions which are better for disposing of back into the sea.
The pH usually changes to about 6 but when it is returned to the sea the change is negligible.
Logannet found this process the BPEO because there are no solid wastes produced, no by-products to be handled and the sea water needed is very available because logannet is situated on the bank of the firth of forth.
The BPEO chosen to reduce NOX emissions at logannet is the most recent development.
There are 3 burning zones in the boiler furnace.
Primary combustion zone
Powdered coal is oxidised in less air than is normally used so the combustion rate is lower and less NOx is formed in a given time.
Reburning zone
Natural gas is injected into the boiler. The NOx reacts with the alkanes to produce Nitrogen, Carbon dioxide and water vapour.
CH4 (g) + 4NO (g) → 2N2 (g) + CO2 (g) + 2H2O (g)
Burning zone
Any excess alkane or CO produced is oxidised. 7
Research is still taking place to explain the complex chemistry involved in photochemical smog formation
Chemists are monitoring tropospheric pollutants to find what pollutants are present and how their concentrations vary. There are several monitoring stations across the country.
Individual reactions are also being studied in laboratories to find out what reactions can take place and how quickly they occur much of this work is done in computer simulation studies to reproduce the behaviour of pollutants in a photochemical episode.
We can also do laboratory experiments of a larger scale in smog chamber simulations.
Primary pollutants are mixed together in a clear plastic bag called a smog chamber and exposed to sunlight. Probes monitor the concentration of various species as the smog builds up.8
