Thursday, 28 April 2011

What is smog?

Over the last few days, we've seen in the news how some of the cities in Britain (well, we've had pictures of London, and on the television London = Britain) have been covered in smog.  The word "smog" comes from a combination of smoke and fog, but what causes it and where does it come from?

Smog usually comes from sources like car exhaust fumes, and fumes which have reacted in sunlight to produce other pollutants (called 'secondary pollutants').  Worse still, the primary pollutants (directly from car exhaust fumes or from burning coal) can react with the secondary pollutants to produce a real mix of gases, called photochemical smog.  "Photochemical" reactions are ones that use light to make them go - in the case of smog, it's sunlight that drives the reactions.

Photochemical smog is produced by the chemical reaction of sunlight, nitrogen oxides (NOx) and other organic compounds in the atmosphere.  Smog also contains airborne particles (called particulate matter - bits of dust, ash and smoke) and ground-level ozone.  Ozone is best kept in the upper atmosphere; at ground level it's toxic (despite what you might have heard about invigorating ozone at the seaside - that's just misinformed nonsense).

Nitrogen oxides are formed when nitrogen and oxygen in the air react together under high temperature such as in the engines of cars and trucks, coal power plants, and industrial manufacturing factories.  Nitrogen and oxygen make up about 98% of the atmosphere between them, and when these are drawn into a hot internal combustion engine, they react together to product nitrogen oxide (NO) and nitrogen dioxide (NO2).  Both NO and NO2 are harmful to human health.

So, smog contains ozone (toxic) and NOx (harmful), along with dust, smoke and so on.  It builds up when the air is still - very little wind - and can occur at times when there's high air pressure.  London has suffered serious instances of smog in the past, in particular in 1952.  Work has been done to reduce smog in Britain and in Europe, in particular reducing the sulphur content of fuels - burning fuels that contain sulphur leads to the formation of sulphur oxides which contribute to smog - and also, incidentally, to acid rain.  This is why smog is generally rare in London... that, and the fact that we rarely get the still, sunny weather that's needed to produce it!

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