Clearing the Air: The Beginning and the End of Air Pollution

by Tim Smedley

PM2.5 is mostly secondary. A big one is sulphur dioxide emitted from burning coal or in some cases high-sulphur diesel … That sulphur oxidises all the way to sulphuric acid and condenses into the particle sulphate. Most sulphate is secondary, the vast majority I’d say.’ I wasn’t expecting that response. It’s one worth bearing in mind whenever anyone talks about filters for trapping particulate matter, for car engines or anywhere else – filters may be able to trap solid particles, but they can do nothing about the ones that are emitted as gases and turn into particles later on. Ammonia gas, for example, reacts with NOx gases in the atmosphere to form solid particles such as ammonium nitrate. Ammonia may be a gas, but it contributes some 10 to 20 per cent of the PM2.5 mass in Europe through secondary PM formation, even in urban areas: the London haze in March 2014, when my daughter was born, was formed from ammonia released on farms mixing with the NOx from London traffic to form ammonium nitrate particles. Paul Agnew at the UK Met Office tells me, ‘When we have a bad air quality episode … it’s generally due to a big rise in the formation and transport of secondary PM2.5, and in particular ammonium nitrate … it’s a very effective way of producing fine particulates.’

PM2.5 is mostly secondary