The contribution of diesel exhaust to summer smog has been a topic of hot debate over the last six years.
Not all air pollution comes directly from sources such as traffic, heating and industry. Pollutants are also created from chemical reactions in the atmosphere. This is especially true in summer when stronger sunlight provides extra energy to drive the chemistry. During this July’s hot weather, air drifted over France and the UK, accumulating pollutants that then reacted in the sunshine. The resulting summertime smog reached six on the UK government’s ten point warning scale.
Many thousands of different types of pollutant gases and particles can be formed under these conditions. Exhaust from petrol cars played a major role in creating the summer smog that plagued Californian cities in the 1950s and the UK in the 1970s and 1980s. Our traffic has changed a lot since this time. Petrol exhausts have been controlled by catalysts but we now have much more diesel traffic.
Four years ago studies around US cities pointed to particle pollution being created by chemical reactions from petrol exhaust, but later US investigations have suggested that diesel is the culprit. Attention has now focused on highly chemically reactive hydrocarbons in diesel exhaust that react to produce other pollutants. These are hard to measure, but latest estimates in London’s air show that some emissions are over 70 times more than expected and that these gases are not removed by exhaust technologies. Vehicle manufactures need to address these to eliminate the health-harm from the diesel vehicles that they sell.
