Guardian: 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 Julys hot weather, air drifted over France and the UK, accumulating pollutants that then reacted in the sunshine. The...