Professor Nick Hewitt and Dr Emily House in LEC have been contracted by consultants Ricardo-AEA plc, on behalf of Defra, to review how the emissions of reactive volatile organic compounds from the biosphere are treated in models of air quality in the UK, and to advise on how they should be included in the future. These compounds, some of which we are familiar with as the characteristic smells of woodlands, meadows and cut grass, play a central role in the chemistry of the lower atmosphere, influencing the formation of ozone and small particles, both of which are important air pollutants. Modellers of air quality need to include the emissions of these compounds in their calculations, but there are considerable variations in the way this is done, resulting in differences in their predictions of ozone and particles. In their report Professor Hewitt and Dr House suggest that more consistency is needed in this area and make suggestions for best practice. In particular, this will allow the different models of ground level ozone pollution used by Defra contractors to be inter-compared on the basis of their treatment of atmospheric chemical reactions, rather than on their treatment of biogenic emission rates.