Measuring the atmosphere

As well as the familiar constituents of nitrogen, oxygen, carbon dioxide and water vapour, the atmosphere contains hundreds of other gaseous constituents that are present in very low concentrations (typically parts per billion, ppb, or less; Box 7.1) but which nevertheless have an impact on the environment and human health.1,2 In addition, airborne particulate matter (PM) constitutes an important nongaseous component of the atmosphere. This encompasses any solid, liquid or mixed solid-liquid material capable of remaining suspended in the atmosphere for at least a few hours (but often for days or weeks).