Chemical processes in the troposphere are driven by a combination of factors. Emissions from the Earth’s surface, either natural or anthropogenic, are the primary source of trace gases in the troposphere. The energy driving many chemical reactions is derived from the Sun, hence the reactions are “photochemical”. Transport processes can distribute longerlived trace gases to locations remote from their origin. In this article, Dr Anna Jones, from the BritishAntarctic Survey, describes some of the highly surprising phenomena that have been discovered in the polar troposphere over the past twenty years.