Untangling the influence of air-mass origin in interpreting composition trends

The UK Met office's NAME atmospheric dispersion model has been used to develop station footprints for the Weybourne and Cape Verde Observatories, as well as two central London atmospheric stations to be used in the CleARFLo project. Back trajectory output was captured every 3 hours for several years, tracking the air mass origin for 10 days backwards in time. Monthly integrated footprints of these outputs showed a distinct seasonal variation of the air mass origins and pathways. The most common air mass pathways for each station were defined and each output was assigned to one of these trajectory types. Classification of the pathways of the air masses reaching the measurement stations at any given time helps to link short-term changes in atmospheric composition with local pollution sources and long range transport from various areas around the stations. Long term and seasonal trends can be extracted from this and changes in the ratios and concentrations of these trace gases can be studied in a consistent manner. This classification and generation of a series of footprints can be used a resource for users of the Weybourne and Cape Verde observatories and for analysis in the Clean Air For London (CleArFLo) project.