Using A Lagrangian Chemistry Transport Model To Investigate Transport and Transformation of Pollutants From The European Boundary Layer.

The transport of pollutants and pollutant precursors from the continental boundary layer to the mid/upper troposphere has been recognised for some time as being po- tentially critical for the photochemistry of this region. Rapid transport by convective systems can transport fresh boundary layer air to the UT in a matter of hours. The ozone production potential of an uplifted air mass may continue to be significant as it is transported far away from its PBL source, due to the increased lifetimes of ozone precursors in the free troposphere. An understanding of the mechanisms which trans- fer boundary layer air to the free troposphere, and the subsequent chemical and mixing processes acting on this air, is therefore vital in order to gain an understanding of the ozone budget of the UT region.

Here we use a Lagrangian chemistry transport model (CiTTyCAT) to investigate the chemistry of air masses observed during the NERC supported EXPORT (European eXport of Precursors of Ozone by long-Range Transport) and ACTO (Atmospheric Chemistry and Transport of Ozone) campaigns. The model uses 3 dimensional tra- jectories calculated from large-scale winds output from the ECMWF. The chemical model is initialised using 3D global fields from the TOMCAT CTM, and comprises 80 species and over 200 gas-phase reactions. Surface emissions and deposition pro- cesses are included along the trajectories.

The main advantage of the Lagrangian method is the ability to follow individual air masses and study the evolution of chemistry in air masses which have been uplifted by different transport events. The lack of sub-grid transport processes (convection, tur- bulent mixing) in the Lagragian advection has allowed the identification of observed air masses where these processes appear to be important. The relative importance of mixing processes has been derived from tracer correlations in the campaign data. The Lagrangian model allows mixing to be investigateded in a controlled fashion and its

1 effects on the chemistry can be directly studied. Here, we present examples of these types of study using data taken on board the UK Met Office C130 aircraft flown during the two measurement campaigns.

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