Steve Arnold and Martyn Chipperfield
Institute for Atmospheric Science, School of Environment, University of Leeds.

Fiona O’Connor, Kathy Law and John Pyle
Centre for Atmospheric Science, University of Cambridge

John Methven
Department of Meteorology, University of Reading

Introduction
The transport of ozone and its precursors between the continental regions has been recognised for some time as a factor influencing tropospheric composition on a global scale (eg Stohl and Trickl, 1999; Jaffe et al., 1999; Wild and Akimoto, 2001). The coupling of strong precursor emissions to large-scale transport mechanisms can mean that ozone production is transported rapidly in air masses uplifted to the free troposphere to regions many hundreds of kilometres down-stream. Here, we use a Lagrangian chemistry transport model to simulate the transport and chemistry in air masses advected out of the European region during August 2000. Measurements made on board the C130 aircraft of the UK Met Office during the EXPORT (European eXport of Precursors and Ozone by long-Range Transport) campaign are used to initialise chemistry in the air masses observed, which are then advected forwards in time using analysed winds from the ECMWF.

Model Description

• The CiTTyCAT model is a zero dimensional chemical box model, which follows a 3D Lagrangian air parcel trajectory calculated from ECMWF analysed winds.
  
• The temperature, humidity, pressure and surface pressure in the box model are forced from the ECMWF analyses interpolated to the trajectory location.
  
• The chemistry model comprises a gas-phase chemistry scheme of 90 species, including full oxidation of C2-C6 hydrocarbons.
  
• Detailed anthropogenic emissions are included for UK (NAEI) and Europe (SuperEMEP). Biogenic emissions are taken from Guenther et al. (1995).
  
• Chemical initialisation taken from aircraft data, TOMCAT 3D CTM or output from back trajectory model run.

Results
Ozone production in forward trajectories is presented for two flights from the EXPORT campaign (A773 - 02/08/00 and A775 – 09/08/00). Chemistry in the trajectories was initialised in three different ways: 1) Using measurements from the C130 aircraft; 2) Using output from the TOMCAT 3D CTM interpolated to the aircraft flight path; 3) Using results from back trajectories arriving on the flight path.

Flight A773 sampled a warm conveyor belt associated with a cold front passing over the central European region. Enhancements in tracers that point to recent boundary layer history are seen throughout the WCB system and in air masses above the WCB. Inferred timescales for air mass uplift to the mid troposphere point to rapid ascent of PBL air in convective cells embedded in the frontal system (Purvis et al. (2003)). Back trajectory reconstructions of the WCB show very limited tracer enhancements compared with the measurements, demonstrating the lack of small scale vertical transport included in the resolved ECMWF winds used to drive the Lagrangian advection (Fig 1). Consequently, forward trajectories initialised from CiTTyCAT back trajectory results show ozone loss over the WCB region, except when initialised in the PBL under influence from recent precursor emissions. In contrast, the convective parameterisation included in the TOMCAT CTM gives very strong ozone precursor enhancements throughout the frontal zone and strong ozone production is seen in forward trajectories. Forward trajectories initialised from measurements show spurious strong ozone production associated with regions of precursor enhancement intercepted during the flight. Trajectories initialised in the upper levels of the WCB region tend to be transported rapidly eastwards in the UT while producing ozone (Fig. 2). Air masses that produce ozone from lower altitudes tend to be transported more slowly and remain in the PBL over Eastern Europe (Fig 3).

Fig 1: Initialisations used in forward trajectories for flight A773. Black:data; red:CiTTyCAT back traj run; blue: TOMCAT CTM.	Fig.2&3: Examples of forward trajectories leaving the A773 flight path in which ozone is produced over the subsequent 5 days.

Flight A775 sampled a wide variety of air mass types including air which appears to have been transported across the Atlantic in long-range transport from the North American PBL, and during an extended flight through the Eastern European boundary layer. A CiTTyCAT simulation using backward trajectories arriving on the flight path shows good agreement with observed tracer concentrations. This demonstrates
that the large-scale winds from the ECMWF coupled to the model emissions are sufficient for reproducing the observed tracer structures, in contrast to flight A773. However, initialisation of forward trajectories with the back trajectory output tends to give low ozone production when not recently influenced by fresh emssions, due to the rapid conversion of NOx to HNO3 over the course of the 5 day back trajectory. Forward trajectories initialised with measurements in the Eastern European PBL produce ozone, which is transported eastwards at low altitudes towards Russia. Some ozone producing air masses are entrained into a large-scale ascent out of the PBL over central Russia (Fig 4). Ozone producing air masses that are initialised at higher altitudes tend to be transported rapidly eastwards in the upper troposphere. There is also a descending flow of ozone production towards the North African boundary layer (Fig. 5). Precursors in these air masses appear to have been uplifted rapidly off the east coast of North America, the air masses again becoming photochemically active as they descend over Southern Europe. The storage of NOx in reservoir form as peroxyacetyl nitrate (PAN) during long-range transport in the UT, which is then released as the air masses descend into the warm lower troposphere over the Mediterranean, appears to play a key role in producing ozone after many days of transport.

Fig.4&5: Examples of forward trajectories leaving the A775 flight path in which ozone is produced over the subsequent 5 days

Conclusions
Ozone production in the CiTTyCAT trajectory model is highly sensitive to chemical initialisation. The use of back trajectory simulations for initialisation away from recent influence of emissions tends to give net ozone loss due to the low ozone production potential of the initial state, arising from the rapid NOx to HNO3 conversion occurring during the back trajectory run. The large-scale winds used to drive the Lagrangian advection are not sufficient to resolve the rapid vertical transport events responsible for the majority of tracer enhancements seen in a WCB associated with a cold frontal system. Ozone production from air masses sampled in the mid/upper troposphere over Europe tends to be transported rapidly eastwards in the UT. Air masses producing ozone originating in the European PBL tend to be transported slowly eastwards at low altitudes and some were seen to be entrained in large-scale ascent towards the east. There is evidence that precursors originating from the North American boundary layer may produce ozone in air masses that descend into the North African PBL from the UT over Europe. The role of PAN in storing up NOx during transport at high altitudes appears to play a key role in this process.