Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 3 2006 10.5194/acpd-6-5057-2006 http://www.atmos-chem-phys-discuss.net/6/5057/2006/ http://www.atmos-chem-phys-discuss.net/6/5057/2006/acpd-6-5057-2006.html http://www.atmos-chem-phys-discuss.net/6/5057/2006/acpd-6-5057-2006.pdf 5057 5094 2006-06-21 Photochemical modelling in the Po basin with focus on formaldehyde and ozone L. Liu F. Flatøy C. Ordóñez G. O. Braathen C. Hak W. Junkermann S. Andreani-Aksoyoglu J. Mellqvist B. Galle A. S. H. Prévôt I. S. A. Isaksen Norwegian Institute for Air Research, Kjeller, Norway University of Oslo, Oslo, Norway Bjerknes Centre for Climate Research, University of Bergen, Bergen, Norway Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland Institute of Environmental Physics, Heidelberg, University of Heidelberg, Germany Research Centre Karlsruhe, Institute for Meteorology and Climate Research, Garmisch Partenkichen, Germany Department of Radio and Space, Chalmers University of Technology, Göteborg, Sweden As part of the EU project FORMAT (Formaldehyde as a Tracer of Oxidation in the Troposphere), a field campaign was carried out in the vicinity of Milan during the summer of 2002. Results from a 3-D regional chemical transport model (NILU RCTM) were used to interpret the observations focusing primarily on HCHO and ozone. The performance of the model was assessed by comparing model results with ground based and aircraft measurements. The model results show good agreement with surface measurements, and the model is able to reproduce the photochemical episodes during fair weather days. The comparison indicates that the model can represent well the HCHO concentrations as well as their temporal and spatial variability. The relationship between HCHO and (O₃×H₂O) was used to validate the model ability to predict the HCHO concentrations. Further analysis revealed the importance of the representativity of different instruments: in-situ concentrations might be locally enhanced by emissions, while long path measurements over a forest can be influenced by rapid formation of HCHO from isoprene. The model is able to capture the plume from the city of Milan and the modelled levels agree generally well with the aircraft measurements, although the wind fields used in the model can lead to a displacement of the ozone plume. During the campaign period, O₃ levels were seldom higher than 80 ppbv, the peak surface ozone maxima reached 90 ppbv. Those relatively low values can be explained by low emissions during the August vacation and unstable weather conditions in this period. The modelled ΔO₃/ΔNO_z slope at Alzate of 5.1 agrees well with the measured slope of 4.9.