Articles | Volume 7, issue 1
Atmos. Chem. Phys., 7, 121–137, 2007

Special issue: Formaldehyde as a tracer of photooxidation in the troposphere...

Atmos. Chem. Phys., 7, 121–137, 2007

  11 Jan 2007

11 Jan 2007

Photochemical modelling in the Po basin with focus on formaldehyde and ozone

L. Liu2,1, F. Flatøy3, C. Ordóñez4, G. O. Braathen1, C. Hak5, W. Junkermann6, S. Andreani-Aksoyoglu4, J. Mellqvist7, B. Galle7, A. S. H. Prévôt4, and I. S. A. Isaksen2 L. Liu et al.
  • 1Norwegian Institute for Air Research, Kjeller, Norway
  • 2University of Oslo, Meteorology and Oceanography Section, Department of Geosciences, Oslo, Norway
  • 3Bjerknes Centre for Climate Research, University of Bergen, Bergen, Norway
  • 4Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland
  • 5Institute of Environmental Physics, Heidelberg, University of Heidelberg, Germany
  • 6Research Centre Karlsruhe, Institute for Meteorology and Climate Research, Garmisch Partenkichen, Germany
  • 7Department of Radio and Space, Chalmers University of Technology, Göteborg, Sweden

Abstract. 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 representativeness 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 ppb, the peak surface ozone maxima reached 90 ppb. Those relatively low values can be explained by low emissions during the August vacation and unstable weather conditions in this period. The modelled ΔO3/ΔNOz slope at Alzate of 5.1 agrees well with the measured slope of 4.9.

Final-revised paper