Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 5 4 2005 10.5194/acp-5-909-2005 http://www.atmos-chem-phys.net/5/909/2005/ http://www.atmos-chem-phys.net/5/909/2005/acp-5-909-2005.html http://www.atmos-chem-phys.net/5/909/2005/acp-5-909-2005.pdf 909 918 2005-03-21 MAX-DOAS measurements of formaldehyde in the Po-Valley A. Heckel A. Richter T. Tarsu F. Wittrock C. Hak I. Pundt W. Junkermann J. P. Burrows Institute of Environmental Physics (IUP), P.O. Box 33 04 40, University Bremen, 28334 Bremen, Germany IUP, Im Neuenheimer Feld 229, University Heidelberg, 69120 Heidelberg, Germany IMK-IFU, Kreuzeckbahnstrasse 19, 82467 Garmisch-Partenkirchen, Germany During the first measurement campaign of the EU FORMAT project in summer 2002 near Milan, northern Italy, ground-based scattered light differential optical absorption spectroscopy (DOAS) measurements were performed using a new multi-axis instrument. From the data set of this four week measurement period, the detailed analysis results of three days, 12–14 August, are presented exemplary. Slant column densities for formaldehyde (HCHO) and the oxygen dimer (O4) have been retrieved, employing fitting windows from 335 nm to 357 nm and 350 nm to 360 nm respectively. In order to convert slant into vertical columns radiative transfer calculations were perfomred using aerosol parameters derived from the actual O4 measurements. By analysing the measurements from different viewing directions (zenith, 4x off-axis) vertical profile information, and in particular mixing ratios for the boundary layer have been derived for the first time for HCHO with a multi-axis DOAS (MAX-DOAS) instrument. HCHO vertical columns are in the range of 5 to 20·1015 molec/cm2 with an relative error of about 15%. This corresponds to HCHO mixing ratios in the boundary layer of 0.7 ppb to 4.2 ppb, which is in excellent agreement with simultaneous measurements from both a Hantzsch in-situ and a long-path DOAS instrument operated at the same place.