ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-5-909-2005 MAX-DOAS measurements of formaldehyde in the Po-Valley Heckel A. 1 Richter A. 1 Tarsu T. 1 Wittrock F. 1 Hak C. 2 Pundt I. 2 Junkermann W. 3 Burrows J. P. 1 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 21 03 2005 5 4 909 918 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/5/909/2005/acp-5-909-2005.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/909/2005/acp-5-909-2005.pdf

During the first measurement campaign of the EU F<SMALL>ORMAT</SMALL> 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&ndash;14 August, are presented exemplary. Slant column densities for formaldehyde (HCHO) and the oxygen dimer (O<sub>4</sub>) have been retrieved, employing fitting windows from 335&nbsp;nm to 357&nbsp;nm and 350&nbsp;nm to 360&nbsp;nm respectively. In order to convert slant into vertical columns radiative transfer calculations were perfomred using aerosol parameters derived from the actual O<sub>4</sub> measurements. By analysing the measurements from different viewing directions (zenith, 4x&nbsp;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&middot;10<sup>15</sup>&nbsp;molec/cm<sup>2</sup> with an relative error of about 15%. This corresponds to HCHO mixing ratios in the boundary layer of 0.7&nbsp;ppb to 4.2&nbsp;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.