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The results from a simulation chamber study on the formaldehyde (HCHO)absorption cross section in the UV spectral region are presented. Weperformed 4 experiments at ambient HCHO concentrations withsimultaneous measurements of two DOAS instruments in the atmospheresimulation chamber SAPHIR in Jülich. The two instruments differ intheir spectral resolution, one working at 0.2 nm (broad-band,BB-DOAS), the other at 2.7 pm (high-resolution, HR-DOAS). Bothinstruments use dedicated multi reflection cells to achieve long lightpath lengths of 960 m and 2240 m, respectively, inside the chamber.During two experiments HCHO was injected into the clean chamber bythermolysis of well defined amounts of para-formaldehyde reachingmixing rations of 30 ppbV at maximum. The HCHO concentrationcalculated from the injection and the chamber volume agrees with theBB-DOAS measured value when the absorption cross section ofMeller and Moortgat (2000) and the temperature coefficient ofCantrell (1990) were used for data evaluation. In two furtherexperiments we produced HCHO in-situ from the ozone + ethenereaction which was intended to provide an independent way of HCHOcalibration through the measurements of ozone and ethene. However, wefound an unexpected deviation from the current understanding of theozone + ethene reaction when CO was added to suppress possibleoxidation of ethene by OH radicals. The reaction of the Criegeeintermediate with CO couldbe 240 times slower than currently assumed. Based on the BB-DOASmeasurements we could deduce a high-resolution cross section for HCHOwhich was not measured directly so far

Investigation of the formaldehyde differential absorption cross section at high and low spectral resolution in the simulation chamber SAPHIR