Intercomparison of instruments for tropospheric measurements using differential optical absorption spectroscopy

The results of an intercomparison campaign of eight different long path UV-visible DOAS instruments measuring NO2, O-3 and SO2 concentrations in a moderately polluted urban site are presented. For effective optical path lengths of 230 and 780 m the overall spread of these measurements (+/- 1 sigma) are 5 x 10(10), 6 x 10(10) and 1 x 10(10) molec . cm(-3) (2.0, 2.4, and 0.4 ppb) for these molecules respectively when all instruments used a common set of absorption cross sections. The remaining differences are not completely random and the systematic differences are attributed to the different retrieval methods used for each instrument.