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In order to promote the development of the passive DOAS techniquethe Multi Axis DOAS – Comparison campaign for Aerosols and Trace gases(MAD-CAT) was held at the Max Planck Institute for Chemistry in Mainz,Germany, from June to October 2013. Here, we systematically compare thedifferential slant column densities (dSCDs) of nitrous acid (HONO) derivedfrom measurements of seven different instruments. We also compare thetropospheric difference of SCDs (delta SCD) of HONO, namely the difference ofthe SCDs for the non-zenith observations and the zenith observation of thesame elevation sequence. Different research groups analysed the spectra fromtheir own instruments using their individual fit software. All the fit errorsof HONO dSCDs from the instruments with cooled large-size detectors aremostly in the range of 0.1 to 0.3  ×  1015 molecules cm−2for an integration time of 1 min. The fit error for the mini MAX-DOAS isaround 0.7  ×  1015 molecules cm−2. Although the HONOdelta SCDs are normally smaller than6  ×  1015 molecules cm−2, consistent time series of HONOdelta SCDs are retrieved from the measurements of different instruments. Bothfits with a sequential Fraunhofer reference spectrum (FRS) and a daily noonFRS lead to similar consistency. Apart from the mini-MAX-DOAS, the systematicabsolute differences of HONO delta SCDs between the instruments are smallerthan 0.63  ×  1015 molecules cm−2. The correlationcoefficients are higher than 0.7 and the slopes of linear regressions deviatefrom unity by less than 16 % for the elevation angle of 1°. Thecorrelations decrease with an increase in elevation angle. All theparticipants also analysed synthetic spectra using the same baseline DOASsettings to evaluate the systematic errors of HONO results from theirrespective fit programs. In general the errors are smaller than0.3  ×  1015 molecules cm−2, which is about half of thesystematic difference between the real measurements. The differences of HONO delta SCDs retrieved in the selected three spectralranges 335–361, 335–373 and 335–390 nm are considerable (up to0.57  ×  1015 molecules cm−2) for both real measurementsand synthetic spectra. We performed sensitivity studies to quantify thedominant systematic error sources and to find a recommended DOAS setting inthe three spectral ranges. The results show that water vapour absorption,temperature and wavelength dependence of O4 absorption, temperaturedependence of Ring spectrum, and polynomial and intensity offset correctionall together dominate the systematic errors. We recommend a fit range of335–373 nm for HONO retrievals. In such fit range the overall systematicuncertainty is about 0.87  ×  1015 molecules cm−2, muchsmaller than those in the other two ranges. The typical random uncertainty isestimated to be about 0.16  ×  1015 molecules cm−2, whichis only 25 % of the total systematic uncertainty for most of theinstruments in the MAD-CAT campaign. In summary for most of the MAX-DOASinstruments for elevation angle below 5°, half daytime measurements(usually in the morning) of HONO delta SCD can be over the detection limit of0.2  ×  1015 molecules cm−2 with an uncertainty of ∼  0.9  ×  1015 molecules cm−2

atmospheric measurement techniques

MAX-DOAS measurements of HONO slant column densities during the MAD-CAT campaign: inter-comparison, sensitivity studies on spectral analysis settings, and error budget