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

In order to promote the development of the passive DOAS technique the 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 the differential slant column densities (dSCDs) of nitrous acid (HONO) derived from measurements of seven different instruments. We also compare the tropospheric difference of SCDs (delta SCD) of HONO, namely the difference of the SCDs for the non-zenith observations and the zenith observation of the same elevation sequence. Different research groups analysed the spectra from their own instruments using their individual fit software. All the fit errors of HONO dSCDs from the instruments with cooled large-size detectors are mostly in the range of 0.1 to 0.3 × 10 15 molecules cm −2 for an integration time of 1 min. The fit error for the mini MAX-DOAS is around 0.7 × 10 15 molecules cm −2 . Although the HONO delta SCDs are normally smaller than 6 × 10 15 molecules cm −2 , consistent time series of HONO delta SCDs are retrieved from the measurements of different instruments. Both fits with a sequential Fraunhofer reference spectrum (FRS) and a daily noon FRS lead to similar consistency. Apart from the mini-MAX-DOAS, the systematic absolute differences of HONO delta SCDs between the instruments are smaller than 0.63 × 10 15 molecules cm −2 . The correlation coefficients are higher than 0.7 and the slopes of linear regressions deviate from unity by less than 16% for the elevation angle of 1°. The correlations decrease with an increase of elevation angle. All the participants also analysed synthetic spectra using the same baseline DOAS settings to evaluate the systematic errors of HONO results from their respective fit programs. In general the errors are smaller than 0.3 × 10 15 molecules cm −2 , which is about half of the systematic difference between the real measurements. The differences of HONO delta SCDs between retrieved in the selected three spectral ranges 335–361 nm, 335–373 nm and 335–390 nm are considerable (up to 0.57 × 10 15 molecules cm −2 ) for both real measurements and synthetic spectra. We performed sensitivity studies to quantify the dominant systematic error sources and to find a recommended DOAS setting in the three spectral ranges. The results show that water vapour absorption, temperature and wavelength dependence of O4 absorption, temperature dependence of Ring spectrum, and polynomial and intensity offset correction all together dominate the systematic errors. We recommend a fit range of 335–373 nm for HONO retrievals. In such fit range the total systematic uncertainty from different sources is about 0.87 × 10 15 molecules cm −2 , much smaller than those in the other two ranges. Meanwhile the systematic bias of the fitted from the simulated real HONO delta SCDs is also smallest in 335–373 nm (about 0.02 × 10 15 molecules cm −2 ). The typical random uncertainty is estimated to be about 0.16 × 10 15 molecules cm −2 , which is only 25 % of the total systematic uncertainty for most of the instruments in the MAD-CAT campaign. In summary most of the MAX-DOAS instruments can well observe the signals of atmospheric HONO absorption in case of HONO delta SCDs higher than 0.2 × 10 15 molecules cm −2 . However, systematic uncertainties limit the reliability of the results.