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

evaluate the systematic errors of HONO results from their respective fit programs. In general the errors are smaller than 0.3×1015 molecules cm^-2, which is about half of the systematic difference between the real measurements. The differences of HONO delta SCDs retrieved in the selected three spectral ranges 335-361, 335-373 and 335-390 nm are considerable (up to 0.57×10¹⁵ 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 overall systematic uncertainty is about 0.87×10¹⁵ molecules cm^-2, much smaller than those in the other two ranges. The typical random uncertainty is estimated to be about 0.16×10¹⁵ molecules cm^-2, which is only 25% of the total systematic uncertainty for most of the instruments in the MAD-CAT campaign. In summary for most of the MAX-DOAS instruments for elevation angle below 5, half daytime measurements (usually in the morning) of HONO delta SCD can be over the detection limit of 0.2