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

摘要

pstrongAbstract./strong In order to promote the development of the passive DOAS technique the Multi Axis DOAS a?? 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 0.3span class="thinspace"/spana????a??span class="thinspace"/span10sup15/supspan class="thinspace"/spanmoleculesspan class="thinspace"/spancmsupa??2/sup for an integration time of 1span class="thinspace"/spanmin. The fit error for the mini MAX-DOAS is around 0.7span class="thinspace"/spana????a??span class="thinspace"/span10sup15/supspan class="thinspace"/spanmoleculesspan class="thinspace"/spancmsupa??2/sup. Although the HONO delta SCDs are normally smaller than 6span class="thinspace"/spana????a??span class="thinspace"/span10sup15/supspan class="thinspace"/spanmoleculesspan class="thinspace"/spancmsupa??2/sup, 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 0.63span class="thinspace"/spana????a??span class="thinspace"/span10sup15/supspan class="thinspace"/spanmoleculesspan class="thinspace"/spancmsupa??2/sup. The correlation coefficients are higher than 0.7 and the slopes of linear regressions deviate from unity by less than 16span class="thinspace"/span% for the elevation angle of 1?°. The correlations decrease with an increase in 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 0.3span class="thinspace"/spana????a??span class="thinspace"/span10sup15/supspan class="thinspace"/spanmoleculesspan class="thinspace"/spancmsupa??2/sup, which is about half of the systematic difference between the real measurements.brbrThe differences of HONO delta SCDs retrieved in the selected three spectral ranges 335a??361, 335a??373 and 335a??390span class="thinspace"/spannm are considerable (up to 0.57span class="thinspace"/spana????a??span class="thinspace"/span10sup15/supspan class="thinspace"/spanmoleculesspan class="thinspace"/spancmsupa??2/sup) for both real measurements and synthetic spectra. We performed