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

Abstract

Abstract. 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 in elevation angle. All the participants also analysed synthetic spectra using the same baseline DOAS settings toPublished by Copernicus Publications on behalf of the European Geosciences Union. Y. Wang et al.: MAX-DOAS measurements of HONO slant column densitiesevaluate 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 retrieved in the selected three spectral ranges 335-361, 335-373 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 O 4 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 × 10 15 molecules cm −2 , much smaller than those in the other two ranges. The typical random uncertainty is estimated to be about 0.16 × 10 15 molecules cm −2 , which is only 25 % of the total syst...

Is a scaling factor required to obtain closure between measured and modelled atmospheric O<sub>4</sub> absorptions? – A case study for two days during the MADCAT campaign

“…The derived O 4 (d)AMFs are almost identical to those from the standard analysis (within 1%). For the spectral retrieval of HONO in a similar spectral range, a significant impact of water vapour absorption around 363 nm was found in Wang et al (2017c) and Lampel et al (2017). In Fig.…”

Detection of O<sub>4</sub> absorption around 328 nm and 419 nm in measured atmospheric absorption spectra

“…We suggest addressing at the same time uncertainties in recent water vapour absorption cross sections (see e.g. Lampel et al, 2015b;Volkamer et al, 2015;Wilson et al, 2016 andLampel et al, 2017a) at around 419 and 360 nm in a controlled laboratory environment with wellknown light path lengths at different pressure and humidity values. The availability of such a data set has the potential to improve the spectral retrieval of various trace gases substantially as pointed out below.…”

The tilt effect in DOAS observations

“…Even though the tilt effect is a general effect and not restricted to a certain wavelength range, here we adapted the HONO retrieval settings suggested by Wang et al (2017) for the spectral analysis (see Table 1). Similar results were obtained in other wavelength intervals (e.g.…”

“…Data from this campaign have been already published, e.g. in Ortega et al (2015), Lampel et al (2015), Peters et al (2017) and Wang et al (2017). …”

“…3.2. A more detailed description, also of the concentration height profiles, can be found in Wang et al (2017).…”