Study of different operational modes of the IAP 2-port-DOAS instrument for investigation of atmospheric trace gases during CINDI-2 campaign

An instrument for measuring atmospheric trace gases by DOAS method using scattered solar radiation was developed in A.M.Obukhov IAP RAS. The instrument layout is based on the lab Shamrock 303i spectrograph supplemented by 2-port radiation input system employing optical fiber. Optical ports may be used with a fixed telescope or with a scanning MAX-DOAS unit. In September 2016 the IAP instrument participated in the 2nd Cabauw Intercomparison of Nitrogen Dioxide Measuring Instruments (CINDI-2) campaign, held in the Netherlands. About 40 instruments from 26 different organizations performed DOAS measurement of NO2, HCHO and other trace gases during the campaign. During the campaign the optical ports of IAP instrument had telescopes A and B with similar field of view of about 0.3°. Telescope A was always directed to the zenith. Telescope B was directed at 5° elevation angle. Two gratings were installed in the spectrometer. They provide different spectral resolution (FWHM ~0.4 and 0.8 nm respectively) and spectral window width (~70 and ~140 nm respectively). During CINDI-2 campaign we performed test measurements in UV and visible wavelength ranges to investigate instrument stability and retrieval errors of NO2 and HCHO contents. We test four operational mode of the instrument in which we used fixed grating position in UV or visible spectral region, or rotate grating between UV and visible, or change gratings using a turret. Here we present preliminary results of comparison of gas content measured by IAP instrument with the median gas content obtained on the basis of measurements of all groups participated in CINDI-2 campaign. The main goal of the study was the detailed comparison of all four modes of our instrument and understanding of probable biases and errors typical for these modes. It was found that rotation of grating turret does not significantly affected on quality of NO2 DSCD retrieval and is much more significant for HCHO DSCD retrieval.