Intercomparison of Pandora stratospheric NO<sub>2</sub> slant column product with the NDACC-certified M07 spectrometer in Lauder, New Zealand

Abstract

Abstract. In September 2014, a Pandora multi-spectral photometer operated by the SAGE-III project was sent to Lauder, New Zealand, to operate side-by-side with the National Institute of Water and Atmospheric Research's (NIWA) Network for Detection of Atmospheric Composition Change (NDACC) certified zenith slant column NO 2 instrument to allow intercomparison between the two instruments and for evaluation of the Pandora unit as a potential SAGE-III validation tool for stratospheric NO 2 . This intercomparison spanned a full year, from September 2014 to September 2015. Both datasets were produced using their respective native algorithms using a common reference spectrum (i.e., 12:00 NZDT (UTC + 13) on 26 February 2015). Throughout the entire deployment period both instruments operated in a zenith-only observation configuration. Though conversion from slant column density (SCD) to vertical-column density is routine (by application of an air mass factor), we limit the current analysis to SCD only. This omission is beneficial in that it provides an intercomparison based on similar modes of operation for the two instruments and the retrieval algorithms as opposed to introducing an air mass factor dependence in the intercomparison as well. It was observed that the current hardware configurations and retrieval algorithms are in good agreement (R > 0.95). The detailed results of this investigation are presented herein.

Nitrogen dioxide and formaldehyde measurements from the GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator over Houston, Texas

“…The retrieved slit function in the NO 2 fitting window is nearly symmetric and very similar in width to the FWHM = 0.58 nm (Kowalewski and Janz, 2014) measured in the laboratory. Using in-flight data, the retrieved FWHM is 0.57 nm at the nadir center position, expanding to 0.58 nm at the edges of the swath.…”

“…We perform spectral fitting to derive slant columns using radiometrically calibrated spectra (Level 1B), which are geolocated and derived from raw (Level 0) data using characteriza-tion data collected in the laboratory before the campaign, as described in detail by Kowalewski and Janz (2014). The firstguess wavelength calibration was determined from spectra collected in the laboratory using mercury-argon, cadmium, neon and krypton discharge lamps as sources.…”

“…For CH 2 O, we fit parameters describing the shape and width of an asymmetric Gaussian (Cai et al, 2012;Nowlan et al, 2016). While the super-Gaussian works well for NO 2 , it results in a very small increase (∼ 5 %) in fitting residuals for CH 2 O over an asymmetric Gaussian, possibly due to the presence of a double shoulder to one side of the slit function shape, as measured in the laboratory at wavelengths less than 380 nm (Kowalewski and Janz, 2014).…”

“…The instrument has dimensions of 48 cm × 48 cm × 46 cm and a mass of 36 kg. We briefly describe the GCAS instrument below; a more detailed description of the instrument and laboratory characterization can be found in Kowalewski and Janz (2014).…”

“…The Vis-NIR ocean color channel uses a 1004 × 1002 CCD array to collect spectra with a spectral sampling of 0.8 nm and resolution of 2.8 nm, and has an order sorting filter to reduce second-order grating effects. The spectrometer units are operated at a temperature of 20 • C and are stable to 0.25 • C 40 min after a nominal takeoff to a typical cruise altitude (Kowalewski and Janz, 2014). In addition to the two spectrometers, a video camera is also included in the housing for the purpose of collecting relevant scene information.…”