Intercomparison of NO<sub>2</sub>, O<sub>4</sub>, O<sub>3</sub> and HCHO slant column measurements by MAX-DOAS and zenith-sky UV–visible spectrometers during CINDI-2

Abstract

Abstract. In September 2016, 36 spectrometers from 24 institutes measured a number of key atmospheric pollutants for a period of 17 d during the Second Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI-2) that took place at Cabauw, the Netherlands (51.97∘ N, 4.93∘ E). We report on the outcome of the formal semi-blind intercomparison exercise, which was held under the umbrella of the Network for the Detection of Atmospheric Composition Change (NDACC) and the European Space Agency (ESA). The three major goals of CINDI-2 were (1) to characterise and better understand the differences between a large number of multi-axis differential optical absorption spectroscopy (MAX-DOAS) and zenith-sky DOAS instruments and analysis methods, (2) to define a robust methodology for performance assessment of all participating instruments, and (3) to contribute to a harmonisation of the measurement settings and retrieval methods. This, in turn, creates the capability to produce consistent high-quality ground-based data sets, which are an essential requirement to generate reliable long-term measurement time series suitable for trend analysis and satellite data validation. The data products investigated during the semi-blind intercomparison are slant columns of nitrogen dioxide (NO2), the oxygen collision complex (O4) and ozone (O3) measured in the UV and visible wavelength region, formaldehyde (HCHO) in the UV spectral region, and NO2 in an additional (smaller) wavelength range in the visible region. The campaign design and implementation processes are discussed in detail including the measurement protocol, calibration procedures and slant column retrieval settings. Strong emphasis was put on the careful alignment and synchronisation of the measurement systems, resulting in a unique set of measurements made under highly comparable air mass conditions. The CINDI-2 data sets were investigated using a regression analysis of the slant columns measured by each instrument and for each of the target data products. The slope and intercept of the regression analysis respectively quantify the mean systematic bias and offset of the individual data sets against the selected reference (which is obtained from the median of either all data sets or a subset), and the rms error provides an estimate of the measurement noise or dispersion. These three criteria are examined and for each of the parameters and each of the data products, performance thresholds are set and applied to all the measurements. The approach presented here has been developed based on heritage from previous intercomparison exercises. It introduces a quantitative assessment of the consistency between all the participating instruments for the MAX-DOAS and zenith-sky DOAS techniques.

Validation of tropospheric NO<sub>2</sub> column measurements of GOME-2A and OMI using MAX-DOAS and direct sun network observations

“…Different data products have been generated for each satellite instrument, using different assumptions for each of the three aforementioned steps (see Boersma et al, 2004;Richter et al, 2011;Lin et al, 2014;Bucsela et al, 2013;Lamsal et al, 2014;van Geffen et al, 2015;Krotkov et al, 2016;Lorente et al, 2017;Liu et al, 2019a. In addition to instrument-specific differences, structural uncertainties arising from the application of different retrieval methodologies to the same satellite observations (sometimes also called forward model uncertainties) can introduce differences in the retrieved tropospheric NO 2 columns (VCD tropo ) of 10 %-50 % (e.g., van Noije et al, 2006;Lorente et al, 2017;Zara et al, 2018).…”

Comparison of formaldehyde tropospheric columns in Australia and New Zealand using MAX-DOAS, FTIR and TROPOMI

“…Differential slant column densities (dSCDs) of O 4 , used in MAX-DOAS aerosol retrievals, were determined using the wavelength range from 338 to 370, as in studies such as Ryan et al (2018) and Kreher et al (2020). A simple sensitivity study was run to determine the appropriate wavelength range for formaldehyde retrieval given that two wavelength ranges are common in previous papers: 324.5-359 and 336-359 nm.…”

“…Because satellite instruments and MAX-DOAS share the same spectroscopic technique for retrieving UV and visible absorbing trace gases, MAX-DOAS is an ideal validation tool as demonstrated for HCHO in several previous papers (e.g. Chance et al, 2000;Thomas et al, 1998;Hoque et al, 2018b;De Smedt et al, 2015;Vigouroux et al, 2009;Lee et al, 2015;Kurosu et al, 2007). However, no such validation studies have been published for the Australasian region to date.…”

“…An EnviMeS MAX-DOAS has been operational at Lauder since November 2016, allowing a significant period of overlap between the Lauder and Melbourne time series. The National Institute of Water and Atmospheric Research (NIWA) EnviMeS MAX-DOAS demonstrated good performance at the CINDI-2 international comparison campaign held in the Netherlands in 2016 (Kreher et al, 2020).…”

Inter-comparison of MAX-DOAS measurements of tropospheric HONO slant column densities and vertical profiles during the CINDI-2 campaign

“…Baseline DOAS analysis settings for the HONO fit.Wavelength calibration Calibration based on Fraunhofer lines of Kurucz solar spectrum(Kurucz et al, 1984) (10 17 molec. cm −2 )Taylor terms (see Pukīte et al, 2010) with respect to σ NO 2 at 298 K: λσ NO 2 , σ 2…”