Abstract
Abstract. The differential optical absorption spectroscopy (DOAS) method is a well-known remote sensing technique that is nowadays widely used for measurements of atmospheric trace gases, creating the need for harmonization and characterization efforts. In this study, an intercomparison exercise of DOAS retrieval codes from 17 international groups is presented, focusing on NO2 slant columns. The study is based on data collected by one instrument during the Multi-Axis DOAS Comparison campaign for Aerosols and Trace gases (MAD-CAT) in Mainz, Germany, in summer 2013. As data from the same instrument are used by all groups, the results are free of biases due to instrumental differences, which is in contrast to previous intercomparison exercises.While in general an excellent correlation of NO2 slant columns between groups of > 99.98 % (noon reference fits) and > 99.2 % (sequential reference fits) for all elevation angles is found, differences between individual retrievals are as large as 8 % for NO2 slant columns and 100 % for rms residuals in small elevation angles above the horizon.Comprehensive sensitivity studies revealed that absolute slant column differences result predominantly from the choice of the reference spectrum while relative differences originate from the numerical approach for solving the DOAS equation as well as the treatment of the slit function. Furthermore, differences in the implementation of the intensity offset correction were found to produce disagreements for measurements close to sunrise (8–10 % for NO2, 80 % for rms residual). The largest effect of ≈ 8 % difference in NO2 was found to arise from the reference treatment; in particular for fits using a sequential reference. In terms of rms fit residual, the reference treatment has only a minor impact. In contrast, the wavelength calibration as well as the intensity offset correction were found to have the largest impact (up to 80 %) on rms residual while having only a minor impact on retrieved NO2 slant columns.
Highlights
In this study, the consistency of differential optical absorption spectroscopy (DOAS) retrievals of tropospheric nitrogen dioxide (NO2) from ground-based scattered light observations is evaluated
Results of test TL0 appear to be identical to the reference fit of the IUPB retrieval code, both using a singular value decomposition (SVD) for inversion of the DOAS matrix
Very small differences exist between TL0 and the reference fits, which are < 0.006 % for NO2 slant columns and < 0.07 % for rms
Summary
The consistency of differential optical absorption spectroscopy (DOAS) retrievals of tropospheric nitrogen dioxide (NO2) from ground-based scattered light observations is evaluated. NO2 is a key species in the formation of tropospheric ozone and a prominent pollutant in the troposphere, causing (together with aerosols) the typical brownish colour of polluted air. It is harmful for lung tissue and a powerful oxidant. The lifetime of NO2 is short (several hours) due to reaction with OH and photodissociation; it is mostly found close to its sources, making it a good tracer of local pollution Anthropogenic sources such as burning fossil fuel in industry, power generation and traffic as well as biogenic sources including bush and forest fires contribute to the tropospheric NOx loading. High NO2 amounts are mostly observed above industrialized and urban areas, traffic routes and over bush fires
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