Caution with Spectroscopic NO<sub>2</sub> Reference Cells (Cuvettes)

Abstract

Spectroscopic measurements of atmospheric trace gases, e.g. by Differential Optical Absorption Spectroscopy (DOAS) are frequently supported by recording the trace gas column density (CD) in absorption cells (cuvettes), which are temporarily inserted into the light-path. The idea is to verify the proper working of the instruments, to check the spectral registration 15(wavelength calibration and spectral resolution), and to perform some kind of calibration (absolute determination of trace gas CDs). In principle DOAS applications do not require absorption cell calibration, however in practice measurements with absorption cells in the spectrometer's light path are frequently performed. In addition, trace gas absorption cells are used as a central component in gas correlation spectroscopy instruments. 20Here we show at the example of NO 2 absorption cells that the effective CD seen by the instrument can deviate greatly from expected values (by orders of magnitude). Analytical calculations and kinetic model studies show the dominating influence of photolysis and dimerisation of NO 2 . In particular, this means that the partial pressure of NO 2 in the cell matters. However, problems can be particular severe at high NO 2 pressures (around 10 5 Pa) as well as 25 low NO 2 partial pressures (of the order of a few 100 Pa). Also, it can be of importance whether the cell contains pure NO 2 or is topped up with air or oxygen (O 2 ). Some suggestions to improve the situation are discussed.Atmos. Meas. Tech. Discuss., https://doi.