Correction of the oxygen interference with UV spectroscopic (DOAS) measurements of monocyclic aromatic hydrocarbons in the atmosphere

The measurement of monocyclic aromatic hydrocarbons by Differential Optical Absorption Spectroscopy (DOAS) and Differential Absorption LIDAR (DIAL) in the atmosphere suffers from interference by the three forbidden Herzberg band systems of O2 and a fourth band system due to the dimers O2---O2 and O2---N2 at wavelengths below 287 nm. Due to the lack of reference spectra in digital form, until now the oxygen absorptions were difficult to eliminate from atmospheric absorption spectra. In this work, reference spectra of the Herzberg bands of oxygen are presented, that allow to eliminate this oxygen interference for practical purposes. Two sets of oxygen reference spectra were recorded between 240 and 290 nm with spectral resolutions of 0.15 nm (FWHM) and 0.05 nm. Spectra were taken at 240 and 720 m absorption path lengths in several mixtures of oxygen and nitrogen from 10% O2/90% N2 to 100% pure O2 at atmospheric pressure (O2 column densities from 6×1022 to 1.8×1024 molecules cm-2). At the resolution of the measurements, the rotational structure of the Herzberg I band Q-branches is not resolved. Therefore, saturation effects of individual transitions of the Herzberg I bands can cause the observed band shape to vary with the column density of oxygen. This apparent deviation from Lambert Beer’s law can lead to problems with the oxygen correction of atmospheric DOAS measurements. In the practical application of the oxygen reference spectra, additional problems arise, because the ratio of molecular absorption in the Herzberg bands and dimer absorption changes when the partial pressure of oxygen is varied. Even though this effect is reduced due to the presence of N2 it needs to be accounted for, if the spectra are applied to atmospheric measurements. Solutions to these problems are discussed and demonstrated together with methods to optimize DOAS measurements of aromatic hydrocarbons. As sample application the oxygen reference spectra were used to correct DOAS measurements of monocyclic aromatic hydrocarbons carried out in the urban air of Heidelberg. Simultaneous time series of mixing ratios are presented for benzene, toluene, p-xylene, m-xylene and phenol. Mean concentrations were found to be 1.8, 2.5, 0.8, 1.2 ppb and 77 ppt, respectively. The spectra are available in digital form from the authors upon e-mail request.