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 Op tical Absorption Spectroscopy (DOAS) and Differential Absorption LIDAR (DIAL) in the atmosphere suffers from interference by the three forbi dden Herzberg band systems of O-2 and a fourth band system due to the dimers O-2-O-2 and O-2-N-2 at wavelengths below 287 nm. Due to the lac k of reference spectra in digital form, until now the oxygen absorptio ns were difficult to eliminate from atmospheric absorption spectra. In this work, reference spectra of the Herzberg bands of oxygen are pres ented, that allow to eliminate this oxygen interference for practical purposes. Two sets of oxygen reference spectra were recorded between 2 40 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 severa l mixtures of oxygen and nitrogen from 10% O-2/90% N-2 to 100% pure O- 2 at atmospheric pressure (O-2 column densities from 6 x 10(22) to 1.8 x 10(24) molecules cm(-2)). At the resolution of the measurements, th e rotational structure of the Herzberg I band Q-branches is not resolv ed. Therefore, saturation effects of individual transitions of the Her zberg I bands can cause the observed band shape to vary with the colum n density of oxygen. This apparent deviation from Lambert Beer's law c an lead to problems with the oxygen correction of atmospheric DOAS mea surements. In the practical application of the oxygen reference spectr a, additional problems arise, because the ratio of molecular absorptio n in the Herzberg bands and dimer absorption changes when the partial pressure of oxygen is varied. Even though this effect is reduced due t o the presence of N-2 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 meas urements of aromatic hydrocarbons. As sample application the oxygen re ference spectra were used to correct DOAS measurements of monocyclic a romatic hydrocarbons carried out in the urban air of Heidelberg. Simul taneous time series of mixing ratios are presented for benzene, toluen e, 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 avai lable in digital form from the authors upon e-mail request. (C) 1998 E lsevier Science Ltd. All rights reserved.