Abstract
Aromatic hydrocarbons are recently gaining importance in urban air. Aside their adverse effects on human health, the atmospheric oxidation of aromatics contributes substantially to the formation of secondary pollutants like ozone. Most aromatics exhibit strong absorptions in the ultraviolet spectral range between 240nm and 290 nm allowing in pricniple for a sensitive detection from Differential Optical Absorption Spectrsocopy (DOAS). This however requires elimination of the Herzberg band systems of atmospheric oxygen which superpose aromatic absorptions. To correct for this interference oxygen reference spectra are required, which due to an extremely weak transition probability of the Herzberg bands require use of long absorption-paths.
Within the scope of this work a reference cell of 15m length was designed which allowed operation of an existing multireflection cell in a sealed atmosphere at atmospheric pressure. The absorption path-length is variable between 240 m and 720 m. Oxygen reference spectra were recorded at different O2/N2 mixing-ratios between 240 nm and 810 nm. Absorption of oxygen dimers could not be separated from the monomer absorption, due to a non-resolved ro-vibronic structure of the oxygen triplett transition. For practical purposes however these dimer abosrptions can be eliminated using mathematical highpass-filtering (Savitzky-Golay-smoothing). Optimized filters are presented.
The quadratic pressure dependence of the oxygen dimerabsorptionen was checked in the wavelength range between 280 nm and 810 nm, and absorption cross-sections determined. Results are found to agree well with literature data. The optical density measured of dimerabsorption in pure oxygen was found smaller 3 % for the conditions deployed.
The reference spectra were applied to eliminate the interference from test-spectra recorded at the EUropean PHOto REactor (EUPHORE), Valencia/Spain. It is demonstrated that the spectra are well suited to measure absolute concentrations of aromatic hydrocarbons. The flexible Savitzky-Golay-high-pass filter further prooves advantageous with the elimination of cross-interferences with the detection of narrow band absorptions.
With this work previous problems with the DOAS measurement of aromatic hydrocarbons can be regarded solved. A new spectral range is now available for future sensitive measurements of aromatic hydrocarbons by DOAS.