Effect of spectral resolution on the measurement of monoaromatic hydrocarbons by DOAS.
The excellent response characteristics and detection sensitivity with much lower operational cost and the capability to discriminate between the isomer of some monoaromatic hydrocarbons (MAHCs) make differential optical absorption spectroscopy (DOAS) a powerful tool to trace concentration variation of MAHCs. But due to the similarity in chemical structure, those MAHCs have the similar overlapped characteristic absorption structures, which make the selection of instrumental parameter critical to the accurate detection of MAHCs. Firstly, the spectral resolution used in DOAS system determines the nonlinear absorption of O2 and the mass dependence of characteristic absorption structure; thereby it determines the effect of elimination error of O2 absorption in the atmospheric spectra for the detection of MAHCs. Secondly, spectral resolution determines the differential absorption characteristics of twelve MAHCs representing major constituents in technical solvents used in the automobile industry and the interference of spectral overlapping. Thirdly, the spectral resolution determines the sensitivity, time resolution and linear range. So the spectral resolution range with the best ratio of signal to noise is used to determine the most suitable spectral resolution range, as well as the spectral resolution range that ensure the characteristic absorption structure of MAHCs and the minimization of O2 absorption interference. Finally, 0.15-0.16 nm (FWHM: full width at half maximum) is assumed to be closest to the optimum spectral resolution and it is confirmed by the results of practical measurement of MAHCs by DOAS.
UV-absorption cross sections of a series of monocyclic aromatic compounds
Abstract Aromatic hydrocarbons are of increasing importance as components of urban volatile organic compounds. Besides gas-chromatographic techniques, the detection of aromatic hydrocarbons by Differential Optical Absorption Spectroscopy (DOAS) is a promising alternative technique. The absolute calibration of DOAS-instruments is straightforward: Only the absorption cross sections of the species of interest and the instrument function are required. In this work we present measurements of high-resolution gas phase UV-absorption cross section spectra of benzene (C 6 H 6 ), toluene (C 7 H 8 ), p-, m-, o -xylene (C 8 H 10 ), phenol (C 6 H 5 OH), benzaldehyde (C 7 H 6 O) and the cresol isomers (C 7 H 8 O), p-, m- and o -cresol in the wavelength interval between 230 and 290 nm. The spectral resolution is 0.11 nm (FWHM), except for o - and m -cresol, where the resolution is 0.32 nm (FWHM). These spectra can be used to calibrate DOAS-instruments. From these absorption cross sections typical detection limits for the atmospheric measurements with Long-Path-Differential Optical Absorption Spectroscopy are calculated to be in the range of a few ppt (phenol), to a few 10 ppt (cresol, benzaldehyde, benzene, p -xylene), up to 200 ppt ( o -xylene). The attainability of these detection limits is demonstrated by measurements of atmospheric phenol and benzaldehyde.
Determination of aerosol size distribution from observation of the aureole around a point source. 2: Experimental.
A theoretical basis is presented for a new method for determining the aerosol size distribution from aureole measurements around a point source. The method involves the determination of the angular scattering coefficient from radiance measurements at different observation angles between 1° and 10° around a source in the solar blind spectral region. The results are used to determine the aerosol size distribution by applying the Twomey-Chahine inversion algorithm. The derived size distribution has the desirable features of being accurate for large particles up to radii of 10 μm and being insensitive to their complex refractive index.