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.
Contribution of oxygen to attenuation in the solar blind UV spectral region.
The solar blind ultraviolet (SBUV) spectral region covers the interval between 230 and 290 nm. The lower limit of this interval is given by the edge of the Schumann-Runge band and the upper limit is determined by solar radiation penetrating the stratospheric ozone shield. The SBUV region is interesting from the experimental point of view, since the lack of solar background is favorable in such applications as lidar, atmospheric communications, and remote sensing. The present models (LOWTRAN-6) include as atmospheric attenuators in this region ozone absorption and aerosol and molecular scattering. New theoretical calculations of the Herzberg I oxygen band predict significant absorption by O(2). This prediction is confirmed experimentally in this study. Field measurements at 252, 255, and 264 nm are reported over optical paths of up to 2750 m. Results show that LOWTRAN-6 is inadequate in the SBUV region, as indicated by the present extinction measurements.
Allowed and Forbidden Character in the 3715‐Å π* ←n System of Benzaldehyde
The 3715‐A system of benzaldehyde‐h6, ‐d1, and ‐d6 has been observed in absorption in the vapor phase. In spite of some diffuseness two band types have been distinguished by their characteristic rotational contours. Bands showing one main peak in the contour belong to the electronically allowed part of the system, assigned as 1A″(nπ*)←1A′, and those showing two main peaks split by about 9 cm−1 belong to the electronically forbidden part of the system involving activity of nontotally symmetric (a″) vibrations. Assignment of the wavenumbers of the three most strongly active a″ vibrations to approximate normal coordinates is certain in the case of the CHO‐torsional vibration but less certain in the cases of the out‐of‐plane substituent vibration and an out‐of‐plane ring vibration. Qualitative estimates of intensity borrowed through activity of these three vibrations have been made using undistorted and distorted molecule wavefunctions of the Pople–Santry–Segal type. The calculated n‐electron density is highl...