Absorption cross-sections carbonyl compounds

The alkylnitrites have large UV absorption cross-sections, comparable to that of HONO [31]. These compounds can be prepared in pure form and are reasonably stable both in the vapor and liquid phases when kept in the dark. As will be illustrated later, the selection of a specific RONO for a given experiment is based largely on the consideration of potential chemical and/or analytical interference problems arising from the accompanying carbonyl products. 

One of the objectives of LACTOZ has been to provide absorption cross-sections and quantum yields so that photolysis rates can be calculated under atmospheric conditions. Emphasis has been on (a) refinement of data for the simple, well-characterised molecules (O3, HCHO) involved in HOx radical production, and (b) determination of data for photolysis of organic compounds formed as products in the degradation of alkanes, alkenes and selected multifunctional VOCs, mainly carbonyl compounds and nitrates, which produce HOx indirectly. 

Fig. 7 UV visible absorption cross-sections of several carbonyl compounds.

UV absorption spectra have been measured for the ketonitrates and dinitrates (LACTOZ 89). In the region of atmospheric interest (X > 270 nm) the absorption cross sections of the carbonyl nitrates are approximately a factor of 10 higher than those of the dinitrates. From the measured cross sections photolysis frequencies have been calculated for the organic ketonitrates and dinitrates. Although the photolysis frequencies represent upper limits the results indicate that photolysis will generally be somewhat more important than loss via reaction with OH radicals for saturated difunctional nitrates. However, for unsaturated nitrates loss due to reaction with OH will dominate over photolysis as an atmospheric sink. The product studies show that photolysis of ketonitrates/dinitrates will result in the re-release of NO2 and the formation of PAN-type compounds. 

Absorption Spectrum and Cross Sections of HCHO For carbonyl compounds such as aldehydes and ketones, absorption bands due to the electronic transition called n — r transition, in which the isolated pair of lone-pair electrons on the O atom of carbonyl group (—C = O) is excited to the excited Jt orbital of the double bond, appears around 300 nm. Since this transition is a forbidden transition, the absorption cross sections are not very large ( 10 cm molecule ) in general. However, since the absorption bands extend to near 350 nm where solar actinic flux grows, their photolyses are very important in the troposphere. 

The ultraviolet absorption spectrum of cyclohexanone reflects the n jt transition common to all carbonyls see figure IX-E-1. The data derived from gas-phase measurements of the cross sections for cyclohexanone from two different research groups [National Center for Atmospheric Research (NCAR) and Ford Scientific Laboratories (Ford)] are in reasonable agreement (Iwasaki et al., 2008). The cyclohexanone cross sections as measured in cyclohexane solution by Benson and Kistiakowski (1942) had indicated seemingly low values (cross sections shown here is significantly less than those observed for cyclopropanone, cyclobutanone, and cyclopentanone, and in fact, all other carbonyls considered in this work. It is not obvious why these significant differences exist in the probability for the n -> 7T transition for cyclohexanone and that of the other cyclic ketones and most other carbonyl compounds. Theoretical studies will be important in defining the reasons for these differences. 

Figure IX-G-1. Comparison of the wavelength dependence of the cross sections of some halogen-atom-substituted formaldehyde molecules and the actinic flux as measured for a cloudless day (June 19, 1998) near Boulder, CO at noon, (a) Shows the spectra for the formyl halides (b) gives the spectra for the carbonyl halides. Note that only the HC(0)Br and BrC(0)Br compounds have a significant overlap with the available actinic flux in the lower troposphere with an overhead Sun (gray curve). The long wavelength tail of the absorption of HC(0)C1 and C1C(0)C1 molecules catches the edge of the flux distribution. Spectral data from Keller-Rudek and Moortgat (2005) figures from Calvert et al. (2008).

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