Broadband optical cavity absorption spectroscopy in the near-ultraviolet: applications in atmospheric chemistry

A novel spectroscopic method, incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS), has been modified and extended to measure absorption spectra in the near-ultraviolet with high sensitivity. The near-ultraviolet region extends from 300 to 400 nm and is particularly important in tropospheric photochemistry; absorption of near-UV light can also be exploited for sensitive trace gas measurements of several key atmospheric constituents. In this work, several IBBCEAS instruments were developed to record reference spectra and to measure trace gas concentrations in the laboratory and field. An IBBCEAS instrument was coupled to a flow cell for measuring very weak absorption spectra between 335 and 375 nm. The instrument was validated against the literature absorption spectrum of SO2. Using the instrument, we report new absorption cross-sections of O3, acetone, 2-butanone, and 2-pentanone in this spectral region, where literature data diverge considerably owing to the extremely weak absorption. The instrument was also applied to quantifying low concentrations of the short-lived radical, BrO, in the presence of strong absorption by Br2 and O3. A different IBBCEAS system was adapted to a 4 m3 atmosphere simulation chamber to record the absorption cross-sections of several low vapour pressure compounds, which are otherwise difficult to measure. Absorption cross-sections of benzaldehyde and the more volatile alkyl nitrites agree well with previous spectra; on this basis, the cross-sections of several nitrophenols are reported for the first time. In addition, the instrument was also used to study the optical properties of secondary organic aerosol formed following the photooxidation of isoprene. An extractive IBBCEAS instrument was developed for detecting HONO and NO2 and had a sensitivity of about 10-9 cm-1. This instrument participated in a major international intercomparison of HONO and NO2 measurements held in the EUPHORE simulation chamber in Valencia, Spain, and results from that campaign are also reported here.

Broadband cavity-enhanced absorption spectroscopy with incoherent light

Although broadband incoherent light does not efficiently couple into a high-finesse optical cavity, its transmission is readily detectable and enables applications in cavity-enhanced absorption spectroscopy in the gas phase, liquid phase and on surfaces. This chapter gives an overview of measurement principles and experimental approaches implementing incoherent light sources in cavity-enhanced spectroscopic applications. The general principles of broadband CEAS are outlined and general “pros and cons” discussed, detailing aspects like cavity mirror reflectivity calibration or the establishment of detection limits. Different approaches concerning light sources, cavity design and detection schemes are discussed and a comprehensive overview of the current literature based on a methodological classification scheme is also presented.

Chamber investigations of atmospheric mercury oxidation chemistry

Mercury is a potent neurotoxin even at low concentrations. The unoxidised metal has a high vapour pressure and can circulate through the atmosphere, but when oxidised can deposit and be accumulated through the food chain. This work aims to investigate the oxidation processes of atmospheric Hg0(g). The first part describes efforts to make a portable Hg sensor based on Cavity Enhanced Absorption Spectroscopy (CEAS). The detection limit achieved was 66 ngm−3 for a 10 second averaging time. The second part of this work describes experiments carried out in a temperature controlled atmospheric simulation chamber in the Desert Research Institute, Reno, Nevada, USA. The chamber was built around an existing Hg CRDS system that could measure Hg concentrations in the chamber of<100 ngm−3 at 1 Hz enabling reactions to be followed. The main oxidant studied was bromine, which was quantified with a LED based CEAS system across the chamber. Hg oxidation in the chamber was found to be mostly too slow for current models to explain. A seven reaction model was developed and tested to find which parameters were capable of explaining the deviation. The model was overdetermined and no unique solution could be found. The most likely possibility was that the first oxidation step Hg + Br →HgBr was slower than the preferred literature value by a factor of two. However, if the more uncertain data at low [Br2] was included then the only parameter that could explain the experiments was a fast, temperature independent dissociation of HgBr some hundreds of times faster than predicted thermolysis or photolysis rates. Overall this work concluded that to quantitatively understand the reaction of Hg with Br2, the intermediates HgBr and Br must be measured. This conclusion will help to guide the planning of future studies of atmospheric Hg chemistry.

Electronic spectroscopy of thiones

The phosphorescence excitation spectra of two thiones, 4-H-1-xanthione (XT) and 4-H-1-pyrane-4-thione (PT), cooled in a supersonic jet were investigated. The vibronic lineshape of the T1z origin of PT measured by cavity ring-down spectroscopy is considered and the excited state rotational constants are calculated. For XT the 3A2(nπ* ) → X1A1 phosphorescence excitation spectrum was investigated in the region 14900-17600 cm-1. The structure observed is shown to be due to the T1← S0 absorption and an assignment in terms of the vibronic structure of the band is proposed. A previous assignment of the S1 ← S0 origin is considered and the transition involved is shown to be most probably due to the absorption of a vibronic tiplet state T1z,v7. An alternative but tentative assignment of the S1,0 ←S0,0 transition is suggested. In the case of PT the phosphorescence excitation spectrum was investigated in the region of the 1A2(ππ*) ← X1A1 absorption band between 27300 and 28800 cm-1. The spectrum exhibits complex features which are typical for the strong vibronic coupling case of two adjacent electronic states. The observed intermediate level structure was attributed to the coupling with a lower lying dark electronic state 1B1(nπ*2), whose origin was estimated to be ~ 825 - 1025 cm-1 below the origin of 1A2(ππ*)0. Consequences of the vibronic coupling on the decay dynamics of 1A2(ππ*) as well as tentative assignments of vibronic transitions 1A2(ππ*)v ← X1A1 are also discussed. In the T1z ← S0 cavity ring-down absorption spectrum of PT, the vibronic lineshape of the T1z origin is analysed. As the T1z line is separated from the T1x,1y lines by a large zero-field splitting it is possible to use an Asyrot-like program to calculate the vibrational-rotational parameters determining the lineshape. It is shown that PT is non-planar in the first excited triplet state and the lineshape is composed of a mixture of A-type and C-type bandshapes. The non-planarity of PT is discussed.