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

A novel spectroscopic method, incoherent broadband cavity enhanced absorption\nspectroscopy (IBBCEAS), has been modified and extended to measure absorption\nspectra in the near-ultraviolet with high sensitivity. The near-ultraviolet region\nextends from 300 to 400 nm and is particularly important in tropospheric\nphotochemistry; absorption of near-UV light can also be exploited for sensitive trace\ngas measurements of several key atmospheric constituents. In this work, several\nIBBCEAS instruments were developed to record reference spectra and to measure\ntrace gas concentrations in the laboratory and field. An IBBCEAS instrument was\ncoupled to a flow cell for measuring very weak absorption spectra between 335 and\n375 nm. The instrument was validated against the literature absorption spectrum of\nSO2. Using the instrument, we report new absorption cross-sections of O3, acetone,\n2-butanone, and 2-pentanone in this spectral region, where literature data diverge\nconsiderably owing to the extremely weak absorption. The instrument was also\napplied to quantifying low concentrations of the short-lived radical, BrO, in the\npresence of strong absorption by Br2 and O3. A different IBBCEAS system was\nadapted to a 4 m3 atmosphere simulation chamber to record the absorption\ncross-sections of several low vapour pressure compounds, which are otherwise\ndifficult to measure. Absorption cross-sections of benzaldehyde and the more volatile\nalkyl nitrites agree well with previous spectra; on this basis, the cross-sections of\nseveral nitrophenols are reported for the first time. In addition, the instrument was\nalso used to study the optical properties of secondary organic aerosol formed\nfollowing the photooxidation of isoprene. An extractive IBBCEAS instrument was\ndeveloped for detecting HONO and NO2 and had a sensitivity of about 10-9 cm-1.\nThis instrument participated in a major international intercomparison of HONO and\nNO2 measurements held in the EUPHORE simulation chamber in Valencia, Spain,\nand results from that campaign are also reported here.