. New Measurements of the Water Vapor Absorption Cross Section in the Blue- Violet Range by Cavity-Enhanced Differential Optical Absorption Spectroscopy

Abstract
The absorption cross section of water vapor in the blue-violet range (415–460 nm) is currently not well known, and many weak spectral lines are not included in either the HIgh resolution TRANsmission molecular absorption (HITRAN) database or its HIgh TEMPerature companion, HITEMP. Direct measurements of the absorption cross section of water vapor in this region have been limited by the slant column density (SCD) of gaseous water molecules that is achievable in a laboratory setting. We use cavity-enhanced differential optical absorption spectroscopy (CE-DOAS) to generate water vapor SCDs comparable to those in remote sensing field measurements under well controlled laboratory conditions. A pair of high-reflectivity (R > 0.99995) mirrors are separated by ∼80 cm to realize effective path lengths up to 16 km; water vapor is generated from deionized water in a double-bubbler system. A heat-exchanger surrounding the optical cavity allows for temperature control of the water vapor; currently, temperatures from room temperature to slightly below boiling can be accessed. Broadband light sources (e.g. an LED with a peak intensity at 455 nm) are used to measure multiple lines at moderately high spectral resolution (0.15 nm). Here we present spectra measured during a month-long measurement period, half of which was devoted to measurements at 29 °C and half of which was devoted to measuring at 59 °C. Our goal is to refine available line lists for gas-phase water by combining laboratory measurements with quantum chemical calculations; the mismatch between measured and calculated cross sections is identified as residual structures, and used to improve the calculations. We present comparisons here with a new calculated linelist called CKAPTEN.