Development of Open-Path Incoherent BroadBand Cavity Enhanced Absorption Spectroscopy (OP-IBB-CEAS) instruments for the measurement of trace gases in the Irish Atmospheric Simulation Chamber

The study of the reactions of volatile organic compounds (VOCs) in the troposphere is important to assess the impact of biogenic and anthropogenic emissions on climate, health and the economy. The challenge of these studies lies, principally, in the difficulty of designing experiments and instruments to measure VOCs and their reaction products under conditions that are significant for the atmosphere (e.g. low concentrations, short lifetimes). The Irish Atmospheric Simulation Chamber (27 m³ Teflon cuboid) is a research facility for the kinetic and mechanistic study of VOC oxidation processes, and a series of open-path incoherent broadband cavity enhanced absorption spectroscopy (OP-IBB-CEAS) setups have been developed for the measurement of different trace gases that are relevant for the study of atmospheric processes. The open-path characteristic of the IBB-CEAS instruments, ensures that there are no sampling losses and that the concentrations measured correspond directly to those of the trace gases in the static chamber. Currently, four OP-IBB-CEAS setups have been developed for measurements of different trace gases: a) A NIR-IBB-CEAS instrument was developed for the measurement of H₂O and CO₂, b) A visible IBB-CEAS instrument centred at 662 nm for the measurement of H₂O, NO₃, and NO₂, c) A visible IBB-CEAS instrument centred at 450 nm for the measurement of (CHO)₂ and NO₂, and d) A UV IBB-CEAS instrument for the measurement of HONO, NO₂ and MACR. The effective mirror reflectivity of the UV-Vis IBB-CEAS instruments was calibrated with NO₂ in a simulated “nighttime” scenario and in a photo-stationary state, using a home-made extractive Cavity Ring-Down Setup. Absorption coefficients for the different instruments range from 10^-9 to 10^-8 cm^-1. A fast retrieval method based on singular value decomposition (SVD) was integrated into an iterative algorithm for fast and robust determination of the concentrations of the different analytes, corresponding to mixing ratios in the range of pptv (e.g. for NO₃), to ppbv (e.g. for MACR). In this presentation we will report on a series of NO_Y production experiments from the reaction of NO₂ and O₃ was conducted to benchmark the response of the IBB-CEAS instruments, and to study NO_Y photolysis. These benchmarking experiments shed some light on the importance of the transition stages between "nighttime" and “daytime” chemistry, and their possible impact on the chemistry of nitrogen oxides.