Monitoring of Atmospheric Hydrogen Peroxide in Houston Using Long Path-Length Laser-Based Absorption Spectroscopy

Abstract:

Hydrogen peroxide (H₂O₂) is a relevant atmospheric species mainly formed by recombination of hydroperoxyl radicals. H₂O₂ participates in the formation of sulfate aerosol by in-cloud oxidation of S(IV) to S(VI) and has been associated with the generation of multi-functional water soluble organic compounds in atmospheric particulate matter. Furthermore, H₂O₂ plays an important role in the oxidative capacity of the atmosphere as it acts as a reservoir for HO_x radicals (OH and HO₂). Particular conditions in the Houston area (e.g. extensive presence of petrochemical industry and high ozone and humidity levels) indicate the potential relevance of this species at this location. Despite its atmospheric relevance, no reports on the levels of H₂O₂ in Houston have been presented previously in the scientific literature. Determination of atmospheric H₂O₂ usually has been conducted based on transfer of the gas-phase H₂O₂ to the liquid phase prior to quantification by techniques such as fluorescence spectroscopy. Although these methods allow detection of H₂O₂ at the sub-ppb level, they present some limitations including the interference from other atmospheric constituents and potential sampling artifacts. In this study, a high sensitivity sensor based on long-path absorption spectroscopy using a distributed-feedback quantum cascade laser was developed and used to conduct direct gas-phase H₂O₂ monitoring in Houston. The sensor, which targets a strong H₂O₂ absorption line (~7.73 µm) with no interference from other atmospheric species, was deployed at a ground level monitoring station near the University of Houston main campus during summer 2014. The performance of this novel sensor was evaluated by side-by-side comparison with a fluorescence-based instrument typically used for atmospheric monitoring of H₂O₂. H₂O₂ levels were determined, and time series of H₂O₂ mixing ratios were generated allowing insight into the dynamics, trends, and atmospheric inter-relations of H₂O₂ in the Houston area.