Measurements of Hydroperoxy and Total Organic Peroxy Radicals in an Indiana Forest Using Laser Induced-Fluorescence

The hydroxyl (OH) and hydroperoxy (HO₂) radicals, as well as organic peroxy radicals (RO₂), play important roles in atmospheric chemistry. The OH radical initiates the oxidation of volatile organic compounds (VOCs), leading to HO₂ and RO₂ radicals whose chemistry, in the presence of nitrogen oxides (NO_x), can lead to the formation of ozone and secondary organic aerosols in the environment. Previous measurements of these radicals in forest environments characterized by high mixing ratios of isoprene and low mixing ratios of NO_x have shown serious discrepancies with modeled predictions, bringing into question our understanding of isoprene oxidation in the atmosphere.

In the summer of 2019, HO_x (=OH+HO₂), and RO_x (=HO₂+RO₂) radicals were investigated using Laser Induced Fluorescence-Fluorescence Assay by Gas Expansion (LIF-FAGE) at the Indiana University Teaching and Research Preserve (IURTP) field laboratory. The LIF-FAGE inlet was modified by attaching a 45-cm reactor to convert RO_x and HO_x species to HO₂, similar to the published ROxLIF technique (Fuchs, Holland et al. 2008). The resulting HO₂ is then converted to OH and detected inside the LIF-FAGE detection cell. Two different modes (RO_x and HO_x) are achieved by the addition of carbon monoxide (CO) and nitrogen oxide (NO), with a high conversion efficiency (~40% conversion) under low pressure conditions (~30 Torr) before reaching the detection cell. The ambient HO_x and RO_x measurements will be compared to model predictions as well as previous measurements of peroxy radical concentrations at this site. In addition to the ambient measurements, laboratory characterizations of the sensitivity of the RO_x instrument to various RO₂ radicals will be presented.

Reference:

Fuchs, H., F. Holland and A. Hofzumahaus (2008). "Measurement of tropospheric RO₂ and HO₂ radicals by a laser-induced fluorescence instrument." Rev Sci Instrum 79(8): 084104.