An Informal Intercomparison of Ambient Measurements of OH, HO2, and RO2 Radicals in an Indiana Forest, Part 1: Comparison of Instrumental Measurements

Odd-hydrogen radicals such as hydroxyl (OH) and hydroperoxy (HO₂),as well as organic peroxy radicals (RO₂), play key roles in atmospheric oxidation. Reactions of these radicals initiate the oxidation of volatile organic compounds (VOCs), which 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. In addition to questioning our understanding of the atmospheric chemistry of isoprene and other biogenic VOCs under low NO_x conditions, these results bring into question the accuracy of the instrumental techniques used to measure these radicals.

In this study, OH, HO₂, and total peroxy radicals (HO₂ +RO₂) were measured during the summer of 2017 at the Indiana University Research and Teaching Preserve (IURTP) using a Laser-Induced Fluorescence-Fluorescence Assay by Gas Expansion (LIF-FAGE) instrument, two Chemical Ionization Mass Spectroscopy (CIMS) instruments, and an Ethane Chemical AMPlifier (ECHAMP) technique. The IURTP is a high-isoprene, low-NO_x forested site near the Indiana University Bloomington campus. Measurements of OH, HO₂ and total RO₂ radicals by the different techniques were compared to each other and to previous measurements of these radicals at this site. In addition, an intercomparison of different calibration techniques will be presented. The results give confidence in the accuracy of HO_x radical measurements by these different techniques.