The hydroxyl radical (OH) and peroxy radicals (HO2 and RO2) are key species that drive the oxidation of trace gases and control the fate of several tropospheric pollutants. Reactions of these radical species with volatile organic compounds (VOCs) and nitrogen oxides (NO + NO2 = NOx) establish a fast radical-propagation cycle that can lead to a net production of harmful secondary pollutants such as ozone (O3) and secondary organic aerosols (SOA). Attempts to develop accurate control strategies for these pollutants require an in-depth understanding of radical chemistry, however recent measurements of radical concentrations have shown discrepancies with predictive models.

This work will describe measurements of radicals made by the Indiana University Laser- Induced Fluorescence-Fluorescence Assay by Gas Expansion (LIF-FAGE) instrument that has been described previously, and will also detail the development and initial measurements made by a Laser-photofragmentation/Laser-induced fluorescence (LP/LIF) instrument capable of nearsimultaneous detection of OH and nitrous acid (HONO), an important radical precursor. In particular, this work will detail measurements of OH, and HO2 made by the LIF-FAGE instrument in a forested environment in northern Michigan as part of the 2016 Program for Research on Oxidants, Photochemistry, and Transport (PROPHET) field campaign. An extensive set of supporting measurements, including photolysis rates and mixing ratios of NOx, VOCs, and other trace gases was used to constrain a zero dimensional box to assess our current understanding of atmospheric chemistry in remote environments characterized by high VOC mixing ratios and low mixing ratios of NOx.

In addition, the development and initial field deployment of the LP/LIF instrument will be detailed. This will include a discussion on a novel calibration method based on determining the photofragmentation efficiency (PE) of HONO by the 355-nm photolysis laser, and also includes initial measurements of HONO and OH in both a forested and urban setting. Lastly, HONO mixing ratios in the indoor environment will be analyzed through measurements made as part of the House Observations of Microbial and Environmental Chemistry (HOMEChem) field campaign. HONO behavior during a variety of common household activities will be assessed and measurements from the LP/LIF instrument are compared to simultaneous measurements from a chemical ionization mass spectrometry (CIMS) instrument. The agreement between the two instruments gives confidence in both measurement techniques and also suggests that HONO concentrations can be well mixed in indoor environments.