The Michigan Observations of the Below Canopy ATmosphere (MOBCAT) field campaign was conducted during the summer of 2022 at the University of Michigan Biological Station near Pellston, MI. Near simultaneous measurements of nitrous acid (HONO) and hydroxyl (OH) radical concentrations were made near the ground surface using a laser-photofragmentation laser-induced fluorescence instrument in addition to other supporting chemical and meteorological measurements. HONO is known to be a significant source of the highly reactive OH radical, and reactions of OH with volatile organic compounds (VOCs) can lead to the production of ozone and secondary organic aerosols (SOA) which can impact human health and welfare. Current models overestimate ozone production over forests, bringing into question our understanding of the production of ozone in these environments. Despite its clear importance for societal and ecosystem health, our understanding of HONO sources and its impact on radical and ozone production is not well understood, with an unknown source commonly reported. Measurements of HONO and OH at this site will be presented, and the measured HONO/OH ratio will be used to estimate the strength of unknown HONO sources in this environment. In addition, a zero-dimensional box model will be used to determine the importance of surface HONO emissions on OH radical production below the forest canopy.

Nitrous acid (HONO) as a source of radicals below a forest canopy: Model and observation comparison

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