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Formic acid (HCOOH) is among the most abundant carboxylic acids in the atmosphere, but its budget is poorly understood. We present eddy flux, vertical gradient, and soil chamber measurements from a mixed forest and apply the data to better constrain HCOOH source/sink pathways. While the cumulative above-canopy flux was downward, HCOOH exchange was bidirectional, with extended periods of net upward and downward flux. Net above-canopy fluxes were mostly upward during warmer/drier periods. The implied gross canopy HCOOH source corresponds to 3% and 38% of observed isoprene and monoterpene carbon emissions and is 15x underestimated in a state-of-science atmospheric model (GEOS-Chem). Gradient and soil chamber measurements identify the canopy layer as the controlling source of HCOOH or its precursors to the forest environment; below-canopy sources were minor. A correlation analysis using an ensemble of marker volatile organic compounds suggests that secondary formation, not direct emission, is the major source driving ambient HCOOH.

Plain Language Summary Formic acid (HCOOH) is one of the most abundant acids in the atmosphere and affects the acidity of precipitation. A number of recent studies have shown that the atmospheric abundance of HCOOH is much higher than predicted, implying some unknown or underrepresented source. Here we present new measurements of HCOOH and related species above, within, and below a mixed forest canopy and use the results to investigate its sources and sinks in this ecosystem. We find that the forest is simultaneously a source and a sink of atmospheric HCOOH, and vertically resolved measurements identify the canopy layer as the major HCOOH source for this environment. Soils have been shown to be a source of HCOOH in some cases, but we show that their influence is unimportant for this ecosystem. The magnitude of the gross HCOOH source from this forest is 15 times higher than predicted in a current atmospheric model. A correlation analysis suggests that the main HCOOH source from this forest is oxidation of other compounds rather than direct emissions from vegetation.