Oxidation of Volatile Organic Compounds as the Major Biogenic Source of Formic Acid in a Mixed Forest Canopy

Formic acid (HCOOH) is the simplest and most abundant carboxylic acid in the global atmosphere; however, its formation pathways and dominant precursors are only poorly understood. While a wide range of primary and secondary HCOOH sources have been identified, none of these is sufficient to explain the high observed atmospheric concentrations across diverse atmospheric environments. Here we present eddy flux, vertical gradient, and soil chamber measurements of HCOOH conducted within a mixed forest canopy during the PROPHET-AMOS 2016 field campaign in Michigan, USA. When aggregated over the entire campaign, the total net above canopy flux was downward (into the canopy). However, the observed fluxes were bidirectional, due to counteracting gross source and sink effects, and exhibited extended periods of net upward as well as net downward exchange. Net above-canopy fluxes were mostly upward during periods of high temperature and low relative humidity. The gradient and chamber measurements reveal the canopy layer as the controlling source of HCOOH and its precursors to the forest environment, with soil and other below-canopy sources observed to be minor. We apply a correlation analysis using an ensemble of primary and secondary marker volatile organic compounds to the temporal and vertical features of the HCOOH time series, and the results point towards secondary chemical formation as the major source driving ambient HCOOH. A comparison of the observed time series with the GEOS-Chem chemical transport model indicates a several-fold model underestimate of the gross canopy HCOOH source.