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

Abstract

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 15× 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.

Keywords: Eddy co‐variance fluxes; Formic acid.

Figure 1

Time series of HCOOH and related measurements during PROPHET‐AMOS. Plotted is the observed (a) photosynthetic photon flux density (PPFD) and friction velocity (u*), (b) temperature and dew point, (c) rainfall and soil moisture, (d) HCOOH mixing ratios, (e) net canopy HCOOH flux, and (f) net canopy HCOOH exchange velocity. Also shown in (d)–(f) are the simulated HCOOH mixing ratios, fluxes, and exchange velocities as predicted by GEOS‐Chem (in red). For (e) and (f), the gray and orange lines show the full observational and model data sets, respectively, while the black and red show the same datasets after filtering for turbulent conditions (u* > 0.3 m/s).

Figure 2

Mean diel profiles of HCOOH (a) mixing ratios, (b) fluxes, and (c) exchange velocities as measured during PROPHET‐AMOS and simulated by GEOS‐Chem. The solid black lines and shaded gray regions show the observed mean and associated 95% confidence interval, with the corresponding model values shown in red. Shaded regions for the model fluxes and exchange velocities span upper and lower limits for in‐canopy production and loss as described in supporting information S1. Along with the measured above‐canopy flux, (b) shows the observed contributions from canopy storage (orange) and soils (brown; the shaded region here spans the range from the three chambers with exposed soil after correcting for wall effects based on the blank chamber). Also shown in (b) are the model‐derived gross upward (green) and downward (blue) fluxes.

Figure 3

Dependence of canopy HCOOH fluxes on environmental drivers. The top row shows measured (black) and modeled (red) HCOOH fluxes plotted as a function of temperature, light (photosynthetic photon flux density [PPFD]), and relative humidity (RH). For the temperature and RH plots, daytime (PPFD > ~1,500 μmol/m²/s; black and red) and other (gray and orange) values are indicated separately. The large symbols and fit lines show the mean fluxes binned by quantile of the independent variable. The thin green and blue lines show the corresponding dependencies for the modeled gross upward and downward HCOOH fluxes, respectively. The bottom row shows curtain plots of the measured HCOOH fluxes as a function of temperature, light, RH, and dew point.

Figure 4

In‐canopy vertical gradients of HCOOH and correlations with tracers of known origin. (a) Curtain plot showing the mean diel cycle of HCOOH vertical gradients for the PROPHET‐AMOS campaign. (b) Mean hourly correlations between the HCOOH vertical gradients and those for selected other VOCs: m/z 33.034 = methanol; m/z 59.050 = acetone; m/z 61.029 = acetic acid/glycolaldehyde; m/z 65.021 = CH₅O₃ ⁺; m/z 69.070 = isoprene; m/z 71.050 = MVK + MACR; m/z 73.027 = methylglyoxal; m/z 75.045 = hydroxyacetone; m/z 89.022 = C₃H₅O₃ ⁺; m/z 113.055 = C₆H₉O₂ ⁺; m/z 137.132 = monoterpenes; m/z 205.196 = sesquiterpenes.