First direct measurements of formaldehyde flux via eddy covariance: Implications for missing in-canopy formaldehyde sources

A median noon upward flux of ~80 μg m⁻² h⁻¹ (~24 pptv m s⁻¹) was observed with a noon range of 37 to 131 μg m⁻² h⁻¹.

Enclosure experiments were performed to determine the HCHO branch (3.5 μg m⁻² h⁻¹) and soil (7.3 μg m⁻² h⁻¹) direct emission rates in the canopy. A zero-dimensional canopy box model, used to determine the apportionment of HCHO source and sink contributions to the flux, underpredicted the observed HCHO flux by a factor of 6. Simulated increases in concentrations of species similar to monoterpenes resulted in poor agreement with measurements, while simulated increases in direct HCHO emissions and/or concentrations of species similar to 2-methyl-3-buten-2-ol best improved model/measurement agreement.