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

Abstract. We report the first observations of formaldehyde (HCHO) flux measured via eddy covariance, as well as HCHO concentrations and gradients, as observed by the Madison Fiber Laser-Induced Fluorescence Instrument during the BEACHON-ROCS 2010 campaign in a rural, Ponderosa Pine forest northwest of Colorado Springs, CO. A median noon upward flux of ~80 μg m⁻² h⁻¹ (~24 ppt_v 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. Given the typical diurnal variability of these BVOC emissions and direct HCHO emissions, this suggests that the source of the missing flux is a process with both a strong temperature and radiation dependence.