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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 &mu;g m&minus;2 h&minus;1 (~24 pptv m s&minus;1) was observed with a noon range of 37 to 131 &mu;g m&minus;2 h&minus;1. Enclosure experiments were performed to determine the HCHO branch (3.5 &mu;g m-2 h&minus;1) and soil (7.3 &mu;g m&minus;2 h&minus;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

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