Abstract. We present the first canopy-scale, continuous, long-term flux measurements of a suite of oxygenated volatile organic compounds (OVOCs). Fluxes were measured above a ponderosa pine plantation, adjacent to the Blodgett Forest Research Station (38ø53'42.9"N, 120ø37'57.9"W, 1315 m elevation), with a fully automated relaxed eddy accumulation (REA) system coupled to a dual GC-FID system. Quantified OVOCs included 2-methyl-3-buten-2-ol (MBO), methanol, ethanol, acetaldehyde, and acetone. These compounds were the most abundant nonmethane VOCs at this site and were highly correlated with each other, especially during daytime. Fluxes were dominated by MBO and methanol with daytime average emissions of ~ 1.3 mg C m -2 h -1. Ethanol, acetaldehyde, and acetone fluxes were approximately a factor of 5 lower. All fluxes showed diurnal cycles with maxima around noon and minima at night. Temperature and light were the main drivers for MBO emission, and the canopy level flux responses were virtually identical with previously measured leaf level fluxes from ponderosa pine trees at the same site. Ambient temperature appeared to be the most important driver of the other OVOC fluxes, but moisture also played a role, particularly for ethanol and acetone emissions, shown for the first time under field conditions. Soil and litter emissions, measured using a Pyrex glass chamber, contributed significantly to the canopy level fluxes of methanol, acetaldehyde, and acetone, and had a much smaller contribution to the canopy fluxes of ethanol. If the magnitude of these OVOC fluxes is similar in other ecosystems, they will have to be considered a major volatile organic compound emission to the atmosphere and a potentially significant carbon loss from the biosphere.

The understanding of oxidation in forest atmospheres is being challenged by measurements of unexpectedly large amounts of hydroxyl (OH). A significant number of these OH measurements were made by laser-induced fluorescence in low-pressure detection chambers (called Fluorescence Assay with Gas Expansion (FAGE)) using the Penn State Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). We deployed a new chemical removal method to measure OH in parallel with the traditional FAGE method in a California forest. The new method gives on average only 40–60% of the OH from the traditional method and this discrepancy is temperature dependent. Evidence indicates that the new method measures atmospheric OH while the traditional method is affected by internally generated OH, possibly from oxidation of biogenic volatile organic compounds. The improved agreement between OH measured by this new technique and modeled OH suggests that oxidation chemistry in at least one forest atmosphere is better understood than previously thought

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Ecosystem ozone uptake can occur through stomatal and surface deposition and through gas phase chemical reactions. In a California pine forest, thinning dramatically enhanced both monoterpene emission and ozone uptake. These simultaneous enhancements provide strong evidence that ozone reactions with unmeasured biogenically emitted volatile organic compounds (BVOCs) dominate ozone uptake, and these unmeasured BVOC emissions are approximately 10 times the measured monoterpene flux. Branch enclosure measurements confirm more than 100 BVOCs are emitted but not typically observed above the forest. These BVOCs likely impact tropospheric composition as a previously unquantified source of secondary oxygenated VOCs, organic aerosols, and OH radicals.