Atmospheric Observations of Abundant Formaldehyde Above a Coniferous Forest Provides Further Evidence of Rapid Oxidation of Arboreal Hydrocarbons

Atmospheric gaseous formaldehyde (HCHO) was measured using Quantum Cascade Laser Spectroscopy (QCLS) directly above a coniferous forest in the California Sierra Nevada during the BEARPEX 2007 (Biosphere Effects on Aerosol and Photochemistry Experiment) from September 16th to October 8th of 2007. Previous work at the site [Holzinger et al., 2005] suggested the presence of large quantities of oxidation products derived from the rapid photochemical processing of biogenic VOCs near the canopy top. The large HCHO fingerprint implicit in such a mechanism was sought. We therefore attempt to explain the observed HCHO concentrations by quantifying the various known photochemical production and loss terms of its budget. Chemical production terms include the oxidation of various biogenic VOCs (isoprene, monoterpenes, MBO, MVK, MACR, PAN, etc.) initiated by both, O3 and OH. The main sinks of HCHO (photolysis, reaction with OH, and dry deposition) are also calculated. In addition, careful consideration of the daytime advection of HCHO from an upwind oak stand, known to be a significant regional isoprene source, is presented. Extrapolation of the afternoon HCHO gradient to the nighttime budget is used to constrain and better estimate the magnitude of advection on the HCHO budget. The budget imbalance could be explained by rapid photochemical oxidation of extremely reactive biogenic VOCs that were not directly observed in the experiment. Finally, a relationship between the observed ozone production rate and the product of the HCHO photolysis rate constant, [NO], and [HCHO] is presented.