Observations and modeling of formaldehyde at the PROPHET mixed hardwood forest site in 2008

We report the first field measurements of gas-phase formaldehyde (HCHO) via laser-induced fluorescence (LIF) obtained August 1-8, 2008 at the PROPHET mixed hardwood forest site at the University of Michigan Biological Station. HCHO mixing ratios above the canopy ranged from a nighttime minimum of~0.5ppb _v to afternoon maxima of ~4.2ppb _v. Daytime HCHO mixing ratios were compared to two zero-dimensional box models: one based on the Master Chemical Mechanism v3.2 (MCM), the other using a simplified treatment of HCHO production. Both models over-predicted HCHO relative to observations. Sensitivity analyses demonstrated that increasing vertical dilution and/or decreasing OH concentrations would result in a significant reduction in this over-prediction for both models, as did reducing NO concentrations in the MCM based model. On average, the two models agreed to within 9%, though sensitivity analyses of changing NO and HO ₂ concentrations result in significant deviation. This displays that modeled HCHO production is heavily-dependent on the yields and rates of RO ₂ destruction pathways under low NO conditions, which may have implications for simplified mechanisms and global predictions of isoprene emission based on satellite HCHO measurements.