A study of formaldehyde chemistry above a forest canopy

Gas-phase formaldehyde (HCHO) Was measured at a mixed deciduous/coniferous forest site as a part of the PROPHET 1998 summer field intensive. For the m easurement period of July 11 through August 20, 1998, formaldehyde mixing r atios ranged from 0.5 to 12 ppb at a height similar to 10 m above the fores t canopy, with the highest concentrations observed in southeasterly air mas ses. Concentrations varied on average from a mid-afternoon maximum influenc ed by photochemical production of 4.0 ppb, to a late night minimum of 2.2 p pb, probably resulting from dry depositional loss. An analysis of local HCH O sources revealed that isoprene was the most important of the measured for maldehyde precursors, contributing, on average, 82% of the calculated midda y HCHO production rate. We calculate that the nighttime HCHO dry deposition velocity is 2.6 times that of ozone, or approximately 0.65 cm/s. In the da ytime, photolysis, dry deposition, and reaction with hydroxyl radical (OH) are roughly equally important as loss processes. Explicit calculations of H CHO chemical behavior highlighted the probable importance of transport and surface deposition to understanding the diel behavior of formaldehyde.