Chemical processing within and above a loblolly pine forest in North Carolina, USA

Hydrocarbon species and related meteorological and chemical variables were measured within and immediately above a loblolly pine forest in North Carolina, USA during 15-18 July 2003. The degree of photochemical processing within the forest canopy of biogenic hydrocarbons emitted at the foliage level is investigated with the aid of a one-dimensional photochemical model. Such in-canopy photochemical processes remain poorly understood largely due to limited observations of plant-emitted gases, chemical reactions, and yields of photochemical reactions inside plant canopies. These hydrocarbons are vented into the overlying atmospheric boundary layer and participate in regional-scale photochemical processes. At the forested site, isoprene was the dominant sink for hydroxyl radicals and ozone precursor among all the volatile organic compounds. Abundances of many hydrocarbons peaked in the early morning and late afternoon/early evening due to local emissions, while reaching minima at mid-day due to intense turbulent mixing and vigorous photochemistry. Methyl vinyl ketone and methacrolein, which were produced mostly from isoprene oxidation, had elevated mixing ratios during noontime in addition to maximum levels in the early morning and early evening. Abundances of species with dominant biogenic origin (e.g., isoprene, α-pinene, β-pinene, and limonene) were higher within the canopy than above the forest. For the species produced in the atmospheric boundary layer due to photochemical reactions, abundances residing away from the canopy were higher than those just above the canopy in response to photochemical production and/or transport associated with advection. Within the forest canopy photochemical reactions destroyed approximately 10 % of the locally emitted isoprene. Chemically more reactive species such as limonene experienced greater rates of removal in response to in-canopy chemical processing. Model sensitivity studies indicated that nitrogen oxides limited the formation of oxidants at the forested study site.