The Fate of Biogenic Hydrocarbons within a Forest Canopy: Field Observation and Model Results

This dissertation examines the importance of within-canopy chemical processing of biogenic volatile organic compounds (BVOCs), particularly isoprene and monoterpenes (i.e., -pinene, -pinene, limonene, and camphene), in order to improve land surface parameterizations of BVOC emissions in landscape-scale models. A forest survey found that the study area in Central Virginia was composed primarily of oak trees (>400f basal area), which are strong isoprene emitters, as well as pine trees (~200f basal area), which emit a variety of monoterpenes. Vertical transects of trace gases collected within and above the forest canopy during August and September 2009 revealed that the canopy environment had high ambient mixing ratios of both isoprene (> 10 ppb) and monoterpenes (> 2 ppb). Measurements also revealed that BVOCs were consumed within the canopy, most notably after sunset. A chemistry-transport-canopy model was developed and demonstrated the potential for large within-canopy losses of both isoprene (9.1%) and monoterpenes (~17.0%) during the daytime. The within-canopy oxidation of isoprene was due primarily to the hydroxyl radical (OH) (~58%), and to lesser extent to ozone (O 3 ) (~35%) and the nitrate radical (NO 3 ) (~7%). For monoterpenes, oxidation occurred mainly with O 3 (~61%) as well as with OH (~21%) and NO 3 (~18%). A canopy reduction factor was developed to describe the net daytime loss of BVOCs, based on a sensitivity test of the model. The factors considered included the fraction of radiation reaching the top of the canopy (R solar ), cumulative leaf area index (LAI), canopy height (H c ), ozone (O 3 ) mixing ratio, and nitrogen oxide (NO x ) mixing ii ratio. On average, positive perturbations of R solar , H c , O 3 , and NO x , and negative perturbations to LAI, resulted in increased BVOC losses. A linear regression model of these factors was found to predict with high confidence (R 2 >0.93) the amount of isoprene and monoterpene lost in the canopy. Overall, this dissertation contributes new evidence that BVOCs are consumed and transformed within a forest canopy. The dynamic canopy reduction factor that was developed was shown to be a good predictor for BVOC losses, and could be applied to improve landscape-scale models of BVOC emissions.

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