Abstract
Diurnal branch-level emission rates of biogenic volatile organic compounds (BVOC) including isoprene, monoterpenes (MT), and sesquiterpenes (SQT) were determined at the University of Michigan Biological Station for the tree species red maple ( Acer rubrum), red oak ( Quercus rubra), paper birch ( Betula papyrifera), white pine ( Pinus strobus), and big tooth aspen ( Populus grandidentata). These emission rates were combined with detailed biomass distribution and meteorological data and incorporated into the canopy model, model of emissions of gasses and aerosols from nature (MEGAN), for estimating whole-canopy fluxes of isoprene. The modeled half-hour fluxes ( mg C m - 2 h - 1 ) and cumulative seasonal fluxes ( mg C m - 2 ) compared favorably with results from direct, canopy-level eddy covariance (EC) isoprene measurements; modeled cumulative seasonal flux deviated less than 30% from the EC results. Significant MT emissions were found from four of the five tree species. MT emissions from three of these were both light- and temperature-dependent and were approximately one order of magnitude greater than light-independent MT emissions. SQT emissions were identified from three of the five tree species. The model was modified to incorporate SQT and both light-dependent and light-independent MT emissions for determining fluxes. Isoprene comprised > 95 % of the total terpenoid flux with MT and SQT comprising 4% and 0.3%, respectively. The average cumulative fluxes (in mg C m - 2 ) from June through September were 2490 for isoprene, 105 for MT, and 7 for SQT. A simple box model analysis was used to estimate the contribution of the isoprene, MT, and SQT emissions to the total OH reactivity. These results confirm that isoprene dominates OH reactions especially during daytime hours. Emissions of reactive MT and SQT increase the BVOC+OH reactivity, but are still lower than estimates of BVOC fluxes at this site necessary for affecting OH reactivity to the significant degree suggested by recent reports.
Highlights
The role of biogenic volatile organic compounds (BVOCs) in atmospheric chemistry has been widely discussed and summarized (e.g. Fehsenfeld et al, 1992; Fuentes et al, 2000; Monson and Holland, 2001)
Branch-level measurements of all terpenoid species were taken from the biomass representing the footprint of the UMBSflux tower and were used to model whole-canopy BVOC fluxes and OH reactivity rates from this forest site
The isoprene emission rates in conjunction with a canopy model demonstrated that the isoprene daily and seasonal fluxes compared favorably with the directly measured values
Summary
The role of biogenic volatile organic compounds (BVOCs) in atmospheric chemistry has been widely discussed and summarized (e.g. Fehsenfeld et al, 1992; Fuentes et al, 2000; Monson and Holland, 2001). Faloona et al (2001) found that OH radical concentrations in a Michigan (MI) forest did not decay at night as rapidly as expected based on known reaction pathways and ambient BVOC measurements They speculated that there must be another OH source, possibly the ozonolysis of biogenic terpenes. Goldstein et al (2004) found that a thinning experiment in a California (CA) ponderosa pine forest greatly enhanced MT emission rates as well as ozone losses They suggested that there were hundreds of biogenic compounds measured at the branch level, only a fraction were observed in forest air and most were likely preferentially lost due to chemical reactions rather than transported out of the canopy. More conclusive BVOC measurements that substantiate the current state of knowledge regarding these processes would be valuable
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