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Quantifying the ecosystem-scale emission and deposition fluxes of biogenic volatile organic compounds (BVOC) and their oxidation products above plant canopies


Biogenic volatile organic compounds (BVOC) are one of the most important chemical classes in tropospheric chemistry associated with ozone and secondary organic aerosol formation. Nevertheless, their emission and deposition rates remain unknown for many potentially important plant species due to lack of measurements. Moreover, the number of BVOC species reported for flux measurement by current techniques has been limited to a small fraction of the number of compounds actually emitted or deposited. Thus, large uncertainties still exist in estimating the ecosystem-scale fluxes of BVOCs and their oxidation products (OVOC).

Two intensive field observation campaigns in different vegetation ecosystems took place in summer 2009 and 2010. The Biosphere Effects on AeRosols and Photochemistry EXperiment field campaign during summer 2009 (BEARPEX 2009, June 15 - July 31) was made in a Ponderosa pine plantation on the western slope of the Sierra Nevada Mountain range in California. During summer 2010 (June 25 - July 26), another intensive BVOC concentration and flux measurement campaign was conducted as part of a one-year continuous field campaign (CITRUS 2010, October 2009 - November 2010) in an orange orchard in the Central Valley of California near Visalia.

Through these two field campaigns, this dissertation presents the ecosystem-atmosphere exchange of a wide range of BVOC and OVOC by coupling state of the art VOC measurement techniques (proton transfer reaction mass spectrometry; PTR-MS, and proton transfer reaction - time of flight - mass spectrometry; PTR-TOF-MS) with a well-established micrometeorological approach (eddy-covariance method; EC). Ambient concentrations of BVOC and OVOC were simultaneously measured for investigating fluxes and vertical gradients in both field sites along with the meteorological parameters.

From BEARPEX 2009, I developed an approach of using the flux and gradient relationship from species which could be measured quickly enough to use the EC flux method (e.g. 5 Hz) to determine the eddy diffusivity (K), the apply that K to estimate the flux of species for which sufficiently fast measurements were not available to use the EC method. In the past, K had been inferred from heat, H2O and CO2 which don't have similar exchange characteristics as BVOC, thus I suggest using a universal K derived from multiple BVOC can more accurately be applied where PTR-MS measurements are conducted.

In the CITRUS field campaign we simultaneously used PTR-MS for flux measurements of 3 species and vertical concentration gradient measurements of 27 species, and PTR-TOF-MS for flux and concentration measurements of a much fuller suite of VOCs above the plant canopy. Using the PTR-TOF-MS we demonstrate for the first time that there were significant emission for 27 VOC species for which concentration is commonly measured by PTR-MS, demonstrating not only the strong potential in use of PTR-TOF-MS in EC flux application but also possible underestimation in current knowledge about BVOC emission from land vegetation. For example, acetic acid was missed in flux measurements by the PTR-MS technique; however, acetic acid was here observed to be the third largest emission of any VOC species in the Orange orchard.

I demonstrate for the first time how the application of PTR-TOF-MS can expand our observational capabilities and changes the scope of understanding of VOC exchanges at atmosphere-biosphere interface. Previous work has mostly focused on quantifying emission strength from vegetation for a limited number of BVOC species. However, the results from flux measurements performed during CITRUS 2010 show the existence of significant deposition flux of BVOC and their oxidation products in addition to their emissions. Additionally, active ecosystem-atmosphere exchanges of the vast majority of the VOCs observed (at least 494 species) were discovered by applying a new approach (time shifted absolute value covariance). Through this measurement and analysis, I proved the existence of ecosystem-scale exchanges of species showing vast majority of VOC have bi-directional flux - both upward and downward. This observation is unprecedented, and revealed that most VOC species have at least some active exchange between the biosphere and atmosphere.

The observational evidence of active VOC exchanges in two independent ecosystems in this dissertation provides significant insights of interaction between the biosphere and atmosphere, coupling the VOC exchange and photochemistry within and above the plant canopy.

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