Saewung Kim - Biogenic Volatile Organic Compounds (BVOC)-exploring their implications in atmospheric chemistry
Many studies have reported that plants emit volatile organic compounds to the atmosphere as by-products of photosynthesis since 60s. The emission of the biogenic volatile organic compounds is estimated around ten times higher than emissions from anthropogenic sectors (Goldstein and Galbally, 2007). Therefore, photochemical oxidation processes of BVOC have significant implications on tropospheric ozone formation and aerosol formation and growth processes. As IPCC (2007) summarized (Figure 1), tropospheric ozone and aerosols are two important radiation forcing agents, affecting global climate change with medium to low scientific understanding. In this context, numerous studies have conducted to address emissions and photochemistry of BVOC during past decades. However, our current understanding is poor to constrain the effects of BVOC and their oxidation products on climate (Goldstein and Galbally, 2007).
Figure 1. A summary of radiative forcing components and their level of scientific understanding (LOSU, IPCC, 2007).
Among research topics to elucidate emissions and their oxidation processes of BVOC in the atmosphere, quantifications of unknown or unmeasured species are one of the urgent topics. Indeed, series of studies have reported direct and indirect evidences, indicating significant roles of unmeasured BVOC for controlling oxidation capacities and aerosol formation processes in forest canopies. This motivates my research, which is a collaborative effort with the biosphere-atmosphere group in the atmospheric chemistry division of NCAR. The research can be summarized in two aspects.
1. Development of measurement techniques for unmeasured BVOC with a Proton-Transfer-Reaction Mass spectrometry (PTR-MS) system.
2. Deployment of PTR-MS in forest canopies to quantify unmeasured BVOC and assess their roles in photochemistry in the forest canopies.
To attain these goals, we carefully characterized analytical characteristics of PTR-MS especially for high molecular weight compounds (atomic mass unit> 150), which are categorized as semi-volatile organic compounds (SVOC). The reason is that measurements of SVOC have been rarely conducted due to technical difficulties despite of their significances in both photochemical ozone and aerosol formation processes. The mass discrimination toward heavy molecules, especially is a one of the great challenges to measure SVOC in the atmosphere. Figure 2 indicates an empirical sensitivity curve of PTR-MS as a function of molecular weight. Mass discrimination of a quadrupole mass filter is the main reason of lower sensitivity in the high mass range. As a quantitive example, the results suggest that the sensitivity of sesquiterpene (C15H24 MW 204), one of the important biogenic SVOC is only around 20 % of benzene (C6H6 MW 78). The fact suggests that an ability to measure ambient level of SVOC (few tens pptv level) by PTR-MS should be evaluated before we deploy PTR-MS in field campaigns. For the assessments, eight different sesquiterpene compounds were investigated. After series of experiments, we have concluded that PTR-MS has enough sensitivity to measure ambient sesquiterpene in forest canopies during the summer season. (Kim et al., 2008)
Figure 2 (left). Mass transmission from three experiments as indicated by different shapes of data points. Two transmission curves, based on the fringing field theory (dashed line), and the fitting curve by a exponential function (solid line) are also indicated.
Figure 3 (right). An aerial map of Woodland Park, CO, which located around 20 miles northwest of Colorado Springs, CO.
During the summer of 2008, we deployed two PTR-MS systems in the Manitou Experimental Forest (Woodland Park, CO, Figure 3) to measure emissions of BVOC from ponderosa pine and ambient distributions of BVOC in the forest canopy. Emissions of BVOC were measured from a branch enclosure sampling system. Representative sensitivity corrected mass spectra from the branch enclosure measurement are shown in Figure 4. The upper panel indicates a mid-day averaged spectrum (11 am to 2 pm MDT) and the bottom panel indicates a mid-night averaged spectrum (11 pm to 2 am MDT). The results indicate that BVOC emissions during the day are around 100 times higher than those of the night due to more physiological activities of the plant. As indicated in the figure, the major emitted species can be identified as MBO, monoterpenes (MT), acetone, oxygenated-MT, sesquiterpenes. In addition, two unidentified species were detected in m/z+ 169 and 181 with relatively small amounts. Figure 5 indicates the mass spectra of ambient air in the forest canopy during the day (the upper panel) and the night (the lower panel). Acetone was the highest peak in the mass range of 50 amu – 210 amu in both the day and the night spectrum since acetone is known as the major oxidation product of MBO and MT, the dominant BVOC species, emitted by Ponderosa Pine. In addition, the mass spectrum of the night indicates comparable amount of oxidation product peaks despite of the lower emissions during the night. We expect that the results provide opportunities to test our current understanding on BVOC distributions and their photochemistry inside of the forest canopy with a simple photochemical box-model study.
References
Goldstein and Galbally (2007) Known and unexplored organic constituents in the Earth’s atmosphere, Environmental Science and Technology, Vol 41, 1515-1521.
IPCC (2007) Climate Change 2007: The physical science basis, Cambridge University Press.
Kim et al (2008) Measurement of atmospheric sesquiterpenes by PTR-MS, Atmospheric Measurement Technique Discussion, Vol 1, 1-33,
ASP Spotlight January 2009
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