MIT Center for Global Change Science
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July 1998
Submitted to the Dept. of Earth, Atmospheric and Planetary Sciences on May 1, 1998 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Atmospheric Science
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Terrestrial ecosystems have long been recognized as important sources of reactive hydrocarbons. One ecosystem type that has not previously been investigated is the subtropical evergreen forest category found in Southern China. To begin a study of this ecosystem, we conducted two campaigns. First, bi-monthly air samples were collected in stainless steel canisters over a 15 month period at Dinghushan Biosphere Reserve (DHSBR) in Guangdong, China and analyzed at MIT using capillary gas chromatography with flame ionization detection. DHSBR is both mountainous and covered 79% by forest and experiences hot, we summers and cool, dry winters. Much of the dominant subtropical vegetation in DHSBR is representative of the natural vegetation in a vast area of interior Southern China (23-32N, 100-120E), while the occasional tropical vegetation is representative of the natural vegetation of coastal Southern China. Seasonal variations in both isoprene and terpene concentrations were observed at DHSBR. Maximum isoprene mole fractions reached 7 ppb in the late summer afternoon compared to wintertime maximums of 0.1 ppb. Second, an in situ study was conducted during the summer of 1996. In addition to hourly NMHC samples, measurements of carbon monoxide (CO), nitrogen oxides (NOx), Photosynthetically Active Radiation (PAR), temperature, relative humidity, wind velocity and daily rainfall were recorded. A study using a vegetation enclosure was also performed in order to estimate NMHC emissions from the individual major tree species in the forest reserve. The resulting isoprene emission estimate for this ecosystem was 8.6 ± 6.1 mg C m-2 h-1. Using these measurements, we have assessed the sensitivity of the emissions of isoprene to environmental variables and ecosystem speciation and the importance of these emissions to regional photochemistry. Estimates of atmospheric hydroxyl radical concentrations were made from the observed decay of reactive hydrocarbons, yielding 5.7 ± 2.8 x 106 molecules cm-3. Measurement of many hydrocarbons in addition to CO, NOx and wind information allowed identification of several characteristic air mass types observed during the in situ campaign.
