Isoprene and Monoterpene Emissions from Duke Forest: A Comparison of Ambient and Elevated CO2 Environments

A three week field campaign was conducted at the Duke Forest FACTS-1 Research Facility in Chapel Hill, NC, from September 8 through 28, 2004. A suite of volatile organic compounds (VOCs), oxygenated VOCs (OVOCs), organic and inorganic aerosols, CO2, O3, and NO were measured above the forest canopy under two different CO2 scenarios: (1) present day (Ring 1, 370 ppmv) and (2) elevated conditions (Ring 2, 570 ppmv). This study was conducted in order to determine how biogenic emissions may change in an elevated CO2 environment and what impact this will have on future air quality predictions. Approximately 700, 2-liter electropolished stainless steel canisters (University of California, Irvine) were filled hourly at both ambient CO2 (Ring 1) and elevated CO2 (Ring 2) of the FACTS-1 Research Facility. Two Proton Transfer Reaction Mass Spectrometer (PTR-MS) systems were also deployed at Duke Forest for on-line monitoring of VOCs, one was located at the Ameriflux tower (Ring 1) while the other was at the Ring 2 tower. Both PTR-MS systems continuously stepped through a series of 30 masses for VOC measurements. Each PTR-MS measured a suite of VOCs from two sampling heights (16 m and 20 m) at each ring. Both the canister samples and PTR-MS measurements revealed that isoprene levels were generally higher in Ring 2 than in Ring 1, and typically ranged from ~0-2 ppbv. On September 22 in Ring 2, levels of isoprene above the canopy from the PTR-MS measurements showed a maximum of ~6.3 ppbv. Isoprene mixing ratios in Ring 1 ranged from ~0.003 to 4.9 ppbv with mean and median values of 0.395 and 0.280 ppbv, respectively, while they varied over a wider range of 0.004-6.3 ppbv in Ring 2 with mean and median values of 0.459 and 0.287 pptv. Daily peaks appeared between 1500-2300 UT (11 AM - 7 PM LT) when the temperature and solar radiation intensity were highest. The daily maximum levels in Ring 2 were generally higher than in Ring 1, indicating enhanced isoprene emissions at the elevated CO2 ring. The isoprene oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) followed the same trend as isoprene, with levels being enhanced at Ring 2 compared to Ring 1. Significant gradients were observed from the PTR-MS measurements for the total monoterpenes in addition to the speciated monoterpenes from the canister samples at both rings. However, in contrast to isoprene, monoterpene mixing ratios were lower in the elevated CO2 environment. Canopy fluxes calculated using the measured CO2 gradient and the eddy covariance data show that isoprene fluxes were higher in Ring 2 while monoterpene fluxes were higher in Ring 1. We returned to the FACTS-1 Research Facility in June 2005 to conduct direct branch flux measurements of isoprene and monoterpenes. Teflon bag enclosures were placed on branches of Pinus taeda (loblolly pine) and Liquidambar styraciflua (sweetgum) over two 48-hour sampling periods. Ambient and post-branch enclosure samples were collected in 2-L electropolished stainless steel canisters at both Rings 1 and 2 approximately every 2 hours for each tree species. The canister analysis revealed significantly enhanced mixing ratios of isoprene and monoterpenes from ambient to post-branch enclosure. Flux calculations based on the emission per leaf area of the branch confirm our 2004 results of enhanced isoprene and suppressed monoterpene mixing ratios in an elevated CO2 environment.