The ability to measure sesquiterpenes (SQT; C 15 H 24 ) by a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) was investigated. SQT calibration standards were prepared by a capillary diffusion method and the PTR-MSestimated mixing ratios were derived from the counts of product ions and proton transfer reaction constants. These values were compared with mixing ratios determined by a calibrated Gas Chromatograph (GC) coupled to a Flame Ionization Detector (GC-FID). Product ion distributions

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... m soft-ionization occurring in a selected ion drift tube via proton transfer were measured as a function of collision energies. Results after the consideration of the mass discrimination of the PTR-MS system suggest that quantitative SQT measurements within 20% accuracy can be achieved with PTR-MS if two major product ions (m/z 149 + and 205 + ), out of seven major product ions (m/z 81 + , 95 + , 109 + , 123 + , 135 + , 149 + and 205 + ), are accounted for. Considerable fragmentation of bicyclic sesquiterpenes, i.e. βcaryophyllene and α-humulene, cause the accuracy to be reduced to 50% if only the parent ion (m/z 205 + ) is considered. These findings were applied to a field dataset collected above a deciduous forest at the PROPHET (Program for Research on Oxidants: Photochemistry, Emissions, and Transport) research station in 2005. Inferred average daytime ecosystem scale mixing ratios (fluxes) of isoprene, sum of monoterpenes (MT), and sum of SQT exhibited values of 15 µg m −3 (4.5 mg m −2 h −1 ), 1.2 µg m −3 (0.21 mg m −2 h −1 ), and 0.0016 µg m −3 (0.10 mg m −2 h −1 ), respectively. A range of MT and SQT reactivities with respect to the OH radical was calculated and compared to an Correspondence to: S. Kim (saewung@ucar.edu) earlier study inferring significantly underestimated OH reactivities due to unknown terpenes above this deciduous forest. The results indicate that incorporating these MT and SQT results can resolve ∼30% of missing OH reactivity reported for this site.