Sesquiterpene emissions from pine trees - Identifications, emission rates and flux estimates for the contiguous United States

Biogenic volatile organic compound (BVOC) emissions were studied using vegetation enclosure experiments. Particular emphasis was given to sesquiterpene compounds (SQT), although monoterpenes (MT) were also characterized. SQT were detected in emissions from seven (out of eight) pine species that were examined. Thirteen SQT compounds were identified; the most abundant ones were β-caryophyllene, α-bergamotene, β-farnesene, and α-farnesene, with emission rates increasing exponentially with temperature. Regression analysis yielded exponential dependencies of both MT and SQT emissions on temperature of the form E = E₀ × exp(β(T - T₀)). This resulted in SQT basal emission rates (E₀ defined at T₀ = 30 °C) ranging between <4 and 620 ng (carbon) gdw^-1 h^-1 (gdw = gram dry weight). The average value of the exponential temperature response factor β for SQT emissions, taken from all experiments, was 0.17 °C^-1, whereas the value for monoterpenes was 0.11 °C^-1. The average, total SQT emissions from pines were estimated to be 9, 16, and 29% of the MT emissions at 20, 30, and 40 °C respectively. The emission factors and β-factors determined from these measurements were used to estimate pine tree MT and SQT emission distributions for the contiguous United States using MEGAN (model of emissions of gases and aerosols from nature, Guenther et al., 2006). SQT fluxes reaching 10-40 mg m^-2 for the month of July were estimated for extensive areas of most western and southern U.S. states.