Biogenic volatile organic compound emissions from nine tree species used in an urban tree-planting program

Abstract

The biogenic volatile organic compound (BVOC) emissions of nine urban tree species were studied to assess the air quality impacts from planting a large quantity of these trees in the City and County of Denver, Colorado, through the Mile High Million tree-planting initiative. The deciduous tree species studied were Sugar maple, Ohio buckeye, northern hackberry, Turkish hazelnut, London planetree, American basswood, Littleleaf linden, Valley Forge elm, and Japanese zelkova. These tree species were selected using the i-Tree Species Selector (itreetools.org). BVOC emissions from the selected tree species were investigated to evaluate the Species Selector data under the Colorado climate and environmental growing conditions. Individual tree species were subjected to branch enclosure experiments in which foliar emissions of BVOC were collected onto solid adsorbent cartridges. The cartridge samples were analyzed for monoterpenes (MT), sesquiterpenes (SQT), and other C10–C15 BVOC using thermal desorption-gas chromatography–flame ionization detection/mass spectroscopy (GC–FID/MS). Individual compounds and their emission rates (ER) were identified. MT were observed in all tree species, exhibiting the following total MT basal emission rates (BER; with a 1−σ lower bound, upper bound uncertainty window): Sugar maple, 0.07 (0.02, 0.11) μg g−1 h−1; London planetree, 0.15 (0.02, 0.27) μg g−1 h−1; northern hackberry, 0.33 (0.09, 0.57) μg g−1 h−1; Japanese zelkova, 0.42 (0.26, 0.58) μg g−1 h−1; Littleleaf linden, 0.71 (0.33, 1.09) μg g−1 h−1; Valley Forge elm, 0.96 (0.01, 1.92) μg g−1 h−1; Turkish hazelnut, 1.30 (0.32, 2.23) μg g−1 h−1; American basswood, 1.50 (0.40, 2.70) μg g−1 h−1; and Ohio buckeye, 6.61 (1.76, 11.47) μg g−1 h−1. SQT emissions were seen in five tree species with total SQT BER of: London planetree, 0.11 (0.01, 0.20) μg g−1 h−1; Japanese zelkova, 0.11 (0.05, 0.16) μg g−1 h−1; Littleleaf linden, 0.13 (0.06, 0.21) μg g−1 h−1; northern hackberry, 0.20 (0.11, 0.30) μg g−1 h−1; and Ohio buckeye, 0.44 (0.06, 0.83) μg g−1 h−1. The following trees exhibited emissions of other C10–C15 volatile organic compounds (VOC): Littleleaf linden, 0.15 (0.10, 0.20) μg g−1 h−1; Ohio buckeye, 0.39 (0.14, 0.65) μg g−1 h−1; and Turkish hazelnut, 0.72 (0.49, 0.95) μg g−1 h−1. All tree species studied in this experiment were confirmed to be low isoprene emitters. Compared to many other potential urban tree species, the selected trees can be considered low to moderate BVOC emitters under Colorado growing conditions, with total emission rates one-tenth to one-hundredth the rates of potential high-BVOC emitting trees. The emissions data were used to estimate the impact of this targeted tree planting on the urban BVOC flux and atmospheric VOC burden. Selecting the low-emitting tree species over known high BVOC emitters is equivalent to avoiding VOC emissions from nearly 500,000 cars from the inner city traffic.