Natural volatile organic compound emission rate estimates for U.S. woodland landscapes

Abstract

Volatile organic compound (VOC) emission rate factors are estimated for 49 tree genera based on a review of foliar emission rate measurements. Foliar VOC emissions are grouped into three categories: isoprene, monoterpenes and other VOCs. Typical emission rates at a leaf temperature of 30°C and a light intensity of 1000 μmol m −2 s −1 range from <0.1 to 70 μg C g −1 h −1 for isoprene, <0.1 to 3 μg C g −1 h −1 for monoterpenes, and < 0.5 to 5 μg C g −1 h −1 for other VOCs. Isoprene emission factors are given for biogenic emission models that incorporate canopy shading effects and thus require leaf-level emission rates and for emission models that do not include a canopy model and therefore require branch-level isoprene emission factors which already account for some shading. Landscape-level emission rates are estimated by combining emission rate factors determined for tree genera with species composition and foliar mass data. Landscape emission rate factors are determined for each of the 91 woodland landscapes in the high resolution (1.1 km) gridded land-cover database compiled by the EROS Data Center (EDC) from satellite and ancillary data. This database covers the entire contiguous United States of America. Landscape emission rates are also be determined using gridded tree distribution data, based on aerial photographs and ground measurements, such as that available in the U.S. Forest Service (USES) Easlwide Forest Inventory Database (EFID). Emission rates are reported for 41 of the 65 tree genera in the EFID including all of the most common genera. Total VOC emission rate factors for the 91 EDC woodland-cover types range from 0.8 to 11 mg C m −2 h −1 at a standard condition of 30°C and 1000 μmol m −2 s −1 . These landscape factors are based on branch-level emission factors and thus already incorporate canopy shading effects. The estimated fluxes of isoprene and monoterpenes are in relatively good agreement with field measurements of area-averaged fluxes if accurate species composition data (e.g. from the EFID) are available. Total VOC emission rate estimates range from 0.8 to 4.3 mg C m −2 h −1 for scrub woodlands and 2.2 to 11 mg C m −2 h −1 for mixed deciduous/coniferous woodlands. The chemical composition of the VOC flux ranges from 8 to 91 isoprene, 1 to 56% for monoterpenes and 8 to 73% for other VOC. On an area-weighted basis, the U.S. average total VOC emission rate factor of 5.1 mg m −2 h −1 for all woodlands is comprised of 58% isoprene, 18% monoterpenes and 24% other VOC. In comparison to previous estimates, these emission rates are generally higher for isoprene and lower for monoterpenes.