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Monoterpene emissions from a Pacific Northwest Old-Growth Forest and impact on regional biogenic VOC emission estimates


Measurements of natural hydrocarbon emission rates are reported for an old-growth Pacific Northwest coniferous forest. The emission data were collected for the two dominant species Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) during the growing season in 1997 and 1998 using branch enclosure techniques. Samples were collected at different heights from 13 to 51m within the canopy using the Wind River Canopy Crane facility. The standard emission factor at a temperature of 30°C and the temperature coefficient for Douglas-fir is Es=0.39±0.14μgCg -1h-1 and β=0.14±0.05°C-1 and for western hemlock Es=0.95±0.17μgCg-1h -1 and β=0.06±0.02°C-1. There was considerable variability among all the emission factors due to seasonal and branch-to-branch variations. Within season emission factors appear to decline from May to September for the Douglas-fir, although there was no corresponding decrease for the western hemlock. There was no significant difference in standard emission factors (Es) or temperature coefficients as a function of sunlit versus shady growth environment (different heights) for Douglas-fir, but western hemlock emission samples collected low in the canopy showed no exponential correlation with temperature. Applying the standard emission factors from this study to a Pacific Northwest domain and comparing the modified emission inventory to the current regulatory-based emission inventory yielded a net decrease of 19% in the domain wide monoterpene emissions. The relatively small difference in biogenic emissions is slightly misleading, as the difference in standard emission rates between this study and current regulatory rates is quite significant, and they offset each other when combined in this domain. When this inventory was input into a regional photochemical air quality simulation using the MM5/CMAQ system, the reduction in biogenic emissions resulted in an insignificant decrease of O3 and a significant decrease in the secondary organic aerosol (domain wide -20%). The emission measurements reported here represent one of the first extensive data sets for an old-growth forest, where sampling conditions are limited to in situ enclosure techniques within the tall, elevated canopy. © 2004 Elsevier Ltd. All rights reserved.

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