Sub-canopy Emissions of VOCs in a Mixed Forest at Borden Ontario

Understanding of the atmosphere/canopy exchange of Volatile Organic Compounds (VOCs) requires insight into deposition, emission, and chemical reactions of VOCs below the canopy. Currently,uncertainty in VOC emission inventories makes it difficult to attribute VOC concentration gradients to stomatal uptake, deposition, or sub-canopy chemistry. In the summer of 2009, over a 4-week period, VOCs were measured with a PTR-MS at 6 heights between 1 and 6 m beneath a 23-m high mixed-forest canopy. Measured VOCs included methanol, isoprene, acetone, methacrolein + methyl vinyl ketone, methylglyoxal, monoterpenes and sesquiterpenes. There are pronounced difference in the behaviour of isoprene and its biproducts and that of the terpenes. Non-terpene fluxes are predominantly downward (although methanol and methyglyoxal demonstrate some bi-directionality). In contrast, the terpene fluxes are significantly upward. This different behaviour may be due to differing chemistry, caused by OH and O3 gradients below the canopy. Alternately, the upward flux of terpenes could suggest a significant source at the surface, possibly due to litter or soil emissions. These measurements will be presented and future modelling efforts to isolate the cause of the terpene/non-terpene behavioural differences will be discussed.