Impact of Prolonged Elevated CO2 on the Emission of Reactive BVOCs from Mature Forest.

Terpenoids are volatile organic compounds (VOCs) with chemical structures of C_5nH_8n. Models estimate that the biosphere directly emits approximately 600 Tg terpenoids into atmosphere annually.¹ Once released, these compounds undergo oxidation reactions with O₃ and OH; leading to the formation of secondary organic aerosols (SOA), which can impact cloud formation and Earth’s albedo.²

 

Whilst observations show that modelled future increases in atmospheric CO₂ will suppress isoprene (C₅H₈) emissions, the impact on the emission of larger terpenoids such as monoterpenes (C₁₀H₁₆) and sesquiterpenes (C₁₅H₂₄) vary between studies.³ Furthermore, not only do larger compounds (particularly sesquiterpenes), often go unidentified in forest VOC studies,⁴ these compounds also have limited measurements for their reaction rate coefficients with OH and O₃, adding uncertainty to the impact of these emissions on atmospheric oxidative capacity.⁵

 

We present a study carried out at a forested  Free Air Carbon Dioxide Enrichment  (FACE) site at the Birmingham Institute for Forest Research. Here, mature 150-year-old Quercus robur (pedunculate oak) trees have been exposed to elevated CO₂ treatment at 150 ppm above ambient for a prolonged 8 year period.  We deployed a Proton-Transfer-Reaction Mass-Spectrometer (PTR-MS) to quantify biogenic VOCs emitted under both ambient and elevated CO₂ (410 ± 10 and 560 ± 20 ppm CO₂ respectively). Measurements of ozone reactivity (k_O3) were also carried out using the homebuilt Total Ozone Reactivity System (TORS).⁶ Our results show a decrease in the emission of C₅H₈ under elevated CO₂, but an increase in the emission of larger, more reactive C₁₅H₂₄ compounds; which drive a doubling in the k_O3 measured. This is in contrast to some model assumptions that elevated CO₂ will decrease all reactive VOC emissions.