Diurnal variability of atmospheric O 2 , CO 2 , and their exchange ratio above a boreal forest in southern Finland

[摘要] The exchange ratio (ER) between atmospheric O 2 and CO 2 is a useful tracer for better understanding the carbon budget on global andlocal scales. The variability of ER (in mol O 2  per mol CO 2 ) between terrestrial ecosystems is not well known, and there is no consensus on how to derivethe ER signal of an ecosystem, as there are different approaches available, either based on concentration ( ER atmos ) or flux measurements( ER forest ). In this study we measured atmospheric O 2 and CO 2  concentrations at two heights (23 and 125  m ) abovethe boreal forest in Hyytiälä, Finland. Such measurements of O 2 are unique and enable us to potentially identify which forest carbonloss and production mechanisms dominate over various hours of the day. We found that the ER atmos  signal at 23  m not only represents the diurnal cycle of the forest exchange but also includes other factors, including entrainment of air masses in the atmospheric boundary layer before midday, with different thermodynamic and atmospheric composition characteristics. To derive ER forest , we infer O 2  fluxesusing multiple theoretical and observation-based micro-meteorological formulations to determine the most suitable approach. Our resulting ER forest shows a distinct difference in behaviour between daytime (0.92  ±  0.17  mol mol −1 ) and nighttime(1.03  ±  0.05  mol mol −1 ). These insights demonstrate the diurnal variability of different ER signals above a boreal forest, and wealso confirmed that the signals of ER atmos and ER forest cannot be used interchangeably. Therefore, we recommend measurements onmultiple vertical levels to derive O 2 and CO 2  fluxes for the ER forest  signal instead of a single level time series of theconcentrations for the ER atmos  signal. We show that ER forest can be further split into specific signals for respiration(1.03  ±  0.05  mol mol −1 ) and photosynthesis (0.96  ±  0.12  mol mol −1 ). This estimation allows us to separate thenet ecosystem exchange (NEE) into gross primary production (GPP) and total ecosystem respiration (TER), giving comparable results to the morecommonly used eddy covariance approach. Our study shows the potential of using atmospheric O 2 as an alternative and complementary method togain new insights into the different CO 2  signals that contribute to the forest carbon budget.

[学科分类] 医学（综合）