Eddy-covariance and biometeorological methods show significant net annual carbon uptake in an old-growth Douglas-fir 64 forest in southwestern Washington, USA. These results contrast with previous assumptions that old-growth forest ecosystems are in carbon equilibrium. The basis for differences between conventional biomass-based carbon sequestration estimates and the biometeorologic estimates are discussed. Annual net ecosystem exchange was comparable to younger ecosystems at the same latitude, as quantified in the AmeriFlux program. Net ecosystem carbon uptake was significantly correlated with photosynthetically active radiation and air temperature, as well as soil moisture and precipitation. Optimum ecosystem photosynthesis occurred at relatively cool temperatures (5°-10°C). Understory and soil carbon exchange always represented a source of carbon to the atmosphere, with a strong seasonal cycle in source strength. Understory and soil carbon exchange showed a Q 10 temperature dependence and represented a substantial portion of the ecosystem carbon budget. The period of main carbon uptake and the period of soil and ecosystem respiration are out of phase, however, and driven by different climatic boundary conditions. The period of strongest ecosystem carbon uptake coincides with the lowest observed values of soil and ecosystem respiration. Despite the substantial contribution of soil, the overall strength of the photosynthetic sink resulted in the net annual uptake. The net uptake estimates here included two correction methods, one for advection and the other for low levels of turbulence.

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Abstract. To understand the Korean Peninsula's carbon dioxide (CO2) emissions and sinks as well as those of the surrounding region, we used 70 flask-air samples collected during May 2014 to August 2016 at Anmyeondo (AMY; 36.53∘ N, 126.32∘ E; 46 m a.s.l.) World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) station, located on the west coast of South Korea, for analysis of observed 14C in atmospheric CO2 as a tracer of fossil fuel CO2 contribution (Cff). Observed 14C ∕ C ratios in CO2 (reported as Δ values) at AMY varied from −59.5 ‰ to 23.1 ‰, with a measurement uncertainty of ±1.8 ‰. The derived mean value Cff of (9.7±7.8) µmol mol−1 (1σ) is greater than that found in earlier observations from Tae-Ahn Peninsula (TAP; 36.73∘ N, 126.13∘ E; 20 m a.s.l., 28 km away from AMY) of (4.4±5.7) µmol mol−1 from 2004 to 2010. The enhancement above background mole fractions of sulfur hexafluoride (Δx(SF6)) and carbon monoxide (Δx(CO)) correlate strongly with Cff (r>0.7) and appear to be good proxies for fossil fuel CO2 at regional and continental scales. Samples originating from the Asian continent had greater Δx(CO) : Cff(RCO) values, (29±8) to (36±2) nmol µmol−1, than in Korean Peninsula local air ((8±2) nmol µmol−1). Air masses originating in China showed (1.6±0.4) to (2.0±0.1) times greater RCO than a bottom-up inventory, suggesting that China's CO emissions are underestimated in the inventory, while observed RSF6 values are 2–3 times greater than inventories for both China and South Korea. However, RCO values derived from both inventories and observations have decreased relative to previous studies, indicating that combustion efficiency is increasing in both China and South Korea.

Local advection of scalar quantities such as heat, moisture, or carbon dioxide occurs not only above inhomogeneous surfaces but also within roughness elements on these surfaces. A two-dimensional advection–diffusion equation is applied to examine the fractionation of scalar exchange into horizontal advection within a canopy and vertical turbulent eddy transport at the canopy top. Simulations were executed with combinations of various wind speeds, eddy diffusivities, canopy heights, and source strengths. The results show that the vertical turbulent fluxes at the canopy top increase along the fetch and approach a limit at some downstream distance. The horizontal advection in the canopy is maximum at the edge of canopy and decreases exponentially along the fetch. All cases have the same features, except the absolute quantities depend on the environmental conditions. When the horizontal axis is normalized by using the dimensionless variable xK/uh2, horizontal diffusion is negligible, and the upwind concentration profile is constant, the curves of horizontal advection and vertical flux collapse into single, unique lines, respectively (x is the horizontal distance from the canopy edge, K is the eddy diffusivity, u is the wind speed, and h is the canopy height). The ratios of horizontal advection to the vertical turbulent flux also collapse into one universal curve when plotted against the dimensionless variable xK/uh2, irrespective of source strength. The ratio R shows a power-law relation to the dimensionless distance [R = a(xK/uh2)−b, where a and b are constant].