Residence times of air in a mature forest: observational evidence from a free-air CO₂ enrichment experiment

Abstract

Abstract. In forests, the residence time of air – the inverse of first-order exchange rates – influences in-canopy chemistry and the exchanges of momentum, energy, and mass with the surrounding atmosphere. Accurate estimates are needed for chemical investigations of reactive trace species, such as volatile organic compounds, some of whose chemical lifetimes are on the order of average residence times. However, very few observational residence-time estimates have been reported. Little is known about even the basic statistics of real-world residence times or how they are influenced by meteorological variables such as turbulence or atmospheric stability. Here, we report opportunistic investigations of residence time of air in a free-air carbon dioxide enrichment (FACE) facility in a mature, broadleaf deciduous forest with canopy height of hc≈25 m. Using nearly 50 million FACE observations, we find that median daytime residence times in the tree crowns range from around 70 s when the trees are in leaf to just over 34 s when they are not. Residence times increase with increasing atmospheric stability, as does the spread around their central value. Residence times scale approximately with the reciprocal of the friction velocity, u∗. During some calm evenings in the growing season, we observe distinctly different behaviour: pooled air being sporadically and unpredictably vented – evidenced by sustained increases in CO2 concentration – when intermittent turbulence penetrates the canopy. In these conditions, the concept of a residence time is less clearly defined. Parameterisations available in the literature underestimate turbulent exchange in the upper half of forest crowns and overestimate the frequency of long residence times. Robust parameterisations of residence times (or, equivalently, fractions of emissions escaping the canopy) may be generated from inverse-gamma distributions, with the parameters 1.4≤α≤1.8 and β=hc/u∗ estimated from widely measured flow variables. In this case, the mean value for τ becomes formally defined as τ‾=β/(α-1). For species released in the canopy during the daytime, chemical transformations are unlikely unless the reaction timescale is on the order of a few minutes or less.

Air-parcel residence times in a mature forest: observational evidence from a free-air CO2 enrichment experiment

“…Turbulent motions transport the air from the boundary layers around the forest elements into the canopy airspace and out into the surrounding atmosphere. The properties of these turbulent motions depend on factors such as a forest's structure and the atmospheric conditions (Bannister et al, 2022a;Brunet, 2020;Finnigan, 2000). The turbulent exchange determines the extent to which a forest is ventilated, i.e.…”

“…In forests and other plant canopies, eddy-diffusivity closure is imperfect and may be unsuitable for certain applications (Bannister et al, 2022a;Finnigan, 2000;Monteith and Unsworth, 2013). However, it remains widely adopted in larger-scale models because it allows for in-canopy turbulent transfer to be estimated from a modest number of variables, without the prohibitive computational expense of more sophisticated closure schemes.…”

“…For example, in real forests, leaf and plant area indices are highly variable, reflecting species distribution, stand demographics, season (for deciduous forests) and planting density, thinning, and harvest techniques (for managed forests). Windthrow and pest or disease outbreaks can also affect LAI and LAD on scales from individual trees to large forest stands (Bannister et al, 2022a;Bréda, 2003). To our knowledge, the influence of canopy density on the residence time of air remains untested in real forests.…”

“…In complex terrain, pressure perturbations around hills induce dynamical patterns in the flow, and differential heating and cooling can cause gravity-driven flows along slopes. These phenomena strongly affect forest-atmosphere exchange (Bannister et al, 2022a;Finnigan et al, 2020), for example, by causing preferential venting in seemingly homogeneous patches of forest (Cook et al, 2004) or evidenced in anomalous fluxes, relative to the landscape averages (Chen et al, 2020). The flow dynamics around forested hills are complex and are sensitive to subtle changes in temperature, slope, and canopy structure; see Finnigan et al (2020) for detail.…”

“…However, LES remains computationally expensive and may underestimate the total boundary-layer turbulence (Grylls et al, 2020). LES is also not easy to configure to simulate conditions found in realworld forests, such as capturing structurally inhomogeneous canopies or variations in the ambient atmospheric conditions (Bannister et al, 2022a). Conversely, Eulerian approaches lend themselves more easily to site observations and physical models, as well as numerical investigations.…”