30 May 2022
30 May 2022
Status: this preprint is currently under review for the journal ACP.

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

Edward J. Bannister1,3, Mike Jesson2,1, Nicholas J. Harper1, Kris M. Hart1, Giulio Curioni1, Xiaoming Cai3,, and A. Rob MacKenzie1,3 Edward J. Bannister et al.
  • 1Birmingham Institute of Forest Research, University of Birmingham, Edgbaston, Birmingham, United Kingdom
  • 2Department of Civil Engineering, University of Birmingham, Edgbaston, Birmingham, United Kingdom
  • 3Department of Geography, Earth, and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, United Kingdom
  • retired

Abstract. In forests, air-parcel residence times – the inverse of first-order exchange rates – influence 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 in 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 air-parcel residence times in a free-air carbon dioxide enrichment (FACE) facility in a mature, broadleaf deciduous forest with canopy height 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. Air-parcel residence times increase with increasing atmospheric stability, as does the dispersion 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 parametrisations of air-parcel 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 𝜏̅  = β / (α−1). For species released in the canopy during the daytime, chemical transformations are unlikely unless the reaction time scale is in the order of a few minutes or less.

Edward J. Bannister et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-318', Tobias Gerken, 23 Jun 2022
  • RC2: 'Comment on acp-2022-318', Anonymous Referee #2, 12 Sep 2022
  • AC1: 'Comment on acp-2022-318', Ed Bannister, 02 Nov 2022

Edward J. Bannister et al.

Data sets

Raw FACE observational data sets BIFoR FACE

Model code and software

Processing and visualisation code supporting this manuscript Edward J. Bannister

Edward J. Bannister et al.


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Short summary
In forests, air-parcel residence times influence canopy chemistry and atmospheric exchange. However, there have been few field observations. We use long-term open-air CO2 enrichment measurements to show median daytime residence times are twice as long when the trees are in leaf versus when they are not. Residence times increase with increasing atmospheric stability and scale inversely with turbulence. Robust parametrisations for large-scale models are available using common distributions.