Residence times of air in a mature forest: observational evidence from a free-air CO<sub>2</sub> enrichment experiment
<p>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 vo...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2023-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/23/2145/2023/acp-23-2145-2023.pdf |
| _version_ | 1811165220527144960 |
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| author | E. J. Bannister E. J. Bannister E. J. Bannister M. Jesson M. Jesson N. J. Harper K. M. Hart G. Curioni X. Cai X. Cai A. R. MacKenzie A. R. MacKenzie |
| author_facet | E. J. Bannister E. J. Bannister E. J. Bannister M. Jesson M. Jesson N. J. Harper K. M. Hart G. Curioni X. Cai X. Cai A. R. MacKenzie A. R. MacKenzie |
| author_sort | E. J. Bannister |
| collection | DOAJ |
| description | <p>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 <span class="inline-formula"><i>h</i><sub>c</sub>≈25</span> 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, <span class="inline-formula"><i>u</i><sub>∗</sub></span>. 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 CO<span class="inline-formula"><sub>2</sub></span>
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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">1.4</mn><mo>≤</mo><mi mathvariant="italic">α</mi><mo>≤</mo><mn mathvariant="normal">1.8</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="65pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="7bd4b5397be209b7a59ff71628e7e3e0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-2145-2023-ie00001.svg" width="65pt" height="11pt" src="acp-23-2145-2023-ie00001.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">β</mi><mo>=</mo><msub><mi>h</mi><mi mathvariant="normal">c</mi></msub><mo>/</mo><msub><mi>u</mi><mo>∗</mo></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="a71e181c13e36b64ffa2cf75b66e68b0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-2145-2023-ie00002.svg" width="50pt" height="14pt" src="acp-23-2145-2023-ie00002.png"/></svg:svg></span></span> estimated from widely measured
flow variables. In this case, the mean value for <span class="inline-formula"><i>τ</i></span> becomes formally
defined as <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mi mathvariant="italic">τ</mi><mo mathvariant="normal">‾</mo></mover><mo>=</mo><mi mathvariant="italic">β</mi><mo>/</mo><mo>(</mo><mi mathvariant="italic">α</mi><mo>-</mo><mn mathvariant="normal">1</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="94837e76ffc260d3bacd73cff44aef35"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-2145-2023-ie00003.svg" width="66pt" height="14pt" src="acp-23-2145-2023-ie00003.png"/></svg:svg></span></span>. 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.</p> |
| first_indexed | 2024-04-10T15:33:44Z |
| format | Article |
| id | doaj.art-be62f70b77584bf39ebf40d2fac0f355 |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-04-10T15:33:44Z |
| publishDate | 2023-02-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-be62f70b77584bf39ebf40d2fac0f3552023-02-13T10:25:35ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242023-02-01232145216510.5194/acp-23-2145-2023Residence times of air in a mature forest: observational evidence from a free-air CO<sub>2</sub> enrichment experimentE. J. Bannister0E. J. Bannister1E. J. Bannister2M. Jesson3M. Jesson4N. J. Harper5K. M. Hart6G. Curioni7X. Cai8X. Cai9A. R. MacKenzie10A. R. MacKenzie11Birmingham Institute of Forest Research, University of Birmingham, Edgbaston, Birmingham, UKDepartment of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, UKnow at: Risk Management Solutions, London, UKBirmingham Institute of Forest Research, University of Birmingham, Edgbaston, Birmingham, UKDepartment of Civil Engineering, University of Birmingham, Edgbaston, Birmingham, UKBirmingham Institute of Forest Research, University of Birmingham, Edgbaston, Birmingham, UKBirmingham Institute of Forest Research, University of Birmingham, Edgbaston, Birmingham, UKBirmingham Institute of Forest Research, University of Birmingham, Edgbaston, Birmingham, UKDepartment of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, UKretiredBirmingham Institute of Forest Research, University of Birmingham, Edgbaston, Birmingham, UKDepartment of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, UK<p>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 <span class="inline-formula"><i>h</i><sub>c</sub>≈25</span> 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, <span class="inline-formula"><i>u</i><sub>∗</sub></span>. 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 CO<span class="inline-formula"><sub>2</sub></span> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">1.4</mn><mo>≤</mo><mi mathvariant="italic">α</mi><mo>≤</mo><mn mathvariant="normal">1.8</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="65pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="7bd4b5397be209b7a59ff71628e7e3e0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-2145-2023-ie00001.svg" width="65pt" height="11pt" src="acp-23-2145-2023-ie00001.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">β</mi><mo>=</mo><msub><mi>h</mi><mi mathvariant="normal">c</mi></msub><mo>/</mo><msub><mi>u</mi><mo>∗</mo></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="a71e181c13e36b64ffa2cf75b66e68b0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-2145-2023-ie00002.svg" width="50pt" height="14pt" src="acp-23-2145-2023-ie00002.png"/></svg:svg></span></span> estimated from widely measured flow variables. In this case, the mean value for <span class="inline-formula"><i>τ</i></span> becomes formally defined as <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mi mathvariant="italic">τ</mi><mo mathvariant="normal">‾</mo></mover><mo>=</mo><mi mathvariant="italic">β</mi><mo>/</mo><mo>(</mo><mi mathvariant="italic">α</mi><mo>-</mo><mn mathvariant="normal">1</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="94837e76ffc260d3bacd73cff44aef35"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-2145-2023-ie00003.svg" width="66pt" height="14pt" src="acp-23-2145-2023-ie00003.png"/></svg:svg></span></span>. 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.</p>https://acp.copernicus.org/articles/23/2145/2023/acp-23-2145-2023.pdf |
| spellingShingle | E. J. Bannister E. J. Bannister E. J. Bannister M. Jesson M. Jesson N. J. Harper K. M. Hart G. Curioni X. Cai X. Cai A. R. MacKenzie A. R. MacKenzie Residence times of air in a mature forest: observational evidence from a free-air CO<sub>2</sub> enrichment experiment Atmospheric Chemistry and Physics |
| title | Residence times of air in a mature forest: observational evidence from a free-air CO<sub>2</sub> enrichment experiment |
| title_full | Residence times of air in a mature forest: observational evidence from a free-air CO<sub>2</sub> enrichment experiment |
| title_fullStr | Residence times of air in a mature forest: observational evidence from a free-air CO<sub>2</sub> enrichment experiment |
| title_full_unstemmed | Residence times of air in a mature forest: observational evidence from a free-air CO<sub>2</sub> enrichment experiment |
| title_short | Residence times of air in a mature forest: observational evidence from a free-air CO<sub>2</sub> enrichment experiment |
| title_sort | residence times of air in a mature forest observational evidence from a free air co sub 2 sub enrichment experiment |
| url | https://acp.copernicus.org/articles/23/2145/2023/acp-23-2145-2023.pdf |
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