Preprints
https://doi.org/10.5194/acp-2021-384
https://doi.org/10.5194/acp-2021-384

  20 May 2021

20 May 2021

Review status: a revised version of this preprint was accepted for the journal ACP.

Transport-driven aerosol differences above and below the canopy of a mixed deciduous forest

Alexander A. T. Bui1, Henry W. Wallace1,a, Sarah Kavassalis2, Hariprasad D. Alwe3, James H. Flynn4, Matt H. Erickson4,b, Sergio Alvarez4, Dylan B. Millet3, Allison L. Steiner5, and Robert J. Griffin1,6 Alexander A. T. Bui et al.
  • 1Department of Civil and Environmental Engineering, Rice University, Houston, TX, 77005, USA
  • 2Department of Chemistry, University of Toronto, Toronto, ON, M5S 3H6, Canada
  • 3Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN 55108, USA
  • 4Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, 77204, USA
  • 5Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, 48109, USA
  • 6Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX, 77005, USA
  • anow at: Washington State Department of Ecology, Lacey, WA, 98503, USA
  • bnow at: TerraGraphics Environmental Engineering, Pasco, WA, 99301, USA

Abstract. Exchanges of energy and mass between the surrounding air and plant surfaces occur below, within, and above a forest's vegetative canopy. The canopy also can lead to vertical gradients in light, trace gases, oxidant availability, turbulent mixing, and properties and concentrations of organic aerosols (OA). In this study, a high-resolution time-of-flight aerosol mass spectrometer is used to measure non-refractory submicron aerosol composition and concentration above (30 m) and below (6 m) a forest canopy in a mixed deciduous forest at the Program for Research on Oxidants: Photochemistry, Emissions, and Transport tower in northern Michigan during the summer of 2016. Three OA factors are resolved using positive matrix factorization: more-oxidized oxygenated organic aerosol (MO-OOA), isoprene-epoxydiol-derived organic aerosol (IEPOX-OA), and 91Fac (a factor characterized with a distinct fragment ion at m/z 91) from both the above- and below-canopy inlets. MO-OOA was most strongly associated with long-range transport from more polluted regions to the south, while IEPOX-OA and 91Fac were associated with shorter-range transport and local oxidation chemistry. Overall vertical similarity in aerosol composition, degrees of oxidation, and diurnal profiles between the two inlets was observed throughout the campaign, which implies that rapid in-canopy transport of aerosols is efficient enough to cause relatively consistent vertical distributions of aerosols at this scale. However, four distinct vertical gradient episodes are identified for OA, with vertical concentration differences (above-canopy minus below-canopy concentrations) in total OA of up to 0.8 μg/m3. The magnitude of these differences correlated with concurrent vertical differences in either sulfate aerosol or ozone. These differences are likely driven by a combination of long-range transport mechanisms, canopy-scale mixing and local chemistry. These results emphasize the importance of including vertical and horizontal transport mechanisms when interpreting trace gas and aerosol data in forested environments.

Alexander A. T. Bui 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-2021-384', Anonymous Referee #1, 09 Jul 2021
  • RC2: 'Comment on acp-2021-384', Anonymous Referee #2, 09 Aug 2021
  • AC1: 'Response to reviews for acp-2021-384', Robert Griffin, 20 Sep 2021

Alexander A. T. Bui et al.

Alexander A. T. Bui et al.

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Short summary
Differences in atmospheric species above and below a forest canopy provide insight into the relative importance of local mixing, long-range transport, and chemical processes on determining vertical gradients in atmospheric particles in a forested environment. This helps to understand the flux of climate-relevant material out of the forest to the atmosphere. We studied this in a remote forest using vertically resolved measurements of gases and particles.
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