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Aerosol particle dry depositionto canopy and forest floor measured by two-layer eddy covariancesystem
Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
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2009 (English)In: Journal of Geophysical Research, ISSN 0148-0227, Vol. 114Article in journal (Refereed) Published
Abstract [en]

We present, to our knowledge for the first time, results from subcanopy (forest trunk space) aerosol flux measurements with the eddy covariance (EC) technique. The measurements were performed at the SMEAR II measurement station located in a Scots pine forest in southern Finland during spring 2003 when the ground was snow covered and in situ biogenic particle formation took place almost every day. Spectral analysis showed that the EC method can be applied to estimate subcanopy aerosol fluxes and thereby ground deposition in a forest. By using the two-layer EC measurements we were able to investigate the fraction of the total aerosol deposition taking place in the canopy and on the forest floor. We found that the ratio of subcanopy to above-canopy flux is dependent on the strength of turbulence. When the friction velocity was low (u * < 0.25 m s−1), approximately 35% of the depositing particles penetrated the canopy and deposited on the ground. When u * increased, the fractional deposition on the forest floor decreased almost linearly, and in strongly turbulent conditions (u *>1.0 m s−1) the ground deposition contributed only around 10% to the total deposition. Thus, it seems that increased turbulence enhances the importance of the canopy for aerosol deposition and the relative importance of the forest floor to the total deposition diminishes.

Place, publisher, year, edition, pages
2009. Vol. 114
URN: urn:nbn:se:su:diva-35715DOI: 10.1029/2008JD010663ISI: 000263612500005OAI: diva2:287818
Available from: 2010-01-19 Created: 2010-01-19 Last updated: 2010-04-19Bibliographically approved
In thesis
1. Aerosol Exchange between Forests and the Atmosphere: Fluxes over a Tropical and a Boreal Forest
Open this publication in new window or tab >>Aerosol Exchange between Forests and the Atmosphere: Fluxes over a Tropical and a Boreal Forest
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main goal of this thesis was to investigate primary biogenic aerosol emission from the Amazon rain forest through measurements of vertical turbulent aerosol number fluxes. In addition, the particle dry deposition sink has been analysed and quantified, and is also compared with the dry deposition sink at a boreal forest site.

The total aerosol number flux of particles with diameter larger than 10 nm was dominated by downward fluxes at the rain forest site, even in the most pristine conditions in the wet season. This is an indication that the primary biogenic aerosol number source is small when considering the total particle size spectrum. However, size resolved aerosol number fluxes indicated net emission for particles with dry diameter 0.5-2.5 μm in clean conditions. These emission fluxes are likely explained by a primary biogenic aerosol source from the rain forest and seemed to be best correlated with horizontal wind speed, peaking during afternoon. Even though there are few particles in this diameter interval, typically one particle per cm3, they could potentially play an important role as giant nuclei in warm rain initiation.

Average particle number based dry deposition velocities over the whole aerosol population were lower at the rain forest site than at the boreal forest site. The reasons are likely the high fraction of accumulation mode particles at the rain forest site and low wind speeds in the tropics compared to the midlatitudes.

This thesis provides a relation describing emission of particles with diameter 0.5-2.5 μm from the rain forest, as a function of wind speed. In addition, linear equations relating average dry deposition velocity of the total aerosol number population to friction velocity are suggested for both the wet and dry season at the rain forest site. Finally, this thesis provides a relationship between dry deposition velocity of particles within the diameter range 0.25-0.45 μm, friction velocity and particle diameter.

Place, publisher, year, edition, pages
Stockholm: Department of Applied Environmental Science (ITM), Stockholm University, 2010. 42 p.
National Category
Natural Sciences
Research subject
Applied Environmental Science
urn:nbn:se:su:diva-38544 (URN)978-91-7447-051-2 (ISBN)
Public defence
2010-05-28, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.Available from: 2010-05-06 Created: 2010-04-19 Last updated: 2011-05-25Bibliographically approved

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