Ozone deposition over a boreal lake by the eddy covariance method

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http://urn.fi/URN:NBN:fi-fe201804208580
Title: Ozone deposition over a boreal lake by the eddy covariance method
Author: Fung, Pak Lun
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Publisher: Helsingin yliopisto
Date: 2018
Language: eng
URI: http://urn.fi/URN:NBN:fi-fe201804208580
http://hdl.handle.net/10138/273491
Thesis level: master's thesis
Discipline: Physics
Fysiikka
Fysik
Abstract: Deposition is the main removal process of ground-level ozone. In some boreal areas, lakes accounts for up to 30% of the areas. However, there have been only few studies on ozone deposition over lake water in the past forty years. So far only one study has measured the ozone deposition velocity over lake (vd) with the eddy covariance techniques. The 42-day campaign described in this thesis was held in August and September in 2012 in the Lake Kuivajärvi at SMEAR II station in Hyytiälä, Finland. The results showed a mean vd of 0.88±0.05 mm s–1, which was one-fifth of that over forest. vd performed a weak diurnal cycle over lake which had a peak during the nighttime, while the forest showed the opposite. The lake data was classified into daytime and nighttime by a threshold of solar elevation angle (–2°). The two sets of data differed statistically by Mann-Whitney U test. A further analysis showed the higher vd at night might be attributed to the more unstable atmospheric condition. Although, there is no evidence supporting a correlation of vd with stability of the mixing layer in the lake, the dominance of mechanically-induced turbulence appeared to suppress vd. By comparing with previous studies, elevated wind did not facilitate the rate of ozone deposition as expected.


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