Articles | Volume 17, issue 18
Atmos. Chem. Phys., 17, 11453–11465, 2017
Atmos. Chem. Phys., 17, 11453–11465, 2017

Research article 26 Sep 2017

Research article | 26 Sep 2017

Canopy uptake dominates nighttime carbonyl sulfide fluxes in a boreal forest

Linda M. J. Kooijmans1, Kadmiel Maseyk2, Ulli Seibt3, Wu Sun3, Timo Vesala4,5, Ivan Mammarella4, Pasi Kolari4, Juho Aalto4,6, Alessandro Franchin4,8, Roberta Vecchi7, Gianluigi Valli7, and Huilin Chen1,8 Linda M. J. Kooijmans et al.
  • 1Centre for Isotope Research, University of Groningen, Groningen, the Netherlands
  • 2School of Environment, Earth and Ecosystem Sciences, The Open University, Milton Keynes, UK
  • 3Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA
  • 4Department of Physics, University of Helsinki, Helsinki, Finland
  • 5Department of Forest Sciences, University of Helsinki, Helsinki, Finland
  • 6SMEAR II, Hyytiälä Forestry Field Station, University of Helsinki, Korkeakoski, Finland
  • 7Department of Physics, Università degli Studi di Milano and INFN, Milan, Italy
  • 8Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA

Abstract. Nighttime vegetative uptake of carbonyl sulfide (COS) can exist due to the incomplete closure of stomata and the light independence of the enzyme carbonic anhydrase, which complicates the use of COS as a tracer for gross primary productivity (GPP). In this study we derived nighttime COS fluxes in a boreal forest (the SMEAR II station in Hyytiälä, Finland; 61°51′ N, 24°17′ E; 181 m a.s.l.) from June to November 2015 using two different methods: eddy-covariance (EC) measurements (FCOS-EC) and the radon-tracer method (FCOS-Rn). The total nighttime COS fluxes averaged over the whole measurement period were −6.8 ± 2.2 and −7.9 ± 3.8 pmol m−2 s−1 for FCOS-Rn and FCOS-EC, respectively, which is 33–38 % of the average daytime fluxes and 21 % of the total daily COS uptake. The correlation of 222Rn (of which the source is the soil) with COS (average R2  =  0.58) was lower than with CO2 (0.70), suggesting that the main sink of COS is not located at the ground. These observations are supported by soil chamber measurements that show that soil contributes to only 34–40 % of the total nighttime COS uptake. We found a decrease in COS uptake with decreasing nighttime stomatal conductance and increasing vapor-pressure deficit and air temperature, driven by stomatal closure in response to a warm and dry period in August. We also discuss the effect that canopy layer mixing can have on the radon-tracer method and the sensitivity of (FCOS-EC) to atmospheric turbulence. Our results suggest that the nighttime uptake of COS is mainly driven by the tree foliage and is significant in a boreal forest, such that it needs to be taken into account when using COS as a tracer for GPP.

Short summary
Carbon cycle studies rely on the accuracy of models to estimate the amount of CO2 being taken up by vegetation. The gas carbonyl sulfide (COS) can serve as a tool to estimate the vegetative CO2 uptake by scaling the ecosystem uptake of COS to that of CO2. Here we investigate the nighttime fluxes of COS. The relationships found in this study will aid in implementing nighttime COS uptake in models, which is key to obtain accurate estimates of vegetative CO2 uptake with the use of COS.
Final-revised paper