Nighttime vegetative uptake of carbonyl sulfide (COS) can exist due to theincomplete closure of stomata and the light independence of the enzymecarbonic anhydrase, which complicates the use of COS as a tracer for grossprimary productivity (GPP). In this study we derived nighttime COS fluxes ina 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 twodifferent methods: eddy-covariance (EC) measurements ()and the radon-tracer method (). The total nighttime COSfluxes averaged over the whole measurement period were −6.8 ± 2.2 and−7.9 ± 3.8 pmol m s for and, respectively, which is 33–38 % of the average daytime fluxes and 21 % of the total daily COS uptake. The correlation ofRn (of which the source is the soil) with COS (average = 0.58) was lower than with CO (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 () 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.
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