Monthly record of the Cl and 36Cl fallout rates in a deciduous forest ecosystem in NE France in 2012 and 2013

This study aims at determining the chlorine and chlorine-36 fallout rates in an experimental beech forest site located in NE France (48°31′55″ N, 5°16′8″ E). A monthly record of Cl and 36Cl concentrations in rainfall samples collected above the canopy was performed during two years, from March 2012 to February 2014. The results show that the Cl concentrations mainly originate from sea-spray while the 36Cl concentrations originate from the stratosphere and therefore present a seasonal dependency. Abrupt and important inputs of 36Cl from the stratosphere indeed yield sharp increases of the recorded concentrations during the spring-summer. We also show that a too short sampling period might bias the determined 36Cl fallout rate. To smooth the seasonal and sporadic bursts of 36Cl, a minimum of 6 months sampling period is required. A mean 36Cl fallout rate of (77 ± 21) atoms m−2 s−1 can be deduced from our study, which is 45% higher than the modelled value. This discrepancy suggests more studies aiming at measuring the 36Cl fallout rate worldwide are necessary.