Annual cycle of volatile organic compound exchange between a boreal pine forest and the atmosphere

[摘要] Long-term flux measurements of volatile organic compounds (VOC) over borealforests are rare, although the forests are known to emit considerable amountsof VOCs into the atmosphere. Thus, we measured fluxes of several VOCs andoxygenated VOCs over a Scots-pine-dominated boreal forest semi-continuouslybetween May 2010 and December 2013. The VOC profiles were obtained with aproton transfer reaction mass spectrometry, and the fluxes were calculatedusing vertical concentration profiles and the surface layer profile methodconnected to the Monin-Obukhov similarity theory. In total fluxes thatdiffered significantly from zero on a monthly basis were observed for 13 outof 27 measured masses. Monoterpenes had the highest net emission in all seasonsand statistically significant positive fluxes were detected from March untilOctober. Other important compounds emitted were methanol, ethanol+formicacid, acetone and isoprene+methylbutenol. Oxygenated VOCs showed also deposition fluxesthat were statistically different from zero. Isoprene+methylbutenol andmonoterpene fluxes followed well the traditional isoprene algorithm and thehybrid algorithm, respectively. Emission potentials of monoterpenes werelargest in late spring and autumn which was possibly driven by growth processesand decaying of soil litter, respectively. Conversely, largest emissionpotentials of isoprene+methylbutenol were found in July. Thus, we concludedthat most of the emissions of m/z 69 at the site consisted of isoprenethat originated from broadleaved trees. Methanol had deposition fluxesespecially before sunrise. This can be connected to water films on surfaces.Based on this assumption, we were able to build an empirical algorithm forbi-directional methanol exchange that described both emission term anddeposition term. Methanol emissions were highest in May and June anddeposition level increased towards autumn, probably as a result of increasingrelative humidity levels leading to predominance of deposition.

[学科分类] 地球化学与岩石