Fluxes of Carbonyl Sulfide at Harvard Forest EMS Tower since 2010

Carbonyl sulfide (OCS), the most abundant sulfur compound in the atmosphere, controls the
sulfur budget and aerosol loading of the stratosphere in times of low volcanic activity. OCS is
also closely tied to the vegetative carbon cycle and may provide an independent measure of the
photosynthetic uptake of carbon. However, the detailed nature of the biogeochemical cycling of
OCS throughout the seasons in terrestrial ecosystems has not been thoroughly explored. We
measured the seasonal response of the ecosystem flux of OCS above a deciduous temperate forest
using an infrared laser absorption spectrometer at Harvard Forest (MA, USA) in 2011. Fluxes
were calculated using two complementary approaches: gradient – flux (also known as flux –
gradient; January – July, 2011) and eddy covariance flux (August – December, 2011), which
agreed within the combined error for a period with both measurements. Strong uptake of OCS by
the forest was observed during most of the growing season, but significant uptake also occurred
when the deciduous leaves were not present. A strong diel cycle of mid-day OCS uptake was
observed in May/June and August/September, consistent with OCS uptake by processes parallel to
CO2 photosynthetic uptake, at the ecosystem scale, for these months. The results show that
while overall OCS and CO2 both show evidence of vegetative uptake, the OCS flux cannot be
explained by this process alone and the relationship between OCS and CO2 changes throughout the
year above this mixed deciduous forest. The data imply that terrestrial uptake of OCS, and
hence the stratospheric sulfur cycle, are potentially quite sensitive to extremes in
temperature and soil moisture.

羰基硫（Carbonyl sulfide, OCS）是大气中含量最丰富的含硫化合物，在火山活动低迷时期，它主导着平流层的硫收支与气溶胶载荷。OCS同时与植物碳循环紧密关联，或可独立表征碳的光合吸收过程。然而，陆地生态系统中OCS生物地球化学循环随季节变化的详细机制尚未得到充分探究。
2011年，研究团队于美国马萨诸塞州哈佛森林（Harvard Forest）利用红外激光吸收光谱仪（infrared laser absorption spectrometer），对温带落叶林上方的OCS生态系统通量的季节响应特征开展了观测。通量计算采用两种互补方法：梯度-通量法（亦称通量-梯度法，2011年1月至7月）与涡动协方差通量（eddy covariance flux，2011年8月至12月），两种方法在共享测量时段内的结果误差处于联合误差范围内，具有一致性。
观测期内，温带落叶林在多数生长季均表现出对OCS的强吸收效应，但在落叶覆盖缺失的时段也存在显著吸收。在5月/6月与8月/9月，观测到显著的午间OCS吸收日循环特征，这与该时段生态系统尺度上与CO₂光合吸收平行的OCS吸收过程相一致。
研究结果表明，尽管OCS与CO₂均表现出植物吸收的相关特征，但OCS通量无法仅通过该过程解释，且该温带混交落叶林上方的OCS与CO₂的季节关联关系存在动态变化。本数据集的结果暗示，陆地生态系统对OCS的吸收（进而影响平流层硫循环）可能对极端温度与土壤湿度条件具有显著敏感性。