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，它调控着平流层的硫收支与气溶胶载荷。羰基硫同时也与植物碳循环紧密关联，或可作为衡量二氧化碳（CO₂）光合吸收的独立指标。然而，陆地生态系统中羰基硫随季节变化的生物地球化学循环细节尚未得到充分探究。2011年，研究团队在美国马萨诸塞州哈佛森林（Harvard Forest），借助红外激光吸收光谱仪（infrared laser absorption spectrometer），针对温带落叶林上方的羰基硫生态系统通量开展了季节响应观测。通量计算采用两种互补方法：梯度通量法（gradient-flux，亦称通量梯度法，2011年1月至7月）与涡度协方差通量法（eddy covariance flux，2011年8月至12月），两种方法在重叠观测期的合并误差范围内结果一致。观测发现，在大部分生长季，森林对羰基硫存在强烈吸收，但在落叶林无叶期也出现了显著吸收现象。在5月/6月与8月/9月，观测到清晰的正午羰基硫吸收日循环模式，这与生态系统尺度上与CO₂光合吸收并行的羰基硫吸收过程相符。研究结果表明，尽管整体上羰基硫与CO₂均表现出植被吸收的特征，但仅靠该过程无法完全解释羰基硫通量的变化，且该温带混交林上方的羰基硫与CO₂的相关关系随季节发生改变。该数据表明，陆地生态系统对羰基硫的吸收——进而影响平流层硫循环——可能对极端温度与土壤湿度条件极为敏感。