Guo Xuehui, Pan Da, Daly Ryan W, Chen Xi, Walker John T, Tao Lei, McSpiritt James, Zondlo Mark A
Department of Civil and Environmental Engineering, Princeton University, Engineering Quad Room E208, 59 Olden St., Princeton, NJ 08544, USA.
The Engineering Research Center on Mid-InfraRed Technologies for Health and the Environment (MIRTHE), Bowen Hall, 70 Prospect Avenue, Princeton, NJ 08544, USA.
Agric For Meteorol. 2022 Nov 15;326. doi: 10.1016/j.agrformet.2022.109128.
Gas-phase ammonia (NH), emitted primarily from agriculture, contributes significantly to reactive nitrogen (N) deposition. Excess deposition of N to the environment causes acidification, eutrophication, and loss of biodiversity. The exchange of NH between land and atmosphere is bidirectional and can be highly heterogenous when underlying vegetation and soil characteristics differ. Direct measurements that assess the spatial heterogeneity of NH fluxes are lacking. To this end, we developed and deployed two fast-response, quantum cascade laser-based open-path NH sensors to quantify NH fluxes at a deciduous forest and an adjacent grassland separated by 700 m in North Carolina, United States from August to November, 2017. The sensors achieved 10 Hz precisions of 0.17 ppbv and 0.23 ppbv in the field, respectively. Eddy covariance calculations showed net deposition of NH (-7.3 ng NH-N m s) to the forest canopy and emission (3.2 ng NH-N m s) from the grassland. NH fluxes at both locations displayed diurnal patterns with midday peaks and smaller peaks in the afternoons. Concurrent biogeochemistry data showed over an order of magnitude higher NH emission potentials from green vegetation at the grassland compared to the forest, suggesting a possible explanation for the observed flux differences. Back trajectories originating from the site identified the upwind urban area as the main source region of NH. Our work highlights that adjacent natural ecosystems sharing the same airshed but different vegetation and biogeochemical conditions may differ remarkably in NH exchange. Such heterogeneities should be considered when upscaling point measurements, downscaling modeled fluxes, and evaluating N deposition for different natural land use types in the same landscape. Additional in-situ flux measurements accompanied by comprehensive biogeochemical and micrometeorological records over longer periods are needed to fully characterize the temporal variabilities and trends of NH fluxes and identify the underlying driving factors.
主要源自农业的气相氨（NH₃）对活性氮（N）沉降有显著贡献。过量的氮沉降到环境中会导致酸化、富营养化以及生物多样性丧失。陆地与大气之间的氨交换是双向的，当底层植被和土壤特征不同时，这种交换可能具有高度的异质性。目前缺乏评估氨通量空间异质性的直接测量方法。为此，我们开发并部署了两个基于量子级联激光的快速响应开路氨传感器，用于量化美国北卡罗来纳州一片落叶林和相距700米的相邻草地在2017年8月至11月期间的氨通量。这些传感器在实地分别实现了0.17 ppbv和0.23 ppbv的10赫兹精度。涡度协方差计算表明，氨向森林冠层的净沉降为（-7.3 ng NH₃-N m⁻² s⁻¹），草地的氨排放为（3.2 ng NH₃-N m⁻² s⁻¹）。两个地点的氨通量都呈现出日变化模式，中午出现峰值，下午峰值较小。同时期的生物地球化学数据显示，与森林相比，草地绿色植被的氨排放潜力高出一个数量级以上，这可能解释了观测到的通量差异。源自该地点的后向轨迹确定上风方向的城市区域是氨的主要源区。我们的工作强调，共享同一空气流域但植被和生物地球化学条件不同的相邻自然生态系统在氨交换方面可能存在显著差异。在对不同自然土地利用类型的点测量进行尺度放大、模型通量进行尺度缩小以及评估氮沉降时，应考虑这种异质性。需要进行更多的原位通量测量，并伴随更长时间的综合生物地球化学和微气象记录，以全面表征氨通量的时间变异性和趋势，并确定潜在的驱动因素。
