Applicability of an eddy covariance system based on a close-path quantum cascade laser spectrometer for measuring nitrous oxide fluxes from subtropical vegetable fields

The soil of subtropical vegetable fields is an important source of the atmospheric greenhouse gas nitrous oxide（N2O）. In a field study in subtropical China, the authors used an eddy covariance（EC）system based on a close-path quantum cascade laser（QCL） spectrometer to measure N2O fluxes from a vegetable field. During the experimental period from 9 October 2014 to 18 February 2015,the observed half-hourly N2O fluxes ranged from.10.7 to 1077.4 μg N m^-2h^-1, with a mean value of99.3 μg N m^-2h^-1. The detection limit（95% confidence level） of the EC system for half-hourly fluxes was estimated at 18.5 μg N m^-2h^-1, i.e. smaller than 97.5% of all measured fluxes, and within the range of the lower limit of reported N2O emissions from subtropical vegetable fields. The random uncertainties in the half-hourly fluxes were estimated at 60% on average, of which 62% was due to stochastic variations caused by turbulence and 38% by instrumental noise. The flux systematic uncertainties were estimated at.18% on average, mainly due to the spectral attenuation; however,this negative bias had already been corrected for by calculating half-hourly fluxes. In conclusion,the close-path QCL-based EC technique is capable of measuring the N2O fluxes from the subtropical vegetable fields of China with high reliability and accuracy.

亚热带蔬菜地是氧化亚氮(N_2O)的重要排放源,本研究首次采用基于闭路量子级联激光吸收光谱仪的涡动相关(QCLAS-EC)法观测亚热带蔬菜地秋冬季非施肥阶段的N_2O排放通量,以评估QCLAS-EC法测量亚热带蔬菜地N_2O通量的适用性。结果表明,QCLAS-EC观测系统在野外条件下能长期稳定运行,可观测到N_2O排放的季节变化趋势,其检测限为18.5μg N m^(-2) h^(-1)(95%置信水平),试验期间获得的97.5%的N_2O通量大于此检测限,表明QCLAS-EC观测系统可有效并准确测量亚热带蔬菜地的N_2O排放通量,仪器本身的灵敏度不是通量检测的限制因素。