Harris Eliza, Nelson David D, Olszewski William, Zahniser Mark, Potter Katherine E, McManus Barry J, Whitehill Andrew, Prinn Ronald G, Ono Shuhei
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts, 02139 United States.
Anal Chem. 2014 Feb 4;86(3):1726-34. doi: 10.1021/ac403606u. Epub 2014 Jan 16.
Nitrous oxide is an important greenhouse gas and ozone-depleting-substance. Its sources are diffuse and poorly characterized, complicating efforts to understand anthropogenic impacts and develop mitigation policies. Online, spectroscopic analysis of N2O isotopic composition can provide continuous measurements at high time resolution, giving new insight into N2O sources, sinks, and chemistry. We present a new preconcentration unit, "Stheno II", coupled to a tunable infrared laser direct absorption spectroscopy (TILDAS) instrument, to measure ambient-level variations in (18)O and site-specific (15)N N2O isotopic composition at remote sites with a temporal resolution of <1 h. Trapping of N2O is quantitative up to a sample size of ∼4 L, with an optimal sample size of 1200-1800 mL at a sampling frequency of 28 min. Line shape variations with the partial pressure of the major matrix gases N2/O2 and CO2 are measured, and show that characterization of both pressure broadening and Dicke narrowing is necessary for an optimal spectral fit. Partial pressure variations of CO2 and bath gas result in a linear isotopic measurement offset of 2.6-6.0 ‰ mbar(-1). Comparison of IR MS and TILDAS measurements shows that the TILDAS technique is accurate and precise, and less susceptible to interferences than IR MS measurements. Two weeks of measurements of N2O isotopic composition from Cambridge, MA, in May 2013 are presented. The measurements show significant short-term variability in N2O isotopic composition larger than the measurement precision, in response to meteorological parameters such as atmospheric pressure and temperature.
一氧化二氮是一种重要的温室气体和消耗臭氧层物质。其来源分散且特征不明，这使得了解人为影响和制定减排政策的工作变得复杂。对一氧化二氮同位素组成进行在线光谱分析可以提供高时间分辨率的连续测量，从而为一氧化二氮的来源、汇和化学性质提供新的见解。我们展示了一种新的预浓缩装置“斯忒诺二号”，它与可调谐红外激光直接吸收光谱仪（TILDAS）相连，用于在偏远地区测量（18）O和特定位置的（15）N一氧化二氮同位素组成的环境水平变化，时间分辨率小于1小时。一氧化二氮的捕集在样品量达到约4升时是定量的，在采样频率为28分钟时，最佳样品量为1200 - 1800毫升。测量了主要基质气体N2/O2和CO2的分压对线形的影响，结果表明，为了实现最佳光谱拟合，同时表征压力展宽和狄克窄化是必要的。CO2和辅助气体的分压变化导致线性同位素测量偏移为2.6 - 6.0‰ mbar-1。红外质谱和TILDAS测量结果的比较表明，TILDAS技术准确且精确，并且比红外质谱测量更不易受到干扰。本文展示了2013年5月在马萨诸塞州剑桥对一氧化二氮同位素组成进行的为期两周的测量结果。这些测量结果表明，一氧化二氮同位素组成存在显著的短期变化，其变化幅度大于测量精度，这是对诸如大气压力和温度等气象参数的响应。
