光腔衰荡光谱法走航连续观测海表大气中CH_4

High-resolution measurement of CH_4 in sea surface air based on cavity ring-down spectroscopy technique: the first trial in China Seas

以基于高精度光腔衰荡光谱法(Cavity Ring-Down Spectroscopy,CRDS)的Picarro G2301型分析仪为核心,自主设计、组装,建立了适用于船载走航连续观测大气CH4的系统.采用可溯源至世界气象组织全球大气观测网(WMO-GAW)的标准气序列开展多次测试,并建立了外标法线性校正方程.测试表明,该系统对CH4响应精密度优于0.5×10-9,准确度优于0.1%,5个月内漂移小于0.3×10-9.同时,结合船载走航连续观测的特点,优化建立了观测数据处理与质量控制方法,以保证所获数据的高质量和国际可比性.分别于2011年7月和2012年5月,率先在北黄海及渤海海峡区域开展了海表大气CH4的走航连续观测.实践证明,该系统操作简便,运行稳定,适用于船载走航连续海表大气CH4的研究,可获取高精度高时空分辨的观测数据.其中,2011年7月观测到的CH4浓度范围为1823.8×10-9～2020.7×10-9,2012年5月为1887.2×10-9～2136.2×10-9,均呈渤海海峡区域浓度高,北黄海中部区域浓度低的分布特征.

For the purpose of high-resolution measurement of CH4 in sea surface air, a low drift, high-precision cavity ring-down analyzer （model： Picarro G2301） is integrated into a continuously sampling system. A set of ship-based protocols for system operation and data processing are also developed. Briefly, air is pumped from a bow intake installed 10~15 m above the sea surface to avoid contamination from the ship. With an air cleaning unit and an airflow regulator, CH4 concentrations in the sea surface air are continuously determined. This is the first trial in China Seas. Using four WMO-GAW traceable gas standards, the underway measuring system is calibrated and carefully tested over the concentration range of atmospheric methane of 1601×10^-9-2317×10^-9（dry volume ratio, the same below）. The results verify the fact that the cavity ring-down analyzer has a typical precision of 0.5×10^-9 and a low drift of 0.3×10^-9in 5 months. In July 2011 and May 2012, the newly-developed measuring system has been applied to field surveys in the Northern Yellow Sea and the Bohai Strait. Results show that the underway measuring system can easily obtain high-quality CH4 data with the relative error of less than 0.1%. The concentration ranges of atmospheric CH4 in the regions under survey are revealed at 1823.8×10^-9-2020.7×10^-9 in July 2011 and 1887.2×10^-9-2136.2×10^-9 in May 2012, respectively. The two surveys show similar distribution pattern of atmospheric CH4 that the concentration of CH4 in sea surface air above the region of Bohai Strait is significantly higher than that above the central area of the Northern Yellow Sea.