地基高分辨率傅里叶变换红外光谱反演环境大气中的CH4浓度变化

CH₄在大气中的浓度较低(～1.8 ppmv)且混合较为均匀, 不同区域浓度差较小, 其在大气中微量变化的精确观测对反演技术提出了很高要求. 基于高分辨率(0.02 cm^-1)傅里叶变换直射太阳光谱, 研究一种高精度、大尺度的CH₄浓度反演方法, 高灵敏地观测CH₄在大气强背景下的浓度变化. 先利用先验参数实现测量光谱的准确建模, 再采用非线性最小二乘光谱拟合和非线性逐次迭代相结合的方法反演CH₄的垂直柱浓度(vertical column density, VCD), 并以7885 cm^-1 O₂ 吸收窗口为参考, 反演CH₄的柱平均干空气混合比浓度(column-averaged dry air mixing ratios) XCH₄. CH₄ VCD和XCH₄拟合误差均小于1%, 且绝大多数XCH₄反演值均位于Total Carbon Column Observing Network (TCCON)规定的<0.5%区间内. 基于典型的日观测值, 研究了CH₄浓度的日变化规律, CH₄ VCD随时间变化而减少, XCH₄的日变化量小于0.02 ppmv.

Observation of ambient CH4 variations using ground-based high resolution Fourier transform solar spectrometry

Concentration of CH₄ in ambient atmosphere is quite low (～1.8 ppmv) and mixed well. So observation of CH₄ variation with high precision has to rely on high-precision of inversion technique. This paper demonstrates the observation of CH₄ variations using high resolution (0.02 cm^-1) Fourier transform solar spectrometry, in which a high precision and large scale of retrieval algorithm is investigated. In the CH₄VCD (vertical column density) retrieval, both a nonlinear least-square method and an iterative scheme are applied after the measured spectrum is modeled with various a-priori parameters. The XCH₄ (column-averaged dry air mixing ratio) is calculated by using the O₂ VCD derived from the 7885 cm^-1 O₂-A band. Fitting errors for both CH₄ VCD and XCH₄ are less than 1%, and most XCH₄ retrievals are within the <0.5% error threshold of the TCCON (total carbon column observing network). Furthermore, the daily CH₄ variations are also investigated based on one typical daily observation. It shows that the daily CH₄ VCD decreases with time and the variations of the XCH₄ is within 0.02 ppmv.