Improved instrumental line shape monitoring for the ground-based, high-resolution FTIR spectrometers of the Network for the Detection of Atmospheric Composition Change

摘要

We propose an improved monitoring scheme for the instrumental line shape(ILS) of high-resolution, ground-based FTIR (Fourier Transform InfraRed)spectrometers used for chemical monitoring of the atmosphere by the Networkfor Detection of Atmospheric Composition Change (NDACC). Good ILS knowledgeis required for the analysis of the recorded mid-infrared spectra. The newmethod applies a sequence of measurements using different gas cells insteadof a single calibration cell. Three cells are used: cell C1 is a refillablecell offering 200 mm path length and equipped with a pressure gauge (filledwith 100 Pa NO), cells C2 and C3 are sealed cells offering 75 mm pathlength. C2 is filled with 5 Pa of pure NO. Cell C3 is filled with 16Pa NO in 200 hPa technical air, so provides pressure-broadenedNO lines. We demonstrate that an ILS retrieval using C1 improvessignificantly the sensitivity of the ILS retrieval over the currentcalibration cells used in the network, because this cell provides narrowfully saturated NO lines. The NO columns derived from C2 and C3allow the performance of a highly valuable closure experiment: adopting theILS retrieved from C1, the NO columns of C2 and C3 are derived.Because NO is an inert gas, both columns should be constant on longtimescales. Apparent changes in the columns would immediately attractattention and indicate either inconsistent ILS results or instrumentalproblems of other origin. Two different cells are applied for the closureexperiment, because the NDACC spectrometers observe both stratospheric andtropospheric gases: C2 mimics signatures of stratospheric gases, whereas C3mimics signatures of tropospheric gases.