Calibration and instrumental line shape characterization of a set of portable FTIR spectrometers for detecting greenhouse gas emissions

摘要

A comprehensive calibration procedure for mobile, low-resolution,solar-absorption FTIR spectrometers, used for greenhouse gases observations,is developed. These instruments commend themselves for campaign use anddeployment at remote sites. The instrumental line shape (ILS) of eachspectrometer has been thoroughly characterized by analyzing the shape ofHO signatures in open path spectra. A setup for the external sourceis suggested and the invariance of derived ILS parameters with regard tochosen path length is demonstrated. The instrumental line shapecharacteristics of all spectrometers were found to be close to nominal.Side-by-side solar observations before and after a campaign, which involvedshipping of all spectrometers to a selected target site and back, are appliedfor verifying the temporal invariability of instrumental characteristics andfor deriving intercalibration factors for XCO and XCH,which take into account residual differences of instrumental characteristics.An excellent level of agreement and stability was found between the differentspectrometers: the uncorrected biases in XCO and XCH aresmaller than 0.01 and 0.15 %, respectively, and the drifts aresmaller than 0.005 and 0.035 %. As an additional sensitivedemonstration of the instrumental performance we show the excellent agreementof ground pressure values obtained from the total column measurements ofO and barometric records. We find a calibration factor of 0.9700 forthe spectroscopic measurements in comparison to the barometric records and avery small scatter between the individual spectrometers (0.02 %). Asa final calibration step, using a co-located TCCON (Total Carbon Column ObservationNetwork) spectrometer as areference, a common scaling factor has been derived for the XCO andXCH products, which ensures that the records are traceable to theWMO in situ scale.