Long open-path measurements of greenhouse gases in air using near-infrared Fourier transform spectroscopy

Abstract

Abstract. In complex and urban environments, atmospheric trace gas composition is highly variable in time and space. Point measurement techniques for trace gases with in situ instruments are well established and accurate, but do not provide spatial averaging to compare against developing high-resolution atmospheric models of composition and meteorology with resolutions of the order of a kilometre. Open-path measurement techniques provide path average concentrations and spatial averaging which, if sufficiently accurate, may be better suited to assessment and interpretation with such models. Open-path Fourier transform spectroscopy (FTS) in the mid-infrared region, and differential optical absorption spectroscopy (DOAS) in the UV and visible, have been used for many years for open-path spectroscopic measurements of selected species in both clean air and in polluted environments. Near infrared instrumentation allows measurements over longer paths than mid-infrared FTS for species such as greenhouse gases which are not easily accessible to DOAS.In this pilot study we present the first open-path near-infrared (4000–10 000 cm−1, 1.0–2.5 µm) FTS measurements of CO2, CH4, O2, H2O and HDO over a 1.5 km path in urban Heidelberg, Germany. We describe the construction of the open-path FTS system, the analysis of the collected spectra, several measures of precision and accuracy of the measurements, and the results a four-month trial measurement period in July–November 2014. The open-path measurements are compared to calibrated in situ measurements made at one end of the open path. We observe significant differences of the order of a few ppm for CO2 and a few tens of ppb for CH4 between the open-path and point measurements which are 2 to 4 times the measurement repeatability, but we cannot unequivocally assign the differences to specific local sources or sinks. We conclude that open-path FTS may provide a valuable new tool for investigations of atmospheric trace gas composition in complex, small-scale environments such as cities.

Vehicle Ammonia Emissions Measured in An Urban Environment in Sydney, Australia, Using Open Path Fourier Transform Infra-Red Spectroscopy

“…The measured spectra are analysed by the program MALT (Multiple Atmospheric Layer Transmission) [41][42][43] using a nonlinear least squares fit of the measured spectra to synthetic spectra, generated by the program. MALT analyses the spectra immediately after collection, in user defined spectral windows.…”

Performance of an open-path near-infrared measurement system for measurements of CO<sub>2</sub> and CH<sub>4</sub> during extended field trials

“…The instrument described here is based on the instrument described by Griffith et al (2018), which was deployed in Heidelberg, Germany. We describe the instrument, retroreflector array and data processing and highlight the modifications relative to Griffith et al (2018).…”

“…At these path lengths, repeatability of 1 ppm CO 2 and 5 ppb for CH 4 can be achieved. Tomographic techniques for quantifying fluxes, such as described by Humphries et al (2012), could be applied to open-path measurements. Griffith et al (2018) detailed a pilot study extending OP-FTIR to measurements in the near-infrared (NIR) spectral region (4000-13 500 cm −1 ), where higher source brightness results in better collimation of the beam and theoretically longer paths.…”

“…Tomographic techniques for quantifying fluxes, such as described by Humphries et al (2012), could be applied to open-path measurements. Griffith et al (2018) detailed a pilot study extending OP-FTIR to measurements in the near-infrared (NIR) spectral region (4000-13 500 cm −1 ), where higher source brightness results in better collimation of the beam and theoretically longer paths. In addition, there are fewer interfering species, though the absorptions of CO 2 and CH 4 are weaker relative to MIR as they are overtone and combination bands.…”

Measurement of Atmospheric CO2 Column Concentrations Based on Open-Path TDLAS

“…Since the 1990s, optical spectroscopy techniques based on open-path have been employed to measure trace gas column concentrations, such as Fourier transform infrared (FTIR) spectroscopy and differential optical absorption spectroscopy (DOAS). With sunlight, a lamp, or LEDs as the light source, researchers have developed FTIR and DOAS systems for measuring column concentrations of various trace gases simultaneously [ 16 , 17 , 18 , 19 ]. However, the broadband light source is disadvantageous to the improvement of spectral resolution, which limits gas detection sensitivity.…”