Near-infrared open-path measurement of CO_2 concentration in the urban atmosphere

Abstract

Average concentration of carbon dioxide (CO2) has been measured over a path length of 5.1 km in the lower troposphere by the method of differential optical absorption spectroscopy (DOAS) using a near-infrared light source based on amplified spontaneous emission. The analysis of CO2 absorption intensity around 1575 nm observed during 10 days over the Chiba city area has revealed that the CO2 concentration varied in the range of around 360-450 ppmv, with presumable influence of air mass advection from nearby industrial facilities. In addition, a good correlation has been found in relative humidity values between the DOAS and meteorological station data. As a whole, the present result indicates the usefulness of such a DOAS approach for measuring the concentration of CO2 averaged over an optical path of a few kilometers in the lower troposphere.

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

“…Data from recent work using broadband DOAS and laserbased long open-path techniques are shown for comparison in Table 4. Compared to conventional DOAS with a grating monochromator, array detector and the same long path fibre-telescope optics (Sommer, 2012;Saito et al, 2015;Somekawa et al, 2011), the FTS system achieves greatly improved repeatability. Compared to more recent work with dual frequency comb laser spectroscopy (Rieker et al, 2014;Waxman et al, 2017), the repeatability is less by about a factor of two.…”

“…Previous work to extend DOAS into the NIR region, using a conventional whitelight source, monochromator and detector array, was limited by the weak absorptions and interfering spectral structures to a repeatability of approximately 30 % and uncertain accuracy for CO 2 and CH 4 (Sommer, 2012). More recently, DOAStype NIR measurements using broadband laser sources (Saito et al, 2015;Somekawa et al, 2011), and frequency comb spectroscopy (Rieker et al, 2014;Waxman et al, 2017) have been described to measure CO 2 and CH 4 in the NIR over path lengths of up to 5 km. These methods achieved a measurement repeatability of 1-4 ppm with absolute bias of up to 7 ppm for CO 2 when compared to point in situ measurements.…”

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

“…Compared to conventional DOAS with a grating monochromator, array detector and the same long path fibretelescope optics (Sommer, 2012;Saito et al, 2015;Somekawa et al, 2011), the FTS system achieves greatly improved precision and accuracy. Compared to more recent work with dual frequency comb laser spectroscopy (Rieker et al, 2014;Waxman et al, 2017), the precision is similar.…”

“…The NIR allows the use of a high temperature, bright white light source (such as quartz halogen or Xe lamp) allowing good beam collimation over kilometrescale pathlengths, but absorption strengths of the available overtone and combination vibrational spectral bands are much 30 lower than for the fundamental transitions in the mid IR. Previous work to extend DOAS into the NIR region using a conventional white-light source, monochromator and detector array was limited by the weak absorptions and interfering spectral structures to an accuracy of approximately 30% for CO 2 and CH 4 (Sommer, 2012 measurements using broadband laser sources (Saito et al, 2015;Somekawa et al, 2011), and frequency comb spectroscopy (Rieker et al, 2014;Waxman et al, 2017) have been described to measure CO 2 and CH 4 in the NIR over pathlengths of up to 5 km. These methods achieved measurement precisions of 1-4 ppm with absolute differences of up to 7 ppm for CO 2 when compared to point in situ measurements.…”

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

“…IPDA is a variant of DIAL, using topographical targets to allow integrated-path measurements. DIAL (Queisser et al, 2016) and IPDA (Wagner and Plusquellic, 2016) can be applied to integrated-path measurements of CO 2 and CH 4 . In general, laser-based techniques can have low power requirements but are only tuned around a single wavelength and therefore can typically only detect a single gas.…”