Differential column measurements using compact solar-tracking spectrometers

Abstract

Abstract. We demonstrate the use of compact solar-tracking Fourier transform spectrometers (Bruker EM27/SUN) for differential measurements of the column-averaged dry-air mole fractions of CH 4 and CO 2 within urban areas. Using Allan variance analysis, we show that the differential column measurement has a precision of 0.01 % for X CO 2 and X CH 4 with an optimum integration time of 10 min, corresponding to Allan deviations of 0.04 ppm and 0.2 ppb, respectively. The sensor system is very stable over time and after relocation across the continent. We report tests of the differential column measurement, and its sensitivity to emission sources, by measuring the downwind-minus-upwind column difference X CH 4 across dairy farms in the Chino area, California, and using the data to verify emissions reported in the literature. Ratios of spatial column differences X CH 4 / X CO 2 were observed across Pasadena within the Los Angeles basin, indicating values consistent with regional emission ratios from the literature. Our precise, rapid measurements allow us to determine significant short-term variations (5-10 min) of X CO 2 and X CH 4 and to show that they represent atmospheric phenomena.Overall, this study helps establish a range of new applications for compact solar-viewing Fourier transform spectrometers. By accurately measuring the small differences in integrated column amounts across local and regional sources, we directly observe the mass loading of the atmosphere due to the influence of emissions in the intervening locale. The inference of the source strength is much more direct than inversion modeling using only surface concentrations and less subject to errors associated with small-scale transport phenomena.

Intercomparison of open-path trace gas measurements with two dual-frequency-comb spectrometers

“…Here, we find agreement between two DCS instruments that is an order of magnitude better and is comparable to that achieved with highly calibrated, state-of-the-art solar-looking FTS systems that retrieve vertical column measurements (Messerschmidt et al, 2011;Frey et al, 2015;Hedelius et al, 2016); how-ever, open-path DCS does not require instrument-specific calibrations (e.g., of the instrument line shape) and provides a very different capability by retrieving the dry mole fractions across regional, kilometer-scale paths over day and night on a mobile platform. Moreover, as the agreement between openpath DCS instruments is below the level of natural background fluctuations, future measurements can facilitate accurate inverse modeling to identify sources and sinks of carbon emission over regions.…”

“…8. Initially, the Allan deviations do follow this slope, but the atmospheric concentrations, especially of CO 2 , vary over this 6 h period, and the Allan deviations reach a floor at ∼ 1000 s. As in Chen et al (2016) it is also useful to plot the Allan deviation of the difference in retrieved concentration between the instruments, e.g., XCO 2 and XCH 4 of Fig. 6.…”

Methane emissions from dairies in the Los Angeles Basin

“…The instruments were then deployed to Chino to develop a methodology to estimate greenhouses gas emissions and improve the uncertainties on flux estimates from this major local source. Descriptions of the capacities and limitations of the mobile EM27/SUN instruments have been published in Chen et al (2016) and Hedelius et al (2016). Using Allan analysis, it has been found out that the precision of the differential column measurements ranges between 0.1 and 0.2 ppb with a 10 min averaging time .…”

“…This technique has been used to quantify emissions from regional to urban scales (Wunch et al, 2009;Stremme et al, 2013;Kort et al, 2014;Lindenmaier et al, 2014;Hase et al, 2015;Franco et al, 2015;Wong et al, 2015;Chen et al, 2016;Kille et al, 2017).…”

“…The Chino area was once home to one of the largest concentrations of dairy farms in the United States (USA), however rapid land-use change in this area may have caused CH 4 fluxes from the dairy farms change rapidly in both space and time. Chen et al (2016) used differential column measurements (downwind minus upwind column gradient X CH 4 across Chino) recorded on favorable meteorological conditions (e.g., constant wind direction) to verify emissions reported in the literature. In this study, the same column measurement network is employed in conjunction with meteorological data and a high-resolution model to estimate CH 4 emissions at Chino for several different days, including more varying wind conditions.…”