A cryogen-free automated measurement system of 2 stable carbon isotope ratio of atmospheric methane 3

Although methane plays an important role in climate change and atmospheric 35 chemistry, its global budget remains quantitatively uncertain due mainly to a wide variety 36 of source types. The stable carbon isotope ratio of atmospheric methane ( δ 13 C-CH 4 ) is 37 useful for separating contributions of different source categories, but due to the complex 38 and laborious analysis, limited measurement data exists. We present a new system for 39 δ 13 C-CH 4 measurement, optimized for the automated analysis of air samples. Although 40 the system is designed in principle similarly to those in previous studies, we successfully 41 set up the system with no use of cryogens (e.g. liquid nitrogen) and attained 42 reproducibility sufficient to analyze atmospheric variations (~0.1 ‰). We performed 43 automated continuous measurements of ambient air outside our laboratory at about 44 hourly intervals for 2 months, which characterized imprint of local methane sources well. 45 Future measurement operation for flask air samples from existing atmospheric 46 monitoring programs will provide a large number of atmospheric δ 13 C-CH 4 data.

isotope ratio δ 13 C (and similarly for δ 2 H) is commonly reported using the delta notation: 67 (1) 68 where R represents the atomic ratio 13 C/ 12 C in the sample or the standard. Measured atmospheric chemistry transport models (e.g. Mikaloff Fletcher et al. 2004). However, given 80 the heterogenous spatiotemporal distribution of CH4 sources and their co-locations, more 81 data to fill the data scarce regions, not only from conventional remote marine boundary 82 layer-based stations but also from sites with substantial imprint from regional sources, are 83 needed so as to detect signals of CH4 emission changes. 84 Measurements of atmospheric δ 13 C-CH4 has been made by offline conversion of CH4 to 85 carbon dioxide (CO2) and subsequent dual-inlet isotope ratio mass spectrometry (DI-IRMS) 86 until the 1990s (e.g. Stevens and Rust, 1982;Lowe et al. 1991;Quay et al. 1999). From the   Here we present a new CF-IRMS measurement system for δ 13 C-CH4 in air installed at the 95 National Institute for Environmental Studies (NIES). The advantage of our system is the        The coiled trap tubing with the temperature sensor has direct surface contact to the cold 158 block inside an insulated enclosure, which was developed based on the work by Saito and 159 Yokouchi (2008). A cooler-based system is not disrupted by the replenishment of liquid 160 nitrogen, and therefore has an advantage in automated and unattended operation.     187 We use a GS-CarbonPLOT column (0.32 mm ID, 30 m length, 3 μm film thickness, Agilent  line that was eventually converted to CO2, appears at retention time ~1060 sec. Before the 207 CH4-derived peak, peaks of air (N2 and O2) and Kr origins appear at ~880 and ~970 sec, 208 Fig. 2 10 respectively, and a peak from CO2 in sample air that was not completely removed by the 209 analytical line appears at ~1150 sec. The rectangular peaks repeated 10 times until ~600 210 sec are injections of CO2 reference gas from the reference open split and the last peak is 211 used for reference of the sample peak. Currently, we determine sample δ 13 C-CH4 values 212 against a δ 13 C value assigned for the CO2 reference gas. The CO2 reference gas was 213 calibrated by dual-inlet IRMS measurements on the NIES δ 13 C-CO2 scale (Mukai 2005).

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Our calibration strategy to reference δ 13 C-CH4 measurements to the VPDB scale will be 215 described in section 3.5.  To examine preconcentration efficiency in terms of HayeSep D amount and of the trap's 227 shape, we made prototypes of T1 and tested them with a flame ionization detector (FID).

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The FID was connected after the main separation column (replaced combustion furnace 229 and the subsequent components of Fig. 1), which precludes uncertainty that could originate  Figure 3 shows the response of the current T1 trap to the T1 setpoint temperature. We find 244 stable δ 13 C-CH4 and peak area (m/z = 44) values at −125 °C and below. This trap was also 245 examined for response to sample air volume passing through T1 at −130 °C (Fig. 4). The 246 peak area increases linearly with volume of sample air that passes through T1 when the   In our set up, we found that the CH4 adsorbed on T1 can be released adequately at 259 temperature of −80 °C and above, this temperature of CH4 release being consistent with 260 −85 °C reported by Brass and Röckmann (2010). Some earlier studies adopted a stepwise In this respect, we experimentally found that the peak of air (N2 and 265 O2) detected on IRMS is reduced by setting the T1-to-T2 transfer temperature lower (closer 266 to −80 °C). On the other hand, it was also found that a rapid increase of T1 temperature to 267 13 0 °C allows more efficient and stable transfer of CH4 from T1 to T2 at a same transfer flow. 268 We avoid stepwise temperature control during the transfer, but instead we reduce the   with decreasing temperature below −140 °C. We found that the air (N2 and O2) peak that 300 appears before the CH4 peak (see Fig. 2) increases with decreasing T2 temperature, 301 especially temperatures below −140 °C, and its tailing interferes with the CH4 peak.   frequently the system is used for sample measurement. It was found that the system tends 318 to provide stable measurement during periods when daily operation is continued. We 319 empirically surmise that regular eluting of small amounts of O2 contained in sample air 320 helps condition the furnace (Miller et al. 2002). In this respect, automation of CPR is 321 advantageous for conditioning the system constantly, since the system can be operated 322 even during periods not used for sample analysis (e.g. nighttime).

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A typical chromatogram shows significant intensity of the air peak (Fig. 2). We confirmed 342 appearance of the air peak even with the combustion furnace kept at room temperature as 343 reported by previous studies; it has been suggested that this peak is due to NO, N2O and    As an indicator of reproducibility of our measurement system, Fig. 8 shows a histogram of 371 δ 13 C-CH4 deviations from the average measured value for the working standard air. The 372 measurements of the standard air were made between 9 November 2018 and 21 January 373 2019 (N=158). The standard deviation of all the measurements is calculated to be 0.12 ‰. 374 We also note that standard deviation from repeated working standard air measurements 375 within one day or a couple of days is better than 0.1 ‰.

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Upstream of V3, CPR has a line that draws outside air via a metal bellows pump (MB-21, 378 Senior Metal Bellows Corp., USA) (Fig. 1). The outside air inlet is located on the roof of a 379 one-story building at NIES. By opening V3 and continuously introducing outside air, we 380 made automated measurements for outside air in Tsukuba for the period 9 November 381 2018-19 January 2019. The measurements were run mainly during the time when the 382 system was not used for flask samples or test measurements. The measurement cycle (i.e. 383 data interval) was ~1 hour and the standard air was analyzed at every 5 or 10 cycles.  19 mole fraction by our system is estimated to be 1.2 % (~25 ppb). The CH4 mole fraction of 388 the standard air was calibrated against laboratory CH4-in-air gases whose CH4 mole 389 fraction was determined on the NIES-94 CH4 scale (Machida et al. 2008). We note that the 390 results presented here are preliminary, but that they show an example of how our system 391 can provide useful on-line δ 13 C-CH4 data.

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In the course of the two-month measurements, we observed diurnal and day-to-day  To infer CH4 sources contributing to the observed variations, some of the high CH4 events 404 are plotted for δ 13 C-CH4 as a function of the reciprocal of the CH4 mole fraction (Fig. 11,   405 often referred to as a Keeling plot). The isotope signature of the source has often been 406 estimated by regression analysis of the Keeling plot according to the following equation (e.g.  November-December when overnight CH4 increases were obvious but observed changes 423 in δ 13 C-CH4 were smaller. These events showed relatively scattered Keeling plots (not 424 shown) and inferred source δ 13 C-CH4 signatures ranging from −46.6±1.4 to −50.1±3.6 ‰.

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Possible nearby CH4 sources are a pond in the institute garden, natural gas use in nearby 426 21 residential areas, exhaust gas from automobiles, and mixtures of these sources.