Southern Hemisphere atmospheric history of carbon monoxide over the late Holocene reconstructed from multiple Antarctic ice archives

Faïn, Xavier ; Etheridge, David M. ; Fourteau, Kévin ; Martinerie, Patricia ; Trudinger, Cathy M. ; Rhodes, Rachael H. ; Chellman, Nathan J. ; Langenfelds, Ray L. ; McConnell, Joseph R. ; Curran, Mark A. J. ; Brook, Edward J. ; Blunier, Thomas ; Teste, Grégory ; Grilli, Roberto ; Lemoine, Anthony ; Sturges, William T. ; Vannière, Boris ; Freitag, Johannes ; Chappellaz, Jérôme

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<p align=justify>Carbon monoxide (CO) is a naturally occurring atmospheric trace gas, a regulated pollutant, and one of the main components determining the oxidative capacity of the atmosphere. Evaluating climate-chemistry models under different conditions than today and constraining past CO sources requires a reliable record of atmospheric CO mixing ratios ([CO]) that includes data since preindustrial times. Here, we report the first continuous record of atmospheric [CO] for Southern Hemisphere (SH) high latitudes over the past 3 millennia. Our continuous record is a composite of three high-resolution Antarctic ice core gas records and firn air measurements from seven Antarctic locations. The ice core gas [CO] records were measured by continuous flow analysis (CFA), using an optical feedback cavity-enhanced absorption spectrometer (OF-CEAS), achieving excellent external precision (2.8-8.8 ppb; <span class="inline-formula">2<i>σ</i></span>) and consistently low blanks (ranging from <span class="inline-formula">4.1±1.2</span> to <span class="inline-formula">7.4±1.4</span> ppb), thus enabling paleo-atmospheric interpretations. Six new firn air [CO] Antarctic datasets collected between 1993 and 2016 CE at the DE08-2, DSSW19K, DSSW20K, South Pole, Aurora Basin North (ABN), and Lock-In sites (and one previously published firn CO dataset at Berkner) were used to reconstruct the atmospheric history of CO from <span class="inline-formula">∼1897</span> CE, using inverse modeling that incorporates the influence of gas transport in firn. Excellent consistency was observed between the youngest ice core gas [CO] and the [CO] from the base of the<span id="page2288"/> firn and between the recent firn [CO] and atmospheric [CO] measurements at Mawson station (eastern Antarctica), yielding a consistent and contiguous record of CO across these different archives. Our Antarctic [CO] record is relatively stable from <span class="inline-formula">∼835</span> to 1500 CE, with mixing ratios within a 30-45 ppb range (2<span class="inline-formula"><i>σ</i></span>). There is a <span class="inline-formula">∼5</span> ppb decrease in [CO] to a minimum at around 1700 CE during the Little Ice Age. CO mixing ratios then increase over time to reach a maximum of <span class="inline-formula">∼54</span> ppb by <span class="inline-formula">∼1985</span> CE. Most of the industrial period [CO] growth occurred between about 1940 to 1985 CE, after which there was an overall [CO] decrease, as observed in Greenland firn air and later at atmospheric monitoring sites and attributed partly to reduced CO emissions from combustion sources. Our Antarctic ice core gas CO observations differ from previously published records in two key aspects. First, our mixing ratios are significantly lower than reported previously, suggesting that previous studies underestimated blank contributions. Second, our new CO record does not show a maximum in the late 1800s. The absence of a [CO] peak around the turn of the century argues against there being a peak in Southern Hemisphere biomass burning at this time, which is in agreement with (i) other paleofire proxies such as ethane or acetylene and (ii) conclusions reached by paleofire modeling. The combined ice core and firn air [CO] history, spanning <span class="inline-formula">∼835</span> to 1992 CE, extended to the present by the Mawson atmospheric record, provides a useful benchmark for future atmospheric chemistry modeling studies.</p>
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