Abstract
Abstract. We present the first reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO in 1950 was 140–150 nmol mol−1, which is higher than today's values. CO mole fractions rose by 10–15 nmol mol−1 from 1950 to the 1970s and peaked in the 1970s or early 1980s, followed by a ≈ 30 nmol mol−1 decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radicals (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless unrealistically large changes in OH are assumed. We argue that the available CO emission inventories strongly underestimate historical NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe.
Highlights
Carbon monoxide (CO) is a reactive trace gas that plays a key role in global atmospheric chemistry by being a major sink of tropospheric hydroxyl radicals (OH)
We have presented the first firn air record of past atmospheric [carbon monoxide (CO)] for the Northern Hemisphere (NH) high latitudes, covering the period from 1950 to 2008
The good agreement among [CO] records from three different firn air sites (NEEM, NGRIP and Summit) confirms that CO is well preserved in the firn at inland Greenland sites
Summary
Carbon monoxide (CO) is a reactive trace gas that plays a key role in global atmospheric chemistry by being a major sink of tropospheric hydroxyl radicals (OH). The Clark et al (2007) firn air record is from the small Devon ice cap in the Canadian Arctic and shows [CO] values generally increasing with depth in the firn, reaching a value of 186 nmol mol−1 for the oldest sample (mean year of origin = 1941) Such an increasing trend going back in time is unrealistic given growing anthropogenic emissions between 1940 and 1980, and suggests that CO is being produced in situ in the Devon firn. This comparison further indicates that the firn air systems do not have substantial blanks for CO and H2 (see Fig. S1 and further discussion in the Supplement)
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