Records of Northern Hemisphere Atmospheric Carbon Monoxide and Hydrogen Back to ~1950 AD from Greenland Firn Air

Carbon monoxide (CO) plays a key role in global atmospheric chemistry by being the main sink of atmospheric hydroxyl radicals (OH). Records of past [CO] from both hemispheres are essential for understanding past changes in atmospheric [OH]. Earlier attempts at reconstruction of northern hemisphere (NH) [CO] suffered from apparent in-situ CO production in ice and firn. We present a record of high-latitude NH [CO] to ~1950 AD, from measurements by four different laboratories of firn air collected from three different boreholes at the NEEM ice core site in Greenland. Procedural blanks indicate no detectable [CO] contamination from sampling. Excellent agreement with a firn air record from another cold Greenland site (NGRIP) is consistent with the NEEM firn CO record being unaltered. A preliminary analysis of our measurements, interpreted using two different firn air diffusion models, suggests that high-latitude NH [CO] 1) was 140 - 150 ppb in the early 1950s, 2) increased from ~1950 to the 1970s, 3) peaked between 157 and 167 ppb in the 1970s, which is earlier than first direct atmospheric measurements suggest, and 4) gradually declined after the 1970s. In addition, we present the NEEM firn record of molecular hydrogen (H2), from measurements by three laboratories. The main features of the [H2] record are similar to those of the [CO] record, with preliminary results suggesting an increase in [H2] from ~1960 to the early 1980s, followed by a gradual decline. This is consistent with early direct atmospheric measurements. The similarity in the main features of [CO] and [H2] records is expected because of broadly similar sources for the two gases. Further work is in progress to improve the firn air diffusion models and reconstruct atmospheric histories with well-defined uncertainty envelopes.