Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions

Carbon monoxide (CO) is a regulated pollutant and one of the key components determining the oxidizing capacity of the atmosphere. Obtaining a reliable record of atmospheric CO mixing ratios ([CO]) since preindustrial times is necessary to evaluate climate-chemistry models under conditions different from today and to constrain past CO sources. We present high-resolution measurements of CO mixing ratios from ice cores drilled at five different sites on the Greenland ice sheet that experience a range of snow accumulation rates, mean surface temperatures, and different chemical compositions. An optical-feedback cavity-enhanced absorption spectrometer (OF-CEAS) was coupled with continuous melter systems and operated during four analytical campaigns conducted between 2013 and 2019. Overall, continuous flow analysis (CFA) of CO was carried out on over 700g m of ice. The CFA-based CO measurements exhibit excellent external precision (ranging from 3.3 to 6.6g ppbv, 1σ) and achieve consistently low blanks (ranging from 4.1±1.2 to 12.6±4.4g ppbv), enabling paleoatmospheric interpretations. However, the five CO records all exhibit variability that is too large and rapid to reflect past atmospheric mixing ratio changes. Complementary tests conducted on discrete ice samples demonstrate that these variations are not artifacts of the analytical method (i.e., production of CO from organics in the ice during melting) but are very likely related to in situ CO production within the ice before analysis. Evaluation of the signal resolution and co-investigation of high-resolution records of CO and total organic carbon (TOC) suggest that past atmospheric CO variations can be extracted from the records' baselines with accumulation rates higher than 20g cmg w.e.yr-1 (water equivalent per year). Consistent baseline CO records from four Greenland sites are combined to produce a multisite average ice core reconstruction of past atmospheric CO for the Northern Hemisphere high latitudes, covering the period from 1700 to 1957g CE. Such a reconstruction should be taken as an upper bound of past atmospheric CO abundance. From 1700 to 1875g CE, the record reveals stable or slightly increasing values in the 100-115g ppbv range. From 1875 to 1957g CE, the record indicates a monotonic increase from 114±4 to 147±6g ppbv. The ice core multisite CO record exhibits an excellent overlap with the atmospheric CO record from Greenland firn air which spans the 1950-2010g CE time period. The combined ice core and firn air CO history, spanning 1700-2010g CE, provides useful constraints for future model studies of atmospheric changes since the preindustrial period. © Copyright: