Patterns of CO₂ and radiocarbon across high northern latitudes during International Polar Year 2008

High-resolution in situ CO₂ measurements were conducted aboard the NASA DC-8 aircraft during the ARCTAS/POLARCAT field campaign, a component of the wider 2007–2008 International Polar Year activities. Data were recorded during large-scale surveys spanning the North American sub-Arctic to the North Pole from 0.04 to 12 km altitude in spring and summer of 2008. Influences on the observed CO₂ concentrations were investigated using coincident CO, black carbon, CH₃CN, HCN, O₃, C2Cl4, and Δ¹ ⁴CO₂ data, and the FLEXPART model. In spring, the CO₂ spatial distribution from 55°N to 90°N was largely determined by the long-range transport of air masses laden with Asian anthropogenic pollution intermingled with Eurasian fire emissions evidenced by the greater variability in the mid-to-upper troposphere. At the receptor site, the enhancement ratios of CO₂ to CO in pollution plumes ranged from 27 to 80 ppmv ppmv-1 with the highest anthropogenic content registered in plumes sampled poleward of 80°N. In summer, the CO₂ signal largely reflected emissions from lightning-ignited wildfires within the boreal forests of northern Saskatchewan juxtaposed with uptake by the terrestrial biosphere. Measurements within fresh fire plumes yielded CO₂ to CO emission ratios of 4 to 16 ppmv ppmv-1 and a mean CO₂ emission factor of 1698 ± 280 g kg-1 dry matter. From the ¹ ⁴C in CO₂ content of 48 whole air samples, mean spring (46.6 ± 4.4‰) and summer (51.5 ± 5‰) Δ¹ ⁴CO₂ values indicate a 5‰ seasonal difference. Although the northern midlatitudes were identified as the emissions source regions for the majority of the spring samples, depleted Δ¹ ⁴CO₂ values were observed in <1% of the data set. Rather, ARCTAS Δ¹ ⁴CO₂ observations (54%) revealed predominately a pattern of positive disequilibrium (1-7‰) with respect to background regardless of season owing to both heterotrophic respiration and fire-induced combustion of biomass. Anomalously enriched Δ¹ ⁴CO₂ values (101-262‰) measured in emissions from Lake Athabasca and Eurasian fires speak to biomass burning as an increasingly important contributor to the mass excess in Δ¹ ⁴CO₂ observations in a warming Arctic, representing an additional source of uncertainty in the quantification of fossil fuel CO₂.