This award supported measurements of atmospheric O₂, CO₂, Ar, and CO₂ isotope ratios on three global surveys conducted by the NASA DC-8 as part of the Atmospheric Tomography (ATom) mission from 2017-2018. A previous award supported these measurements on the first ATom campaign in 2016.

Our ATom measurements have mapped atmospheric O₂ and CO₂ with unprecedented detail and spatial extent. The 8 north-south transects, 4 each in the Pacific and Atlantic basins spread over the seasonal cycle, reveal the breathing of the Northern Extratropical terrestrial biosphere, the atmospheric signature of marine productivity in both hemispheres, and smaller scale features traceable to forest, surface ocean, and industrial emissions. A major focus of our analyses has been the application of these measurements to derive quantitative estimates of hemispheric scale planetary metabolism, both on land and in the ocean.

We have also used our O₂ and CO₂ measurements in conjunction with reactive gas measurements by other ATom instruments, to develop novel constraints and up-scaling methodologies for estimating marine biogenic gas fluxes in the Southern Ocean. Furthermore, the O₂ and CO₂ measurements have proved useful in conjunction with measurements of other pollutants in constraining the mixture of CO₂ emission sources from Northern Africa. Our additional measurements of deviations in the Ar/N2 ratio have provided a unique window onto lower stratospheric mixing via gravitational fractionation, and in the troposphere these measurements also show the imprint of seasonal air-sea heat fluxes.

Figure 1 shows interpolated altitude-latitude cross-sections from the NCAR AO2 instrument overlain with observations on flasks collected by the NCAR/Scripps Medusa whole air sampler for the southbound Pacific basin portion of ATom-4 in April-May of 2018. The CO₂ cross-section shows concentrations elevated by over 5 ppm throughout the entire northern extratropical troposphere, with enhancements as high as 8 ppm north of 60 N. This reflects the seasonal accumulation of northern extratropical CO₂ emissions over winter from a combination of net terrestrial respiration and fossil fuel burning.

The molar equivalent color scales in Figure 1 reveal larger northern extratropical depletions in O₂ as a result of the greater than 1 oxidation ratio for fossil fuel burning and the additional ocean uptake of O₂ resulting from both ventilation of northern ocean waters with accumulated respiration signatures and the cooling of surface waters. Conversely, at southern high latitudes, ocean heating and net marine productivity lead to O₂ emissions over the austral summer which we observed as a strong accumulated O₂ signal throughout the southern extratropical tropopause. The composite tracer atmospheric potential oxygen (APO = O₂ + 1.1 ? CO₂) is essentially conservative with respect to terrestrial processes and highlights this ocean signal.

Given the relative lack of land plants and industrial emissions at high southern latitudes, the observed southern hemisphere CO₂ field was comparatively flat. APO suggests that the interhemispheric gradient in O₂ at this time of year is approximately half owing to air-sea fluxes. These flights also intercepted stratospheric air poleward of 60 N and below 300 hPa, and in an isolated intrusion at 33 N and 300 hPa, with correspondingly high O₃ and other stratospheric tracers.

North-south cross sections from all four ATom campaigns graphically illustrate the seasonally evolving gradients in CO₂ and O₂, both horizontally and vertically. Higher O₂ and lower CO₂ is seen near the surface in summer in the northern hemisphere. APO also undergoes a large seasonal cycle, mostly reflecting seasonal air-sea exchanges of O₂. These measurements are allowing us to make quantitative estimates of hemispheric scale seasonal productivity for both land and oceans.

This award has directly supported one postdoc and one graduate student, and has indirectly supported an additional two postdocs and three graduate students who are using the measurements in their work. All measurements supported by this award are publicly available in the ATom data repositories at NASA and ORNL.

Last Modified: 12/15/2020
Modified by: Ralph F Keeling