Scientific objectives

The AirCore observations contribute to several research topics concerning the monitoring and understanding of anthropogenic greenhouse gases and, more generally, carbon cycle studies. The main scientific objectives of the AirCore are:

• Understanding of carbon exchanges along the atmospheric column
• Cal/val activities for greenhouse gas space missions (SWIR and TIR)
• Evaluation of atmospheric chemistry and transport models.

1) Understanding of carbon exchanges along the atmospheric column

By providing a full recovery of the atmospheric profiles of CO₂ and CH₄ from the surface up to an altitude of 30-40 km, AirCores provide a means to better understand carbon exchanges along the atmospheric column, as well as the chemistry processes that yield the strong decreasing slope of CH₄ in the stratosphere.

The above figure shows all the profiles that have been acquired as part of the French AirCore network since August 2014. The increase of CO2 throughout these years is well seen. Also seen are the strong differences in the boundary layer, that can be attributed to the season. When measurements are made in spring and summer, the CO2 concentration near the ground is smaller than in the free troposphere, mostly reflecting carbon uptake by vegetation. When measurements are made in fall or winter, the CO2 concentration near the ground is higher than in the free troposphere. This kind of variations give insight on local sources and sinks of carbon.

Stratospheric concentration of CO2 can be used to infer the mean age of stratospheric air can be derived from observations of sufficiently long-lived trace gases with approximately linear trends in the troposphere. Mean age can serve as a tracer to investigate stratospheric transport and long-term changes in the strength of the overturning Brewer–Dobson circulation of the stratosphere.

2) Cal/val activities for greenhouse gas space missions (SWIR and TIR)

In order to fully validate weighted columns retrieved from space-borne instruments or ground-based FTIR (TCCON or EM27/SUN) at the level of precision required by carbon cycle studies, it is necessary to use independent in-situ measurements of GHG profiles. As AirCore provides a full recovery of the atmosphertic profiles of CO₂ and CH₄, they are well suited to validate total columns as retrieved from instruments operating in the short-wave infrared (such as OCO-2, GOSAT, Sentinel5P, MicroCarb, Merlin or CO2M)  or to validare mid-tropospheric columns as retreived from instruments operating in the thermal infrared (such as IASI or IASI-NG).

In addition to providing a primary calibration for retrieved total columns, AirCore profiles benefit creation of a priori profiles for the retrievals. These improvements are important because the shape of the a priori profile used on the retrieval process is retained in the gas retrievals, and any errors in the stratospheric part of the retrieved profile will result in corresponding biases in the troposphere, which may eventually translate into biases in the estimates of surface carbon fluxes.

3) Evaluation of atmospheric chemistry and transport models

Thanks to theri coverage of a large part of the atmospheric column, AirCores can help evaluating simulations made from atmospheric chemistry and transport models, such as the models used in flux atmospheric inversions. In particular, valubale information are given on the upper-troposphere-lower-stratosphere part of the column, which is a region poorly covered by atmospheric measurements.