Description: The CO2 fluxes were measured over the surfaces of snow, ice, water with LI-COR 8100-104 chambers connected to a LI-8100A soil CO2 flux system (LI-COR Inc., USA) during expedition PS122 (MOSAiC Legs 1−5) to the central Arctic in October 2019−September 2020. A chamber was connected via a closed loop to an infrared gas analyzer (LI-8100A, LI-COR Inc., USA) to measure CO2 concentrations with an air pump at a rate of 3 L min−1 during 20-minute intervals. Power was supplied by a battery (8012−254, Optima Batteries Inc., USA). We also used a Teflon-coated metal chamber (0.50 m in diameter and 0.30 m high with a serrated bottom edge) (Nomura et al., 2010, 2012). Every 5 or 10 minutes during an experiment, about 500 mL of air was collected from the chamber using a 50-mL glass syringe with a three-way valve and then transferred to a 3000-mL Tedlar bag (AAK 3L, GL Sciences Inc., Japan). After collection, air samples were quickly transported in a dark container to a laboratory onboard the R/V Polarstern, which was moored near our sampling site. The CO2 concentrations were measured with a CO2 analyzer (Picarro 2132-i, Los Gatos Research Inc., USA) used for continuous measurements of atmospheric CO2/CH4 concentrations on board. The CO2 fluxes (in mmol C m−2 day−1) (a negative value indicates CO2 being absorbed from the atmosphere) were calculated with LI-COR software (model: LI8100PC Client v.3.0.1.) based on the changes of the CO2 concentrations within the headspace of the LI-COR 8100−104 chambers. For the metal chamber, we took into account the changes of CO2 concentrations and the volume of the chamber (Nomura et al., 2010, 2012) to calculate the CO2 fluxes. The detection limit of this system was about +0.1 mmol C m−2 day−1 (Nomura et al., 2010; 2018). An inter-comparison experiment between the metal chamber and the LI-COR 8100−104 chamber in the home laboratory indicated good agreement (Nomura et al., 2022). Data obtained with both methods were therefore comparable. When the surface of the melt ponds/leads was frozen, flux measurements were made over the frozen surface. Then, a 1 m x 1 m hole was cut with a hand saw, and chambers were installed over the water surface with buoyant material (Nomura et al., 2020; 2022). In addition, chambers were installed over snow/slush/frost flower surface, and ice surface after removing snow by shovel. We conducted water mixing experiments at St. 4 (melt pond) on September 2, 2020 and at the ROV lead site on September 5, 2020 to understand how the carbonate chemistry and CO2 fluxes responded to changes in the marine environment caused by agitation in the melt pond and lead by wind and movement of sea ice. We measured the fluxes between the atmosphere and the surfaces of the melt ponds and leads using a floating metal chamber. After the measurements, the water in the melt ponds or leads was mixed for 30 minutes by two persons using a shovel and an oar. After mixing, the pre-mixing measurements were repeated.