Comment on bg-2022-82

Abstract

Reducing uncertainties in the air–sea CO2 flux calculations is one of the major challenges when addressing the oceanic contribution in the global carbon balance. In traditional models, the air–sea CO2 flux is estimated using expressions of the gas transfer velocity as a function of wind speed. However, other mechanisms affecting the variability in the flux at local and regional scales are still poorly understood. The uncertainties associated with the flux estimates become particularly large in heterogeneous environments such as coastal and marginal seas. Here, we investigated the air–sea CO2 exchange at a coastal site in the central Baltic Sea using nine years of eddy covariance measurements. To the best of our knowledge, this is the longest record of direct observations of CO2 fluxes and the corresponding gas transfer velocities (k) in a marine environment. Based on these observations we were able to capture the temporal variability of the air–sea CO2 flux and other parameters relevant for the gas exchange. The analysis of water-side and atmospheric control mechanisms showed that during wind speeds above 8 m s−1 the conditions on both sides of the air–water interface were relevant for the gas exchange. Our findings further suggest that at such relatively high wind speeds, sea spray was an efficient mechanisms for air–sea CO2 exchange. In contrast to high wind-speed conditions, during low wind speeds (< 6 m s−1) only water-side processes were found to be relevant control mechanisms, in particular, water-side convective processes. Altogether, our results show that currently existing wind-based parametrizations of k might be good approximations as long-term averages for environments with coastal characteristics. However, in order to reduce the uncertainty associated to these averages and produce reliable short-term k estimates, additional physical processes must be considered.