Size-resolved Spray-mediated CO₂ Flux: Sensitivity to Fall Speed Parameterization

Abstract. The magnitude of spray-driven air–sea fluxes is highly dependent on the residence time of the droplets in the atmosphere and the degree of evaporation they undergo. However, there is significant uncertainty in modeling spray fall speeds and therefore accurately estimating time aloft across the sea spray size spectrum. We compare the widely used terminal-velocity fall speed with a force-balance acceleration formulation that resolves acceleration over the droplet’s full fall trajectory and assess the impact on modeled spray-driven CO₂ flux across the droplet size distribution over varying wind speed, sea state, and relative humidity (RH). Including force-balance acceleration reduces fall speed for all droplets with the greatest impact on droplets over 200 µm, with fall speed reductions up to 60 % for the largest drops. This change introduces competing effects at the individual droplet level, both increasing the size of droplets that undergo complete evaporation by 2–8 % under modeled conditions and also enhancing the CO₂ uptake of large cooling drops. However, the modest increase in the maximum evaporating droplet size of medium-sized droplets, which are numerous, has a substantially greater impact on the overall spray-driven CO₂ flux than the increase in CO₂ uptake by cooling spume droplets, which are comparatively rare. Moreover, the impact of fall-speed formulation is modulated by RH, with larger relative flux increases occurring at higher RH, where slower evaporation rates make droplets more sensitive to small changes in residence time. These combined effects drive an 18–26 % increase in net spray-driven CO₂ flux under modeled conditions. Fall-speed formulation is therefore an important source of uncertainty in spray-driven CO₂ flux estimates, particularly in the high-wind, developing-sea regime where spray dominates air–sea exchange.