Monitoring, reporting and verification (MRV) of marine carbon dioxide removal (mCDR) from purposeful ocean alkalinity enhancement (OAE) is generally thought to require numerical simulations. While this remains true at regional and global scales, this study seeks to establish that observational methods can be used to verify efficacy of alkalinity enhancement for CDR at local scale to attract investment on mCDR. This is achieved through a field trial conducted in a shallow coastal lagoon connecting Xichong River to Dapeng Bay of Shenzhen, with the longer term and bigger scale effects simulated by a model. The trial lasted ~ 2 weeks with 4 releases of NaOH in the lagoon, each for ~30 h, that led to increases in excess alkalinity ([TA]-[DIC]) up to 101 umol kg^-1 according to high time resolution sampling. Two methods, flux chamber and monitoring of air and water pCO₂ differences (ΔpCO₂) were employed to quantify CO₂ exchanges between air and lagoon water. Due to dosing and/or hydrological fluctuations, the CO₂ exchange fluxes were reduced from a larger atmospheric carbon source of 1014 or 910 tCO₂/km² yr to a small sink of -303 tCO₂/km² yr or a small source of 17 tCO₂/km² yr, quantified by chamber or by air-water ΔpCO₂, responding linearly to a range of excess alkalinity ([TA]-[DIC]) from -39 to 101 umol kg^-1 in the lagoon. Thus we conclude that observation based MRV is feasible for the lagoon-based alkalinity enhancement with the potential to mitigate coastal acidification, which is severe in Hong Kong - Shenzhen coastal sea waters. This potential is demonstrated by high spatially and temporally resolved hydrodynamic modeling with simplified chemistry that simulated the effect of  maintaining a constant + 100 umol kg^-1 excess alkalinity in the lagoon on the 0.45 km² of nearshore embayment water. The simulation shows that removal amounted to 4 tCO₂/km² yr in the simulation year of 2024, with a corresponding pH increase of 0.006 yr^-1. Long-term alkalinity additions to coastal rivers, including the small ones, to meaningfully reverse coastal acidification and CO₂ outgassing, should be explored.
 

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