Buffering, stoichiometry, and the sensitivity of pH to biogeochemical and physical processes: a proton-based model perspective (AGGREGATION CH 5)

Abstract

Mechanistic understanding of factors governing pH is essential, given growing concern about ocean acidification. The classical approach to carbonate chemistry and pH calculations is centred around the alkalinity concept. In this publication, the classical approach is taken one step further: the in uences of biogeochemical and physical processes on the pH are calculated directly, without the detour of alkalinity. The in uence of a given process on pH is expressed by its process rate, modulated by a sensitivity factor. Here, we provide the necessary tools and procedures to calculate this sensitivity factor analytically for an arbitry process. Moreover, we show that the sensitivity can be decomposed as the ratio of a particular stoichiometric coefficient of protons over a buffer factor. The stoichiometric coefficient can be derived by describing the process using a fractional reaction equation at ambient pH and it represents the protons that are released by the process without accounting for re-equilibration. The presented approach thus provides a chemical interpretation of the mechanisms underlying pH changes in aquatic systems. Applying the concept of buffer capacity and pH sensitivities to an averaged global ocean shows that towards the end of the century the ocean will be around four times less buffered than it is today. This demonstrates how the concepts and tools presented here can make a complementary contribution to the existing modelling approaches of ocean acidification.