Surges of acidity in UK rainwater: implications for ocean acidification?

A low-cost project recorded unexpected surges of acidity in UK rainfall events over eight summer weeks. With hindsight we now convert the pH values to xH, thereby showing that these rain events typically have at least one acid spike-and-decay sequence.  We compared the acid decay curves with that in a solution of carbon dioxide (CO₂) exposed to a blustery atmosphere, and have separately recorded similar spikes using CO₂ spectrometry in the headspace above incoming rainwater. For one rain event a suite of twelve ion analyses was made at three intervals showing no other significant acid anhydride, again indicating the identity of the acidic agent as CO₂.

In our most complete week, a frequency analysis showed that 8,840 of the 10,080 records had acidity of less than 3µmols [H⁺] per mol H₂O, representing the local equilibrium state in the sample well between rain events. The remaining 1240 records show active rainfall with acidity averaging 31.3 µmols [H+] per mol water.  Adopting the conversion curve established by Butler (1982), this would represent dissolved CO₂ ten times the measured local equilibrium state, i.e. ten times supersaturated, while including three spikes exceeding thirty-five times supersaturated. 

This kinetic behaviour in dissolved CO₂ seems to have escaped scientific notice. If occurring over an ocean such surges would contribute to acidification, defined as `reduction in the pH of the ocean over an extended period of time, caused by uptake of CO₂ from the atmosphere’ (NOAA accessed 2/11/2025).  This process is monitored on a three-hourly cycle by Global Ocean Acidification Observing Network, and we have therefore downloaded CO₂  measurements for tethered buoys WHOTS and SOTS in case CO₂ spikes coincide with lowered salinity as an indicator of local rainfall.

In speculating a concentrating mechanism for CO₂ within the precipitating atmosphere we review 20^th-century arguments for the capture of anionsby cloud ice against a 21^st-century thermodynamic model of the formation of CO₂ gas hydrate.