A continuous method for measurement of pH and acidity in ice cores

A continuous direct measurement technique has been developed for the accurate determination of pH and acidity in ice cores at monthly to seasonal resolution. A small flow-through bubbling chamber is employed for sample equilibration with a known concentration of carbon dioxide and the measurement is made with a traditional glass pH electrode. The technique provides both high resolution accuracy and efficiency that cannot be found with the existing techniques of electrical conductivity measurement (ECM), Gran titration, or discrete pH measurement. A 63-year acidity record from the Humboldt North ice core in northwest Greenland is presented and compared to the major ion data from the ice core to determine the relative importance of the species that are responsible for the acidity. Sulfuric acid (H2SO4) followed by nitric acid (HNO3) were found to be the primary acid components, with hydrochloric acid (HCl) and organic acids (HOrg) also contributing. Ammonia (NH3) was the primary basic component followed closely by carbonate (CO3). Acidity ranges from 1 to 11 muM (pH 5.5--4.9) with a distinct annual peak in late winter-early spring. Trends in the acid and base species are examined and show that recent decreases in H2SO 4 have been partially compensated for by increases in HNO3 and HCl. The correlation between HNO3 and HCl supports an existing body of aerosol data showing HCl production by acid displacement on sea salt aerosols is greater in the presence of HNO3 than other acids.