Airborne cloud water pH measurements in diverse regions: statistics and relationships with constituents

Abstract

Airborne cloud water measurements are examined in this study, with a focus on pH and interrelationships with influential species for three regions: the Northwest Atlantic (winter and summer 2020–2022), the West Pacific (summer 2019), and the Northeast Pacific (summers between 2011 and 2019). Northwest Atlantic results are categorized into three ways: data closer to the U.S. east coast for (i) winter, (ii) summer, and (iii) summertime measurements over Bermuda. The median pHs are as follows: Northwest Atlantic winter/summer = 4.83/4.96, Bermuda = 4.74, West Pacific = 5.17, and Northeast Pacific = 4.40. The regions exhibit median pH values of ∼4–6 across various altitude bins reaching as high as 6.8 km, with the overall minimum and maximum values being 2.92 and 7.58, respectively (both for the Northeast Pacific). Principal component analysis of species to predict pH shows that the most influential principal component is anthropogenic in nature. Machine leaning modeling suggests that the most effective combination of species to predict pH includes some subset of oxalate, non-sea salt Ca²⁺, NO₃⁻, non-sea salt SO₄²⁻, and methanesulfonate. These results demonstrate that cloud water acidity is relatively well constrained between a pH of 4 and 5.5 and that anthropogenic activities impact regional cloud water pH in the areas examined, with dust offsetting acidity at times.