Contrasting airborne cloud water pH measurements in diverse regions: Statistics and relationships with constituents
Kayla M. Preisler1, Ewan C. Crosbie2,3, Miguel Ricardo A. Hilario1, Grace Betito1, Andrea F. Corral1, Michael A. Shook2, Edward L. Winstead2,3, Luke D. Ziemba2, and Armin Sorooshian1,4
1Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, 85721, USA
2NASA Langley Research Center, Hampton, VA, 23681, USA
3Analytical Mechanics Associates, Inc., Hampton, VA, 23666, USA
4Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, 85721, USA
Cloud water pH is difficult to measure in clouds, yet it is relevant for aqueous chemistry and wet deposition effects. This work leverages an extensive cloud water dataset from various airborne campaigns, with a focus on pH relationships with influential species across three diverse regions: the northwest Atlantic (NW Atlantic, winter and summer 2020-2022), west Pacific (W Pacific, fall 2019), and the northeast Pacific (NE Pacific, summers between 2011-2019). The NW Atlantic results are further categorized into three sets of conditions, with the first two comprising data closer to the U.S. East Coast (winter and summer) and measurements over Bermuda in the summer of 2022. The median (± standard deviation) pHs were as follows: NW Atlantic winter/summer = 4.83±0.06/4.95±0.04, Bermuda = 4.74±0.07, W Pacific = 5.17±0.05, NE Pacific = 4.40±0.02. All regions exhibit a narrow range of median pH values between 4.00 and 5.00 across various altitudes (up to 6 km for W Pacific, 4 km for NW Atlantic, and 1 km for NE Pacific). Using correlation heat maps and machine learning techniques, we identified the most influential species for estimating pH: NW Atlantic = oxalate, non-sea salt (NSS)-Ca2+, NO3-; W Pacific = NSS-SO42-, oxalate, NO3-; NE Pacific = NO3-, methanesulfonic acid (MSA), NSS-Ca2+. pH showed the highest correlation coefficient with oxalate for the NW Atlantic (0.71) and W Pacific (0.54), unlike the NE Pacific, which found NSS-Ca2+ (0.29) as the highest correlation coefficient. These results demonstrated that cloud water acidity is relatively well-constrained between a pH of 4.00-5.00, with species with the best predictive ability being acidic anions (oxalate, NO3-) and a dust tracer cation (NSS-Ca2+).