Measuring the acidity of atmospheric aerosols is critical, as many key multiphase chemical reactions involving aerosols are highly pH-dependent. These reactions impact processes, such as secondary organic aerosol (SOA) formation, that impact climate and health. However, determining the pH of atmospheric particles, which have minute volumes (10-23-10-18 L), is an analytical challenge due to the nonconservative nature of the hydronium ion, particularly as most chemical aerosol measurements are made offline or under vacuum, where water can be lost and acid-base equilibria shifted. Because of these challenges, there have been no direct methods to probe atmospheric aerosol acidity, and pH has typically been determined by proxy/indirect methods, such as ion balance, or thermodynamic models. Herein, we present a novel and facile method for direct measurement of size-resolved aerosol acidity from pH 0 to 4.5 using quantitative colorimetric image processing of cellular phone images of (NH4)2SO4-H2SO4 aqueous aerosol particles impacted onto pH-indicator paper. A trend of increasing aerosol acidity with decreasing particle size was observed that is consistent with spectroscopic measurements of individual particle pH. These results indicate the potential for direct measurements of size-resolved atmospheric aerosol acidity, which is needed to improve fundamental understanding of pH-dependent atmospheric processes, such as SOA formation.
Bibliographical noteFunding Information:
This work was supported by National Science Foundation (NSF) Grants CHE-1654149 (CAREER, A.P.A.) and AGS-1554936 (CAREER, C.S.D.) and University of Michigan (UM) startup funds. R.L.C. was partially supported by the Susan Lipschutz Fellowship Award. R.D.C. was supported by a Rackham Merit Fellowship Award and the Marian P. and David M. Gates UMBS Graduate Student Fellowship. S.C. and M.A.H. were supported by the Detroit Research Internship Summer Experience (D-RISE) program, by NSF Grants CHE-1654149 (A.P.A.) and CHE-1305777 (Dr. Nicolai Lehnert), other UM sources, and Cass Technical High School. Dr. Simon Clegg is thanked for helpful discussions.
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