Direct Determination of Aerosol pH: Size-Resolved Measurements of Submicrometer and Supermicrometer Aqueous Particles

Abstract

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 (10-10 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 (NH)SO-HSO 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.

The acidity of atmospheric particles and clouds

“…For all models that include high latitudes in the domain, extremely acidic particles are predicted in the Arctic and over Greenland (pH F < 1). No estimates of Arctic aerosol pH F are available in the literature, but several studies have inferred low amounts of ammonium and high acidity from proxies (Fisher et al, 2011;Croft et al, 2016), yet sources of NH 3 from seabirds may neutralize particle acidity in the Arctic . Other work has noted that sulfur dioxide is able to escape scavenging more effectively during lifting than ammonia or ammonium sulfate (Park et al, 2004), thus providing for long-range transport of acidity over alkalinity.…”

Aerosol pH and its driving factors in Beijing

“…Inorganic ions in particles present clear size distributions, and the size-resolved chemical composition can change at different pollution levels (Zhao et al, 2017;Ding et al, 2017Ding et al, , 2018, which may result in variations in aerosol pH. Thus, we further investigated the size-resolved aerosol pH at different pollution levels.…”

The sensitivity of PM&lt;sub&gt;2.5&lt;/sub&gt; acidity to meteorological parameters and chemical composition changes: 10-year records from six Canadian monitoring sites

“…Field blanks were routinely performed as the background corrections. The detailed protocol for ambient constituent measurements for the NAPS database can be seen in Dabek-Zlotorzynska et al (2011). In total, there are 1067, 851, 711, 840, 713 and 742 sets of valid data used at the sites of Toronto, Ottawa, Windsor, Montreal, Simcoe and St Anicet, respectively.…”