Aerosols and droplets with high water content are important systems that dictate many atmospheric processes. pH is arguably a key controlling parameter as its effect on aerosol physicochemical properties (e.g., size, surface tension, and mixing state) ultimately impact climate, human health, and aerosol reactivity. pH has been reported to correlate with many atmospheric processes such as secondary aerosol formation, cloud condensation nuclei activity, and liquid-liquid phase separation. Ammonium sulfate (AS) is a major component of atmospheric secondary aerosols. As such, sulfuric acid plays an important role in the formation of new atmospheric particles, and ammonia is significant in the heterogeneous neutralization of acidic aerosols.<br/>Aerosol pH is frequently evaluated via analysis of field-collected aerosol composition or is indirectly predicted by proxy methods such as thermodynamic models. In this study, the impact of ammonium ion on aerosol droplet pH was directly quantified using pH nanoprobes and surface-enhanced Raman spectroscopy (SERS). Sample solutions were prepared by mixing 1 M ammonium sulfate (AS), ammonium nitrate (AN), sodium sulfate (SS), or sodium nitrate (SN) solutions with 1 M phosphate buffer (PB) at different volume ratios. The pH of the 10 – 50 µm aerosol droplets generated from these solutions was measured to determine how ionic composition affects droplet pH. When the ammonium concentration (AS or AN) was increased, the centroid pH within the droplets decreased from ≈11 to 5.5. Such a decrease was not observed in sodium (SS or SN) systems and no pH differences were observed between sulfate and nitrate. The formation of ammonia and its partitioning to the gas phase in ammonium containing droplets appeared to be negligible. The pH difference between ammonium-containing droplets and ammonium-free droplets suggests that the presence of ammonium alters the system ion distribution. The role of the air-water interface and the interfacial preference of different cations and anions in ion distribution were investigated and discussed using surface tension measurements and 2D Raman spectra.

Direct quantification of the effect of ammonium on aerosol droplet pH

ACS Fall 2020 Virtual Meeting

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