Influence of acidity on liquid–liquid phase transitions of mixed secondary organic aerosol (SOA) proxy–inorganic aerosol droplets

The phase state and morphology of aerosol particles play a critical role in determining their effect on climate. While aerosol acidity has been identified as a key factor affecting multiphase chemistry and phase transitions, the impact of acidity on the phase transition of multicomponent aerosol particles has not been extensively studied in situ. In this work, we employed aerosol optical tweezers (AOT) to probe the impact of acidity on the phase transition behavior of levitated aerosol particles. Our results revealed that higher acidity decreases the separation relative humidity (SRH) of aerosol droplets mixed with ammonium sulfate (AS) and secondary organic aerosol (SOA) proxy, such as 3-methylglutaric acid (3-MGA), 1,2,6-hexanetriol (HEXT) and 2,5-hexanediol (HEXD) across aerosol pH in atmospheric conditions. Phase separation of organic acids was more sensitive to acidity compared to organic alcohols. We found the mixing relative humidity (MRH) was consistently higher than the SRH in several systems. Phase-separating systems, including 3-MGA inline-formula $M1inlinescrollmathml/$ 8pt14ptsvg-formulamathimg1b4178c77ca0d4bfee6c9ddd864f3a43 acp-23-10255-2023-ie00001.svg8pt14ptacp-23-10255-2023-ie00001.png  AS, HEXT inline-formula $M2inlinescrollmathml/$ 8pt14ptsvg-formulamathimg527256ea34e0af356380afd605ccefc0 acp-23-10255-2023-ie00002.svg8pt14ptacp-23-10255-2023-ie00002.png  AS and HEXD inline-formula $M3inlinescrollmathml/$ 8pt14ptsvg-formulamathimge653eaf840568ee76bb20ba3bf368ae0 acp-23-10255-2023-ie00003.svg8pt14ptacp-23-10255-2023-ie00003.png  AS, exhibited oxygen-to-carbon ratios (inline-formulaO:C) of 0.67, 0.50 and 0.33, respectively. In contrast, liquid–liquid phase separation (LLPS) did not occur in the high-inline-formulaO:C system of glycerol inline-formula $M6inlinescrollmathml/$ 8pt14ptsvg-formulamathimg073414a2b77546d8d5847ae97897d626 acp-23-10255-2023-ie00004.svg8pt14ptacp-23-10255-2023-ie00004.png  AS, which had an inline-formulaO:C ratio of 1.00. Additionally, the morphology of 42 out of the 46 aerosol particles that underwent LLPS was observed to be a core–shell structure. Our findings provide a comprehensive understanding of the pH-dependent LLPS in individual suspended aerosol droplets and pave the way for future research on phase separation of atmospheric aerosol particles.