Acidity-Driven Shifts in Aerosolized Sodium Sulfate Crystal Structure via Raman Microspectroscopy and Implications for Isoprene-Derived Secondary Organic Aerosol Formation

Sea spray aerosol (SSA) contributes significantly to global aerosol budgets and the NaCl present often heterogeneously reacts with H2SO4 to form Na2SO4 within particles of varying acidity. The acidic particles can subsequently undergo reactive uptake of isoprene-derived oxidation products, such as isoprene epoxydiols (IEPOX), to form significant amounts of secondary organic aerosol (SOA). While the complex pH-dependence of IEPOX-SOA formation with ammonium sulfate aerosol has been widely explored, there has been minimal research on pH-dependent SOA formation with sodium sulfate. Herein, aerosolized solutions of Na2SO4 mixed with different amounts of H2SO4 (pH = 1, 1.3, and 3) were used as seed aerosol to take up IEPOX and form SOA. We unexpectedly observed a distinct crystal structure (phase III) in more acidic particles using Raman spectroscopy, based on characteristic peaks for νas(SO42-) at 1077, 1130, and 1199 cm-1 versus 1101, 1131, and 1152 cm-1 for νas(SO42-) in typical crystalline Na2SO4 (phase V). The presence of phase III in iSOA indicates a more acidic environment. Our results show that reactive uptake of IEPOX to seed particles increases acidity (i.e., pH decreases) over time. This has important implications for IEPOX-SOA formation, particularly on aged SSA in marine and coastal environments.