Acidity-Driven Shifts in Aerosolized Sodium Sulfate Crystal Structure via Raman Microspectroscopy and Implications...

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 ν_as(SO₄^2-) at 1077, 1130, and 1199 cm^-1 versus 1101, 1131, and 1152 cm^-1 for ν_as(SO₄^2-) 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.