Unexpected Gas-Phase Formation of Glycolic Acid Sulfate in the Atmosphere

Organosulfates (OSs) are ubiquitous in atmospheric particulate matter and serve as key tracers for secondary organic aerosols. Traditionally, OSs have been primarily linked to the particle phase, with their presence in the gas phase remaining largely undetected. This study provides compelling observational evidence of a continuously present gas-phase OS, glycolic acid sulfate (GAS), in the urban atmosphere using advanced mass spectrometry techniques. GAS concentrations exhibited distinct seasonal and diurnal patterns, peaking in summer with maximum levels of 4.6 × 10⁴ cm^-3 observed around midday, indicating a photochemical origin. Thermal desorption profile analysis revealed GAS as an extremely low-volatility organic compound, suggesting preferential aerosol partitioning. Remarkably, the observed gas-phase fraction of GAS exceeded predictions based on gas-particle equilibrium theory by 5~7 orders of magnitude, strongly suggesting the existence of a distinct source from gas-phase chemistry. We propose a potential formation mechanism involving the reaction between SO₃ radical and glycolic acid, which correlates nearly linearly with GAS production rates, suggesting a near-collision-limited rate constant (k_field ≈ 2.2 × 10^-10 cm³ s^-1). This study fundamentally reshapes our understanding of OSs sources and underscores the potential involvement of SO₃ in the formation of low-volatility organic compounds in the atmosphere.