Unexpected enhancement of new particle formation by lactic acid sulfate resulting from SO₃ loss in forested and agricultural regions

Abstract. Organosulfates (OSs) are key components of atmospheric aerosols and serve as tracers for secondary organic aerosol (SOA) formation. Among these, lactic acid sulfate (LAS) has been increasingly detected in the atmosphere. However, its molecular formation pathways and its role in new particle formation (NPF) remain poorly understood. In this work, we investigate the gas-phase formation mechanism of LAS via the reaction between lactic acid (LA) and SO₃, and assess its impact on sulfuric acid-ammonia (SA-A) driven NPF using quantum chemical calculations and Atmospheric Cluster Dynamics Code (ACDC) kinetic modeling. Our results show that SA and H₂O significantly catalyze the LA-SO₃ reaction, enhancing the effective rate coefficient by 7–10 orders of magnitude within the temperature range of 280–320 K. Further molecular-level analysis using the ACDC reveals that LAS not only significantly enhances the clustering stability of SA and A up to 10⁸-fold, but also plays a significant and direct role in SA-A nucleation under conditions typical of forested and agricultural regions. Notably, LAS-SA-A clusters contribute to 97 % of the overall cluster formation pathways in regions with high LAS concentrations like Centreville, Alabama. Additionally, our findings show that the nucleation potential of LAS-SA-A clusters is stronger than that of LA-SA-A clusters, aligning with field observations, even though LAS concentrations are typically three orders of magnitude lower than LA. These findings imply that OSs formed through SO₃ consumption may significantly contribute to the enhanced NPF rates observed in continental regions.