Surface Formation Pathway of Nitrogen- and Sulfur-Containing Organic Compounds on Ammonium Sulfate

The formation of nitrogen- and sulfur-containing organic compounds (N-Org and S-Org) is important for atmospheric secondary organic aerosol (SOA) production, thereby influencing air quality and global climate. However, the mechanisms underlying N-Org and S-Org formation on aerosol particle surfaces are poorly understood due to the limited availability of surface-sensitive analytical techniques. This study investigates the surface interactions of glyoxal (GL), a known SOA precursor, with ammonium sulfate (NH4)2SO4, under varying relative humidity (RH) conditions, using ambient-pressure X-ray photoelectron spectroscopy (APXPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and molecular dynamics (MD) simulations. N-Org species, such as imines, a key intermediate in brown carbon (BrC) formation, are identified on the (NH4)2SO4 surface at low RH (<= 13.3%). The formed S-Org species cannot be specified due to the difficulties in distinguishing S-Org from inorganic sulfate in the XPS spectra. Elemental ratios on (NH4)2SO4 surface across the entire probing depth show increased S/O and N/O ratios upon GL exposure, indicating the formation of N-Org and S-Org species. NEXAFS measurements further confirm the surface changes of (NH4)2SO4 associated with the adsorption of GL and water. These findings provide compelling evidence of surface-driven N-Org and S-Org formation pathways, demonstrating that heterogeneous reactions on (NH4)2SO4 particle surfaces could be an active source of atmospheric BrC and SOA.