Influence of Relative Humidity and Seed Particles on Molecular Composition of α‑Pinene Secondary Organic Aerosol

Secondary organic aerosol (SOA), a major component of submicrometer particles, is critical to the climate and human health. SOA can form through nucleation of low-volatility organic compounds, following atmospheric oxidation, or by condensing these vapors onto existing particles. In either of these cases, the formation of SOA particles could be affected by atmospheric conditions (e.g., relative humidity (RH)) and particle liquid water content. This study examines the effects of RH on the formation and composition of SOA from dark α-pinene (C₁₀H₁₆) ozonolysis, as a canonical system, with or without ammonium sulfate (AS, (NH₄)₂SO₄) seed particles across varying RH levels. Using online extractive electrospray ionization mass spectrometry, we identified monomers (C_7-10) and dimers (C_15-20) in the SOA with high chemical and temporal resolution. In both cases, high RH (>90%) promotes dimer formation in the particle phase, while they appear at the beginning of the experiment when (NH₄)₂SO₄ seeds are present. The prompt increase in dimers in high RH seed containing experiments (60-65% dimers), which are absent at low RH (10%), suggests that intraparticle reactions are responsible for the dimer formation.