Formation of Brown Carbon from Aqueous-Phase Limonene SOA Reactions with Reduced Nitrogen Species: Effects of Gas-Phase Precursors and Drying Conditions

Aqueous-phase processing of secondary organic aerosols (SOAs) from limonene oxidation can generate brown carbon (BrC) through reactions with reduced nitrogen species. This study provides laboratory evidence that dark ozonolysis of limonene, in the presence of biogenic (α-pinene) or anthropogenic (toluene) precursors, generates SOAs more effectively in BrC formation when subsequently dried with (NH₄)₂SO₄/glycine in bulk solutions, and toluene had a stronger effect than α-pinene in promoting oligomeric BrC formation. BrC absorption was significantly enhanced in limonene-SOA and the (NH₄)₂SO₄/glycine mixed droplet upon evaporation at ∼75% RH but was suppressed at lower RH, likely due to reduced aerosol liquid water and precursor evaporation. RH-dependent trends were also observed in SOA generated by limonene and toluene mixtures (lim/tol-SOA), with the strongest oligomeric BrC absorption at ∼85% RH for both (NH₄)₂SO₄ and glycine systems. While bulk-dried experiments showed 1–2 orders of magnitude differences in BrC absorption between limonene-SOA and lim/tol-SOA, evaporated droplets exhibited no significant differences. This study underscores the importance of studying the effects of SOA precursors, atmospherically relevant drying conditions, and volatility of BrC precursors on the light absorption properties of complex SOAs via aqueous-phase processing.

This study provides insights into how atmospheric processing of biogenic and anthropogenic emissions influences brown carbon formation in aqueous droplets, and the dynamic behavior of brown carbon formation upon evaporation at different humidity conditions.