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Many climate models treat the light-absorbing SOA component called “brown carbon” (BrC) as non-light absorbing because its formation and transformations are poorly understood. We therefore investigated the influence of reactive nitrogen (NO_x, NH₃)-, acidity (H₂SO₄)-, and water-mediated chemistry on SOA formed by the photo-oxidation of toluene, the subsequent formation and transformation of BrC, and its optical properties. We discovered that nitrogen-poor (NP) SOA is formed when the molar ratio of NO_x to reacted toluene (henceforth, [NO_x/ΔHC]) is 0.15 or less, whereas nitrogen-rich (NR) SOA is formed when [NO_x/ΔHC] > 0.15. NR and NP SOA have markedly different characteristics. The light absorption coefficient (B_abs) and mass absorption cross-section (MAC) of the SOA increased with [NO_x/ΔHC] under both the NP and NR regimes. For NP SOA, the MAC increased with [NO_x/ΔHC] independently of the relative humidity (RH). However, the MAC of NR SOA was RH-dependent. Under both NP and NR regimes, acidity promoted SOA browning while NH₃ increased B_abs and MAC at 80% RH. The highest MAC was observed at the lowest RH (20%) for acidic NR SOA, and it was postulated that the MAC of SOA depends mainly on the pH and the [H⁺]_free/[SOA mass] ratio of the aqueous SOA phase.