###### Abstract  
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\({}_{\mathrm{x}}\), NH\({}_{3}\))-, acidity (H\({}_{2}\)SO\({}_{4}\))-, 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\({}_{\mathrm{x}}\) to reacted toluene (henceforth, [NO\({}_{\mathrm{x}}\)/AHC]) is 0.15 or less, whereas nitrogen-rich (NR) SOA is formed when [NO\({}_{\mathrm{x}}\)/AHC] > 0.15. NR and NP SOA have markedly different characteristics. The light absorption coefficient (B\({}_{\mathrm{abs}}\)) and mass absorption cross-section (MAC) of the SOA increased with [NO\({}_{\mathrm{x}}\)/AHC] under both the NP and NR regimes. For NP SOA, the MAC increased with [NO\({}_{\mathrm{x}}\)/AHC] 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 NH\({}_{3}\) increased B\({}_{\mathrm{abs}}\) 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\({}^{+}\)]\({}_{\mathrm{free}}\)/[SOA mass] ratio of the aqueous SOA phase. s 2022 202202202202202202202202022020220202020202020202020202020202020202020202020202020202020202020202020202020202020213131313131313131313131313131313131313131313131313131313  
aerosol (SOA). The latter processes include nucleation, condensation, and heterogeneous and multiphase chemical reactions that form highly oxidized multifunctional molecules (HOMs) with extremely low volatility and high molecular weights [3; 10; 11], some of which are strongly light-absorbing [12; 13; 14; 15]. Known primary and secondary sources of BrC aerosols include fossil fuel combustion [16], biomass burning [17; 18], biological aerosols (e.g., soil humics and bioaerosols) [19], and SOA formed from anthropogenic or biogenic precursors [13]. While the primary BrC sources are straightforward to identify, the mechanisms that generate light-absorbing SOA are largely unknown [8]. Quantifying BrC's contribution to light absorption is critically important for accurate interpretation of the aerosol optical depth (AOD), i.e., the light extinction within the atmospheric column due to scattering and absorption. Aromatic compounds comprise 20-40% of gasoline by volume and are major anthropogenic volatile organic compounds (VOCs) that play an important role in the formation of tropospheric ozone and SOA [20; 21; 22; 23]. The aromatic compounds toluene and benzene are thought to be some of the most important SOA precursors because of their high anthropogenic emissions and SOA formation potential [20; 22; 24; 25; 26; 27; 28]. The optical properties of aromatic-derived SOA have therefore been studied extensively over the last decade [26; 27; 28; 29; 30; 31; 32; 33; 34]. The highest mass absorption coefficient (MAC) values are observed for toluene SOA formed under high-NO\({}_{\rm x}\) conditions at moderate RH with short photolysis aging times [27]. However, the optical properties of toluene/benzene-derived SOA formed under other atmospherically-relevant conditions are unclear. In particular, there are four major factors whose effects on the light absorption characteristics of toluene SOA are unknown: (1) the molar ratio of NO\({}_{\rm x}\) to reacted toluene ([NO\({}_{\rm x}\)/\(\Delta\)HC]) in the atmosphere, (2) the RH and the dependence of its effects on [NO\({}_{\rm x}\)/\(\Delta\)HC], (3) acidity and the dependence of its effects on [NO\({}_{\rm x}\)/\(\Delta\)HC] and the RH, and (4) aging in the presence of [OH].