Secondary brown carbon from photooxidation of 1-methylnaphthalene and longifolene

An improved understanding of the optical properties of secondary organic aerosol (SOA) particles is needed to better predict their climate impacts. Here, SOA was produced by reacting 1-methylnaphthalene or longifolene with hydroxyl radicals (OH) under variable ammonia (NH₃), nitrogen oxide (NO_x), and relative humidity (RH) conditions. In the presence of NH₃ and NO_x, longifolene-derived aerosols had relatively high single scattering albedo (SSA) values and low absorption coefficients at 375 nm independent of RH, suggesting that the longifolene SOA is mostly scattering. In 1-methylnaphthalene experiments, the resulting SSA and SOA mass absorption coefficient (MAC_org) values suggest the formation of light-absorbing SOA, and the addition of high NO_x and high NH₃ enhanced the SOA absorption. Under intermediate-NO_x dry conditions, the MAC_org values increased from 0.13 m² g⁻¹ in NH₃-free conditions to 0.28 m² g⁻¹ in high-NH₃ conditions. Under high-NH₃ conditions, the MAC^org value further increased to 0.36 m² g⁻¹ with an increase in RH. Under dry high-NO_x conditions, the MAC^org value increased from 0.42 to 0.67 m² g⁻¹ with the addition of NH³, while with elevated RH, the MAC^org value reached 0.70 m² g⁻¹. The time series of MAC_org showed increasing trends only in the presence of NH₃. Composition analysis of SOA suggests that organonitrates, nitroorganics, and other nitrogen-containing organic compounds (NOCs) are potential chromophores in the 1-methylnaphthalene SOA. Significant formation of NOCs was observed in the presence of high-NO_x and NH₃ and was enhanced under elevated RH.