Influence of Ammonia and Relative Humidity on the Formation and Composition of Secondary Brown Carbon from Oxidation of 1-Methylnaphthalene and Longifolene

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 (NOx), and relative humidity (RH) conditions. In the presence of NH3 and NOx, 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 (MACorg) values suggest the formation of light-absorbing SOA, and the addition of high NOx and high NH3 enhanced the SOA absorption. Under intermediate-NOx dry conditions, the MACorg 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 MACorg value further increased to 0.36 m2 g-1 with an increase in RH. Under dry high-NOx conditions, the MACorg value increased from 0.42 to 0.67 m2 g-1 with the addition of NH3, while with elevated RH, the MACorg value reached 0.70 m2 g-1. The time series of MACorg 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-NOx and NH3 and was enhanced under elevated RH.