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 (NH 3 ), nitrogen oxide (NO x ), and relative humidity (RH) conditions. In the presence of NH 3 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 NH 3 enhanced the SOA absorption. Under intermediate-NO x dry conditions, the MAC org values increased from 0.13 m 2 g –1 in NH 3 -free conditions to 0.28 m 2 g –1 in high-NH 3 conditions. Under high-NH 3 conditions, the MAC org value further increased to 0.36 m 2 g –1 with an increase in RH. Under dry high-NO x conditions, the MAC org value increased from 0.42 to 0.67 m 2 g –1 with the addition of NH 3, while with elevated RH, the MAC org value reached 0.70 m 2 g –1 . The time series of MAC org showed increasing trends only in the presence of NH 3 . 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 NH 3 and was enhanced under elevated RH.