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 (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.