Abstract. Light-absorbing organic aerosol, or brown carbon (BrC), has significant but
poorly constrained effects on climate; for example, oxidation in the
atmosphere may alter its optical properties, leading to absorption
enhancement or bleaching. Here, we investigate for the first time the effects
of heterogeneous OH oxidation on the optical properties of a laboratory
surrogate of aqueous, secondary BrC in a series of photo-oxidation chamber
experiments. The BrC surrogate was generated from aqueous resorcinol, or
1,3-dihydroxybenzene, and H2O2 exposed to >300 nm
radiation that is atomized, passed through trace gas denuders, and injected
into the chamber, which was conditioned to either 15 % or 60 %
relative humidity (RH). Aerosol absorption and scattering coefficients and single scattering albedo (SSA) at
405 nm were measured using a photoacoustic spectrometer. At
60 % RH, upon OH exposure,
absorption first increased, and the SSA decreased sharply. Subsequently,
absorption decreased faster than scattering, and SSA increased gradually.
Comparisons to the modelled trend in SSA, based on Mie theory calculations,
confirm that the observed trend is due to chemical evolution, rather than
slight changes in particle size. The initial absorption enhancement is likely
due to molecular functionalization and/or oligomerization and the bleaching
to fragmentation. By contrast, at 15 % RH, slow absorption enhancement
was observed without appreciable bleaching. A multi-layer kinetics model,
consisting of two surface reactions in series, was constructed to provide
further insights regarding the RH dependence of the optical evolution. Candidate parameters suggest
that the oxidation is efficient, with uptake coefficients on the order of
unity. The parameters also suggest that, as RH decreases, reactivity
decreases and aerosol viscosity increases, such that particles are well-mixed
at 60 % RH but not at 15 % RH. These results further the current
understanding of the complex processing of BrC that may occur in the
atmosphere.
Development of a 443 nm diode laser-based differential photoacoustic spectrometer for simultaneous measurements of aerosol absorption and NO2
