Solar Absorption by Aerosol-Bound Nitrophenols Compared to Aqueous and Gaseous Nitrophenols

Nitrophenols
are well-known absorbers of near-UV/blue radiation
and are considered to be a component of solar-absorbing organic aerosol
material commonly labeled brown carbon. Nitrophenols have been identified
in a variety of phases in earth’s atmosphere, including the
gaseous, aqueous, and aerosol bound, and these different environments
alter their UV–vis absorption spectra, most dramatically when
deprotonated forming nitrophenolates. We quantify the impact of these
different absorption profiles by calculating the solar power absorbed
per molecule for several nitrophenols. For instance, aqueous 2,4-dinitrophenol
absorption varies dramatically over the pH range of cloud droplets
with pH = 5.5 solutions absorbing three times the solar power compared
to pH = 3.5 solutions. We also measured the UV–vis spectra
of 2-nitrophenol adsorbed on several aerosol substrates representative
of mineral dust, inorganic salts, and organic aerosol and compare
these spectra to gaseous and aqueous 2-nitrophenol. 2-Nitrophenol
adsorbed on mineral and chloride aerosol substrates exhibits a red-shifted
absorption band (∼450–650 nm) consistent with 2-nitrophenolate
and absorbs twice the solar power per molecule compared to gaseous,
aqueous, and organic aerosol-bound 2-nitrophenol. We also discuss
how different nitrophenol absorption profiles alter important atmospheric
photolysis rate constants [e.g., J(NO2) and J(O3)] by attenuating solar flux.