Effects of inorganic salts on the heterogeneous OH oxidation of organic compounds: insights from methylglutaric acid–ammonium sulfate
Authors: Lam Hoi Ki , Shum Sze Man , Davies James F. , Song Mijung , Zuend Andreas , Chan Man Nin 
Published: July 2019

Abstract. Atmospheric particles, consisting of inorganic salts, organic compounds and
a varying amount of water, can continuously undergo heterogeneous oxidation
initiated by gas-phase oxidants at the particle surface, changing the
composition and properties of particles over time. To date, most studies
focus on the chemical evolution of pure organic particles upon oxidation. To
gain more fundamental insights into the effects of inorganic salts on the
heterogeneous kinetics and chemistry of organic compounds, we investigate
the heterogeneous OH oxidation of 3-methylglutaric acid (3-MGA) particles
and particles containing both 3-MGA and ammonium sulfate (AS) in an
organic-to-inorganic mass ratio of 2 in an aerosol flow tube reactor at a
high relative humidity of 85.0 %. The molecular information of the
particles before and after OH oxidation is obtained using the direct
analysis in real time (DART), a soft atmospheric pressure ionization source
coupled to a high-resolution mass spectrometer. Optical microscopy
measurements reveal that 3-MGA–AS particles are in a single liquid phase
prior to oxidation at high relative humidity. Particle mass spectra show
that C6 hydroxyl and C6 ketone functionalization products are
the major products formed upon OH oxidation in the absence and presence of
AS, suggesting that the dissolved salt does not significantly affect
reaction pathways. The dominance of C6 hydroxyl products over C6 ketone products could be explained by the intermolecular hydrogen
abstraction by tertiary alkoxy radicals formed at the methyl-substituted
tertiary carbon site. On the other hand, kinetic measurements show that the
effective OH uptake coefficient, γeff, for 3-MGA–AS particles
(0.99±0.05) is smaller than that for 3-MGA particles (2.41±0.13) by about a factor of ∼2.4. A smaller reactivity observed in 3-MGA–AS particles might be attributed to a higher surface
concentration of water molecules and the presence of ammonium and sulfate
ions, which are chemically inert to OH radicals, at the particle surface.
This could lower the collision probability between the 3-MGA and OH
radicals, resulting in a smaller overall reaction rate. Our results suggest
that inorganic salts likely alter the overall heterogeneous reactivity of
organic compounds with gas-phase OH radicals rather than reaction mechanisms
in well-mixed aqueous organic–inorganic droplets at a high humidity, i.e.,
85 % relative humidity (RH). It also acknowledges that the effects of inorganic salts on the
heterogeneous reactivity could vary greatly, depending on the particle
composition and environmental conditions (e.g., RH and temperature). For
instance, at lower relative humidities, aqueous 3-MGA–AS droplets likely
become more concentrated and more viscous before efflorescence, possibly
giving rise to diffusion limitation during oxidation under relatively dry or
cold conditions. Further studies on the effects of inorganic salts on the
diffusivity of the species under different relative humidities within the
organic–inorganic particles are also desirable to better understand the
role of inorganic salts in the heterogeneous reactivity of organic
compounds.