Reactivity of aminophenols in forming nitrogen-containing brown carbon from iron-catalyzed reactions

Abstract

Nitrogen-containing organic carbon (NOC) in atmospheric particles is an important class of brown carbon (BrC). Redox active NOC like aminophenols received little attention in their ability to form BrC. Here we show that iron can catalyze dark oxidative oligomerization of o- and p-aminophenols under simulated aerosol and cloud conditions (pH 1–7, and ionic strength 0.01–1 M). Homogeneous aqueous phase reactions were conducted using soluble Fe(III), where particle growth/agglomeration were monitored using dynamic light scattering. Mass yield experiments of insoluble soot-like dark brown to black particles were as high as 40%. Hygroscopicity growth factors (κ) of these insoluble products under sub- and super-saturated conditions ranged from 0.4–0.6, higher than that of levoglucosan, a prominent proxy for biomass burning organic aerosol (BBOA). Soluble products analyzed using chromatography and mass spectrometry revealed the formation of ring coupling products of o- and p-aminophenols and their primary oxidation products. Heterogeneous reactions of aminophenol were also conducted using Arizona Test Dust (AZTD) under simulated aging conditions, and showed clear changes to optical properties, morphology, mixing state, and chemical composition. These results highlight the important role of iron redox chemistry in BrC formation under atmospherically relevant conditions.

Contribution of Carbonyl Chromophores in Secondary Brown Carbon from Nighttime Oxidation of Unsaturated Heterocyclic Volatile Organic Compounds

“…38,64,65 Although triones in ambient aerosols were less reported, 66 they have been suggested as possible contributors to BrC generated from aqueous-phase reactions. 67,68 The collective evidence demonstrates that carbonyl chromophores are ubiquitous constituents of SOA and may play an active role in secondary BrC formation.…”

Iron content in aerosol particles and its impact on atmospheric chemistry

“…There remain unresolved research questions on the role of metals in aqueous and multiphase transformations of NOC and formation of secondary light-absorbing NOC. In the following section, recent studies on the reactivity of soluble and insoluble Fe with phenolic chemical compounds, o - and p -aminophenols ( o AP and p AP) 49 and gallic acid, 50 are presented.…”

“…The net redox potential values () for the reactions between phenol or aniline and Fe( iii ) are negative indicating non-spontaneity under acidic conditions, whereas they are positive between o AP and p AP. In our studies using o AP and p AP 49 conducted at room temperature (296 K), we used soluble Fe( iii ) and insoluble Fe from AZTD to catalyze redox chemistry and analyze soluble and insoluble reaction products for comparison with those obtained from metal free oxidation of o AP and p AP that yields polymeric products referred to as poly( o -aminophenol) and poly( p -aminophenol). 70 We found that soluble Fe( iii ) can catalyze dark oxidative oligomerization of o AP and p AP under simulated aerosol and cloud conditions (pH 1–7) and ionic strength 0.01–1 M adjusted by ammonium sulfate.…”

“…4C). 49 These MEC spectra for the control and reacted slurries were calculated for the colored solid products using eqn (4):where l is the light pathlength. Scattering from the suspended particles is dominant with an absorbance feature around 430 nm for the slurry reacted with o AP assigned to the quinone imine moieties.…”

Formation of insoluble brown carbon through iron-catalyzed reaction of biomass burning organics

“…2,[19][20][21][22][23] Previous laboratory studies have shown that Fe(III) can catalyze oligomerization reactions with known constituents of BBOA and industrial emissions to produce lower volatility secondary brown carbon (BrC) products. [24][25][26][27][28][29][30][31] Reaction solutions were open to air and dissolved O 2 acted as an oxidant. These irreversible, oxidative polymerization reactions formed waterinsoluble, black particles in acidic systems (pH 3) with excess Fe(III).…”