Dark Iron-Catalyzed Reactions in Acidic and Viscous Aerosol Systems Efficiently Form Secondary Brown Carbon

Abstract

Iron-driven secondary brown carbon formation reactions from water-soluble organics in cloud droplets and aerosols create insoluble and soluble products of emerging atmospheric importance. This work shows, for the first time, results on dark iron-catalyzed polymerization of catechol forming insoluble black polycatechol particles and colored water-soluble oligomers under conditions characteristic of viscous multicomponent aerosol systems with relatively high ionic strength (I = 1–12 m) and acidic pH (∼2). These systems contain ammonium sulfate (AS)/nitrate (AN) and C3–C5 dicarboxylic acids, namely, malonic, malic, succinic, and glutaric acids. Using dynamic light scattering (DLS) and ultra high pressure liquid chromatography-mass spectrometry (UHPLC-MS), we show results on the rate of particle growth/agglomeration and identity of soluble oligomeric reaction products. We found that increasing I above 1 m and adding diacids with oxygen-to-carbon molar ratio (O:C > 1) significantly reduced the rate of polycatechol formation/aggregation by a factor of 1.3 ± 0.4 in AS solution in the first 60 min of reaction time. Using AN, rates were too slow to be quantified using DLS, but particles formed after 24 h reaction time. These results were explained by the relative concentration and affinity of ligands to Fe­(III). We also report detectable amounts of soluble and colored oligomers in reactions with a slow rate of polycatechol formation, including organonitrogen compounds. These results highlight that brown carbon formation from iron chemistry is efficient under a wide range of aerosol physical states and chemical composition.

Aging of atmospheric aerosols and the role of iron in catalyzing brown carbon formation

“…China. Table 2 lists the physical properties of C2-C6 dicarboxylic acid that we used to explore their complexation to iron and their effect on the extent of BrC formation using catechol 68,148 and in the absence of catechol. 69 The following sections elaborate more on these cases.…”

“…222 the OHradical plays a role in the first and second steps. Under conditions characteristic of viscous multicomponent aerosol systems with relatively high ionic strength (I = 1-12 m) and acidic pH (~2) that likely affected the kinetics of the polymerization reactions, Al-Abadleh et al 148 detected colored water-soluble oligomers shown in Scheme 2 using ultra high pressure liquid chromatography mass spectrometry (UHPLC-MS).  max = 407 nm HO Scheme 2: Structure of molecules and oligomers and their characteristic absorption wavelength detected in the reaction of catechol with Fe(III) under conditions characteristic of viscous multicomponent aerosol systems with relatively high ionic strength (I = 1-12 m) and acidic pH (~2) using UHPLC-MS.…”

“…35 Uptake of VOCs from biogenic and anthropogenic sources, gas and particle phase oxidation reactions can produce lowvolatility products leading to mixing of organic species with inorganic salts. 35 Iron-catalyzed polymerization of catechol was investigated 148 under conditions characteristic of relatively viscous multicomponent aerosol systems and adsorbed water 70 with high ionic strength (I = 1-12 m), acidic pH (~2), 66,237,239 , and low water activity (0.6-0.97) 240 for comparison with earlier studies completed under conditions typical of cloud chemistry (I = 0.01 M, pH 3-5). 68,70 To vary the pH, I, and water activity, background aqueous phase solutions were prepared by adding either ammonium sulfate (AS) or ammonium nitrate (AN), and ubiquitous C3-C5 dicarboxylic acids (malonic, malic, succinic, and glutaric acids) detected in biomass burning aerosols 163 and used in laboratory studies investigating liquid-liquid phase separation.…”

“…For example, increasing the concentration of AN from 0.01 to 12.3 M reduced the %Fe-CA from 9.5 to 2.4, and adding diacids further reduced %Fe-CA to be in the range 0-0.8 depending on the diacid (see Table S2 in ref. 148 ). The %Fe-CA in AN solutions were considerably higher than those for Rxn 1-3 using AS (Table 6), which ranged from 1.2-0.12.…”

“…This result signifies the role that the type of the inorganic anion plays in the mechanism of polycatechol 6, and (right) AN solution of varying composition per Table S2 in ref. 148 . The line through the data is an exponential decay fit, y = a + b e -c x , with a = 0.5 nm min -1 , b = 31 nm min -1 , and c = 0.2 m -1 .…”