Effects of copper on chemical kinetics and brown carbon formation in the aqueous ˙OH oxidation of phenolic compounds.

Many phenolic compounds (PhCs) in biomass burning and fossil fuel combustion emissions can partition into atmospheric aqueous phases (e.g., cloud/fog water and aqueous aerosols) and undergo reactions to form secondary organic aerosols (SOAs) and brown carbon (BrC). Redox-active transition metals, particularly Fe and Cu, are ubiquitous species in atmospheric aqueous phases known to participate in Fenton/Fenton-like chemistry as a source of aqueous ˙OH. However, even though the concentrations of water-soluble Cu are close to those of water-soluble Fe in atmospheric aqueous phases in some areas, unlike Fe, the effects that Cu have on SOA and BrC formation in atmospheric aqueous phases have scarcely been studied and remain poorly understood. We investigated the effects of Cu(II) on PhC reaction rates and BrC formation during the aqueous oxidation of four PhCs (guaiacol, catechol, syringol, and vanillin) by ˙OH generated from Fenton-like chemistry under different pH conditions. While the PhCs reacted when both H2O2 and Cu(II) were present in the absence (i.e., dark oxidation) and presence (i.e., photooxidation) of light, the reaction rates were at least one order of magnitude higher during photooxidation. Higher PhC reaction rates were measured at higher pH during both dark oxidation and photooxidation as a result of higher ˙OH concentrations produced by Fenton-like chemistry. Only water-soluble BrC was formed during dark oxidation and photooxidation when Cu(II) was present. Mass absorption coefficients (103 to 104 cm2 g-1) comparable to those of biomass burning BrC were measured during dark oxidation and photooxidation when Cu(II) was present. Light absorption was enhanced at higher pH during dark oxidation and photooxidation, which indicated that higher quantities and/or more absorbing BrC chromophores were formed at higher pH. The effects that Cu(II) had on the PhC reaction rates and the composition of SOAs and BrC formed depended on the PhC base structure (i.e., benzenediol vs. methoxyphenol). Overall, these results show how aqueous reactions involving Cu(II), H2O2, and PhCs can be an efficient source of daytime and nighttime water-soluble BrC and SOAs, which can have significant implications for how the atmospheric fates of PhCs are modeled for areas with substantial concentrations of water-soluble Cu in highly to moderately acidic cloud/fog water and aqueous aerosols.

Kinetics of the nitrate-mediated photooxidation of monocarboxylic acids in the aqueous phase.

The photooxidation of organic compounds by hydroxyl radicals (·OH) in atmospheric aqueous phases contributes to both the formation and aging of secondary organic aerosols (SOAs), which usually include carboxylic acids. Hydrogen peroxide (H2O2) and inorganic nitrate are two important ·OH photochemical sources in atmospheric aqueous phases. The aqueous phase pH is an important factor that not only controls the dissociation of carboxylic acids and consequently their ·OH reactivities, but also the production of ·OH and other reactive species from the photolysis of some ·OH photochemical precursors, particularly inorganic nitrate. While many studies have reported on the aqueous pH-dependent photodegradation rates of carboxylic acids with ·OH produced by H2O2 photolysis, the aqueous pH-dependent photodegradation rates of carboxylic acids with ·OH produced by inorganic nitrate photolysis have not been studied. In this work, we investigated the pH-dependent (pH 2 to 7) aqueous photooxidation of formic acid (FA), glycolic acid (GA), and pyruvic acid (PA) initiated by the photolysis of ammonium nitrate (NH4NO3). The observed reaction rates of the three carboxylic acids were controlled by the [NH4NO3]/[carboxylic acid] concentration ratio. Higher [NH4NO3]/[carboxylic acid] concentration ratios resulted in faster photodegradation rates, which could be attributed to the higher concentrations of ·OH produced from the photolysis of higher concentrations of NH4NO3. In addition, the observed photodegradation rates of the three carboxylic acids strongly depended on the pH. The highest photodegradation rate was observed at pH 4 for FA, whereas the highest photodegradation rates were observed at pH 2 for GA and PA. The observed pH-dependent FA and GA photodegradation rates were due to the combined effects of the pH-dependent ·OH formation from NH4NO3 photolysis and the differences in ·OH reactivities of dissociated vs. undissociated FA and GA. In contrast, the observed pH-dependent PA photodegradation rate was due primarily to the pH-dependent decarboxylation of PA initiated by light. These results highlight how the aqueous phase pH and inorganic nitrate photolysis can combine to influence the degradation rates of carboxylic acids, which can have significant implications for how the atmospheric fates of carboxylic acids are modeled for regions with substantial concentrations of inorganic nitrate in cloud water and aqueous aerosols.

Evaluation of Laminated Composite Beam Theory Accuracy.

Carbon fiber-reinforced polymer (CFRP) has been widely implemented in electric vehicle bodies and aircraft fuselage structures. The purpose of CFRP is to reduce the weight and impart rigidity in the final product. A beam structure is typically used to bear the structural load, and the rigidity of the beam can be changed by arranging the laminated fibers at different angles. In this study, a composite I-beam is used as an example in engineering components. Because the theoretical model of the superimposed composite I-beam is established, the theoretical formula is based on the theoretical assumptions of the two-dimensional composite beam, and is combined with the traditional composite plate theory to analyze the maximum bending stress, strain, and deflection. During the theoretical derivation, it is assumed that the flanges of the I-beams are divided into narrow and wide flanges. The beams are considered as structures of beams and flatbeds. When a narrow flange is loaded in the side, the wide flange has no lateral deformation, and the lateral moments are neglected. Therefore, the accuracy of this formula needs to be verified. The purpose of this study is to verify the accuracy of theoretical solutions for the deflection and stress analysis of composite beams. A finite element analysis model is used as the basis for comparing the theoretical solutions. The results indicate that when the aspect ratio of the beam is >15, the theoretical solution will have better accuracy. Without the addition of the material, when 0° ply is placed on the outermost layer of the flange of the nonsymmetric beam, the effective rigidity of the beam is increased by 4−5% compared with the symmetrical beam. The accuracy range of the theoretical solution for the composite beams can be accurately defined based on the results of this study.