Heterogeneous Uptake of Amines by Citric Acid and Humic Acid

Heterogeneous uptake of methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA) onto citric acid and humic acid was investigated using a Knudsen cell reactor coupled to a quadrupole mass spectrometer at 298 K. Acid–base reactions between amines and carboxylic acids were confirmed. The observed uptake coefficients of MA, DMA, and TMA on citric acid at 298 K were measured to be 7.31 ± 1.13 × 10^–3, 6.65 ± 0.49 × 10^–3, and 5.82 ± 0.68 × 10^–3, respectively, and showed independence of sample mass. The observed uptake coefficients of MA, DMA, and TMA on humic acid at 298 K increased linearly with sample mass, and the true uptake coefficients of MA, DMA, and TMA were measured to be 1.26 ± 0.07 × 10^–5, 7.33 ± 0.40 × 10^–6, and 4.75 ± 0.15 × 10^–6, respectively. Citric acid, having stronger acidity, showed a higher reactivity than humic acid for a given amine; while the steric effect of amines was found to govern the reactivity between amines and citric acid or humic acid

Laboratory Study on the Hygroscopic Behavior of External and Internal C₂–C₄ Dicarboxylic Acid–NaCl Mixtures

Atmospheric aerosol is usually found to be a mixture of various inorganic and organic components in field measurements, whereas the effect of this mixing state on the hygroscopicity of aerosol particles has remained unknown. In this study, the hygroscopic behavior of mixtures of C₂–C₄ dicarboxylic acids and NaCl was investigated. For both externally and internally mixed malonic acid–NaCl and succinic acid–NaCl particles, correlation between water content and chemical composition was observed and the water content of these mixtures at relative humidity (RH) above 80% can be well predicted by the Zdanovskii–Stokes–Robinson (ZSR) method. In contrast, a nonlinear relation between the total water content of the mixtures and the water content of each chemical composition separately was found for oxalic acid–NaCl mixtures. Compared to the values predicted by the ZSR method, the dissolution of oxalic acid in external mixtures resulted in an increase in the total water content, whereas the formation of less hygroscopic disodium oxalate in internal mixtures led to a significant decrease in the total water content. Furthermore, we found that the hygroscopicity of the sodium dicarboxylate plays a critical role in determining the aqueous chemistry of dicarboxylic acid–NaCl mixtures during the humidifying and dehumidifying process. It was also found that the hydration of oxalic acid and the deliquescence of NaCl did not change in external oxalic acid–NaCl mixtures. The deliquescence relative humidity (DRHs) for both malonic acid and NaCl decreased in both external and internal mixtures. These results could help in understanding the conversion processes of dicarboxylic acids to dicarboxylate salts, as well as the substitution of Cl by oxalate in the atmosphere. It was demonstrated that the effect of coexisting components on the hygroscopic behavior of mixed aerosols should not be neglected

Heterogeneous Kinetics of <i>cis</i>-Pinonic Acid with Hydroxyl Radical under Different Environmental Conditions

To understand the atmospheric fate of secondary organic aerosol (SOA), heterogeneous degradation behaviors of a specific tracer derived from α-pinene–<i>cis</i>-pinonic acid (CPA), initiated by hydroxyl radicals (OH), were investigated under different environmental conditions using a flow reactor. The second-order rate constant (<i>k</i>₂) of the CPA–OH reaction was determined to be (6.17 ± 1.07) × 10^–12 cm³·molecule^–1·s^–1 at 25 °C and 40% relative humidity (RH). Higher temperature promoted this reaction, while relative humidity had a little inhibiting effect on it. The atmospheric lifetime of CPA varied from 2.1 to 3.3 days under different environmental conditions. Infrared spectrometry (IR), density functional theory (DFT) calculation and gas chromatography coupled mass spectrometry (GC–MS) results indicated that the oxidation products should be ascribed to poly­(carboxylic acid)­s. This study shows that the heterogeneous degradation of CPA initiated by OH radical is appreciable, and the concentrations of CPA measured in field measurements may underestimate the corresponding precursors of SOA

Role of NH₃ in the Heterogeneous Formation of Secondary Inorganic Aerosols on Mineral Oxides

In this work, a relationship between the role of NH₃ and the properties of mineral oxides (α-Fe₂O₃, α-Al₂O₃, CaO, and MgO) in the evolution of NO₃^–, SO₄^2–, and NH₄⁺ has been established. It was found that the promotion effect of NH₃ was more favorable for the formation of NO₃^– (or SO₄^2–) and NH₄⁺ on acidic α-Fe₂O₃ and α-Al₂O₃ due to acid–base interactions between NO₂ with NH₃ or between SO₂ and NH₃, while this effect was weaker on basic CaO and MgO possibly due to their basic nature. The acid–base interaction (NO₂/SO₂ with NH₃) overpowered the redox reaction (SO₂ with NO₂) on Fe₂O₃ owing to its unique redox chemistry. However, the opposite was found on basic CaO and MgO for the formation of SO₄^2– and NO₃^–. Under equivalent concentration conditions, the two synergistic effects did not further strengthen on Fe₂O₃, CaO and MgO due to a competition effect. In NH₃-rich situation, a synchronous increase of SO₄^2–, NO₃^–, and NH₄⁺ occurred on Fe₂O₃. On acidic Al₂O₃, the favorable adsorption of NH₃ on the surface as well as the existence of NO₂ with an oxidizing capability synergistically promoted the formation of SO₄^2–, NO₃^–, and NH₄⁺

SO₂ Photoaging Enhances the Surface Conversion of NO₂‑to-HONO on Elemental Carbon

Chemical interactions between soot and NO2 are believed to play a significant role in the formation of HONO in the atmosphere. Despite extensive studies, the present understanding of how soot chemistry influences HONO formation remains contentious due to the rapid deactivation of surface reactive sites. In this study, we reveal the novel mechanism that the photoaging of SO2 can notably accelerate the reduction of functionalized elemental carbon (EC) in soot by rapidly removing surface hydroxyl functional groups. The reduced EC can further drive continuous HONO formation due to the rejuvenation of the surface reduction reactivity. We verify that the increase in surface vacancy defects created by the removal of OH groups is the key contributing factor and the reactive centers driving NO2 adsorption and reduction. This finding challenges the existing notion that fresh soot is rapidly deactivated due to the decline in reductive capacity. Our work suggests that aged graphene-like EC on soot may have a significant effect on the chemical conversion of NO2-to-HONO in polluted air, contributing to a better understanding of air pollution chemistry

SO₂ Initiates the Efficient Conversion of NO₂ to HONO on MgO Surface

Nitrous acid (HONO) is an important source of hydroxyl radical (OH) that determines the fate of many chemically active and climate relevant trace gases. However, the sources and the formation mechanisms of HONO remain poorly understood. In this study, the effect of SO₂ on the heterogeneous reactions of NO₂ on MgO as a mineral dust surrogate was investigated. The reactivity of MgO to NO₂ is weak, while coexisting SO₂ can increase the uptake coefficients of NO₂ on MgO by 2–3 orders of magnitude. The uptake coefficients of NO₂ on SO₂-aged MgO are independent of NO₂ concentrations in the range of 20–160 ppbv and relative humidity (0–70%RH). The reaction mechanism was demonstrated to be a redox reaction between NO₂ and surface sulfite. In the presence of SO₂, NO₂ was reduced to nitrite under dry conditions, which could be further converted to gas-phase HONO in humid conditions. These results suggest that the reductive effect of SO₂ on the heterogeneous conversion of NO₂ to HONO may have a significant contribution to the unknown sources of HONO observed in polluted areas (for example, in China)

A review on the heterogeneous oxidation of SO₂ on solid atmospheric particles: Implications for sulfate formation in haze chemistry

The oxidation of sulfur dioxide (SO2) to sulfate in the atmosphere is an important concern in regional air quality, global climate change, and human health. While gas-phase and liquid-phase oxidation of SO2 are widely regarded as important sources of sulfate, the contribution of the heterogeneous oxidation process on particle surfaces is controversial. Recently, this heterogeneous chemistry has been considered to be an important mechanism that is missing in current models to explain sulfate concentrations observed in haze episodes in East Asia. Therefore, the heterogeneous oxidation of SO2 on particles under the conditions of complex air pollution needs to be reassessed. This review summarizes the fundamental understanding of the heterogeneous reactions of SO2 on solid particles such as mineral dust, black carbon, sea salts, organic aerosol, and so on. The factors affecting the mechanism and kinetics of the heterogeneous reactions of SO2, including coexisting components (O3, NO2, H2O2, NH3, and VOCs), reactive sites, surface properties, relative humidity, and illumination, are reviewed. Reactive oxygen species involved in the heterogeneous oxidation of SO2 on particles are discussed. To our knowledge, while previous reviews have appeared on the oxidation of SO2 in the aqueous-phase, this is the first review on the atmospheric heterogeneous reactions of SO2 on the surface of solid particles, which can be of help in understanding the sulfur cycle in the atmosphere and its environmental impacts. A number of recommendations for future research are also presented.</p

Additional HONO and OH Generation from Photoexcited Phenyl Organic Nitrates in the Photoreaction of Aromatics and NO_<i>x</i>

HONO acts as a major OH source, playing a vital role in secondary pollutant formation to deteriorate regional air quality. Strong unknown sources of daytime HONO have been widely reported, which significantly limit our understanding of radical cycling and atmospheric oxidation capacity. Here, we identify a potential daytime HONO and OH source originating from photoexcited phenyl organic nitrates formed during the photoreaction of aromatics and NOx. Significant HONO (1.56–4.52 ppb) and OH production is observed during the photoreaction of different kinds of aromatics with NOx (18.1–242.3 ppb). We propose an additional mechanism involving photoexcited phenyl organic nitrates (RONO2) reacting with water vapor to account for the higher levels of measured HONO and OH than the model prediction. The proposed HONO formation mechanism was evidenced directly by photolysis experiments using typical RONO2 under UV irradiation conditions, during which HONO formation was enhanced by relative humidity. The 0-D box model incorporated in this mechanism accurately reproduced the evolution of HONO and aromatic. The proposed mechanism contributes 5.9–36.6% of HONO formation as the NOx concentration increased in the photoreaction of aromatics and NOx. Our study implies that photoexcited phenyl organic nitrates are an important source of atmospheric HONO and OH that contributes significantly to atmospheric oxidation capacity