Heterogeneous Reactions between Toluene and NO₂ on Mineral Particles under Simulated Atmospheric Conditions

Heterogeneous reactions between organic and inorganic gases with aerosols are important for the study of smog occurrence and development. In this study, heterogeneous reactions between toluene and NO₂ with three atmospheric mineral particles in the presence or absence of UV light were investigated. The three mineral particles were SiO₂, α-Fe₂O₃, and BS (butlerite and szmolnokite). In a dark environment, benzaldehyde was produced on α-Fe₂O₃. For BS, nitrotoluene and benzaldehyde were obtained. No aromatic products were produced in the absence of NO₂ in the system. In the presence of UV irradiation, benzaldehyde was detected on the SiO₂ surface. Identical products were produced in the presence and absence of UV light over α-Fe₂O₃ and BS. UV light promoted nitrite to nitrate on mineral particles surface. On the basisi of the X-ray photoelectron spectroscopy (XPS) results, a portion of BS was reduced from Fe³⁺ to Fe²⁺ with the adsorption of toluene or the reaction with toluene and NO₂. Sulfate may play a key role in the generation of nitrotoluene on BS particles. From this research, the heterogeneous reactions between organic and inorganic gases with aerosols that occur during smog events will be better understood

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

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

Secondary Organic Aerosol Formation from Ambient Air at an Urban Site in Beijing: Effects of OH Exposure and Precursor Concentrations

Secondary organic aerosol (SOA) is an important component of atmospheric fine particles (PM_2.5), while the key factors controlling SOA formation in ambient air remain poorly understood. In this work, the SOA formation in Beijing urban ambient air was investigated using an oxidation flow reactor (OFR) with high concentrations of OH radicals. The SOA formation potential increased significantly with the increase of ambient PM_2.5 concentration during the observation. The optimum ambient exposure time, which is the aging time equivalent to atmospheric oxidation (with similar OH exposure) associated with the peak SOA formation, varied between 2 and 4 days in this study. The OA enhancement in this study was much higher than that of developed countries under different environmental conditions. The higher OA enhancement is probably due to the higher concentrations of volatile organic compounds (VOCs) in the urban air of Beijing. This might also have occurred because fragmentation did not dominate in the oxidation of OA, and did not result in negative OA enhancement on highly polluted days compared to relatively clean days with similar exposure time. These results suggested that under typical ambient conditions, high concentrations of VOC precursors might contribute to sustained organic aerosol growth and long duration haze events in Beijing

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⁺

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

Ozonolysis of Trimethylamine Exchanged with Typical Ammonium Salts in the Particle Phase

Alkylamines contribute to both new particle formation and brown carbon. The toxicity of particle-phase amines is of great concern in the atmospheric chemistry community. Degradation of particulate amines may lead to secondary products in the particle phase, which are associated with changes in the adverse health impacts of aerosols. In this study, O₃ oxidation of particulate trimethylamine (TMA) formed via heterogeneous uptake of TMA by (NH₄)₂SO₄, NH₄HSO₄, NH₄NO₃ and NH₄Cl, was investigated with in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and proton transfer reaction mass spectrometry (PTR-MS). HCOOH, HCHO, CH₃NCH₂, (CH₃)₂NCHO, CH₃NO₂, CH₃N­(OH)­CHO, CH₃NHOH and H₂O were identified as products on all the substrates based upon IR (one-dimensional IR and two-dimensional correlation infrared spectroscopy), quantum chemical calculation and PTR-MS results. A reaction mechanism was proposed to explain the observed products. This work demonstrates that oxidation might be a degradation pathway of particulate amines in the atmosphere. This will aid in understanding the fate of particulate amines formed by nucleation and heterogeneous uptake and their potential health impacts during atmospheric aging

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