Atmospheric secondary aerosol formation by heterogeneous reactions of aldehydes in the presence of a sulfuric acid aerosol catalyst

Quantitative kinetics reveal that reactions of HO2 are a significant sink for aldehydes in the atmosphere and may initiate the formation of highly oxygenated molecules via autoxidation

Large aldehydes are widespread in the atmosphere and their oxidation leads to secondary organic aerosols. The current understanding of their chemical transformation processes is limited to hydroxyl radical (OH) oxidation during daytime and nitrate radical (NO3) oxidation during nighttime. Here, we report quantitative kinetics calculations of the reactions of hexanal (C5H11CHO), pentanal (C4H9CHO), and butanal (C3H7CHO) with hydroperoxyl radical (HO2) at atmospheric temperatures and pressures. We find that neither tunneling nor multistructural torsion anharmonicity should be neglected in computing these rate constants; strong anharmonicity at the transition states is also important. We find rate constants for the three reactions in the range 3.2-7.7 × 10-14 cm3 molecule-1 s-1 at 298 K and 1 atm, showing that the HO2 reactions can be competitive with OH and NO3 oxidation under some conditions relevant to the atmosphere. Our findings reveal that HO2-initiated oxidation of large aldehydes may be responsible for the formation of highly oxygenated molecules via autoxidation. We augment the theoretic studies with laboratory flow-tube experiments using an iodide-adduct time-of-flight chemical ionization mass spectrometer to confirm the theoretical predictions of peroxy radicals and the autoxidation pathway. We find that the adduct from HO2 + C5H11CHO undergoes a fast unimolecular 1,7-hydrogen shift with a rate constant of 0.45 s-1. We suggest that the HO2 reactions make significant contributions to the sink of aldehydes.

The role of aldehydes on sulfur based-new particle formation: a theoretical study

Aldehydes play a crucial role in the formation of atmospheric particles, attracting significant attention due to their environmental impact. However, the microscopic mechanisms underlying the formation of aldehyde-involved particles remain uncertain. In this study, through quantum chemical calculations and molecular dynamics (MD) simulations, we investigate the microscopic formation mechanisms of binary and ternary systems composed of three representative aldehydes, two sulfur-based acids, water, and two bases. Our research findings reveal that the most stable structures of acid-aldehyde clusters involve the connection of acids and aldehyde compounds through hydrogen bonds without involving proton transfer reactions, indicating relatively poor cluster stability. However, with the introduction of a third component, the stability of 18 clusters significantly increase. Among these, in ten systems, acids act as catalysts, facilitating reactions between aldehyde compounds and water or alkaline substances to generate glycols and amino alcohols. However, according to MD simulations conducted at 300 K, these acids readily dissociate from the resulting products. In the remaining eight systems, the most stable structural feature involves ion pairs formed by proton transfer reactions between acids and aldehyde compounds. These clusters exhibit remarkable thermodynamic stability. Furthermore, the acidity of the acid, the nature of nucleophilic agents, and the type of aldehyde all play significant roles in cluster stability and reactivity, and they have synergistic effects on the nucleation process. This study offers microscopic insights into the processes of new particle formation involving aldehydes, contributing to a deeper understanding of atmospheric chemistry at the molecular level.

Experimental and theoretical study of Criegee intermediate (CH2OO) reactions with n-butyraldehyde and isobutyraldehyde: kinetics, implications and atmospheric fate

The reactions of the simplest Criegee intermediate (CH2OO) with n-butyraldehyde (nBD) and isobutyraldehyde (iBD) were studied at 253-318 K and (50 ± 2) torr, using Cavity Ring-down spectroscopy (CRDS). The rate coefficients obtained at room temperature were (2.63 ± 0.14) × 10-12 and (2.20 ± 0.21) × 10-12 cm3 molecule-1 s-1 for nBD and iBD, respectively. Both the reactions show negative temperature-dependency, following equations, knBD(T = 253-318 K) = (11.51 ± 4.33) × 10-14 × exp{(918.1 ± 107.2)/T} and kiBD(T = 253-318 K) = (6.23 ± 2.29) × 10-14 × exp{(1051.4 ± 105.2)/T} cm3 molecule-1 s-1. High-pressure limit rate coefficients were determined from theoretical calculations at the CCSD(T)-F12/cc-pVTZ-F12//B3LYP/6-311+G(2df, 2p) level of theory, with <40% deviation from the experimental results at room temperature and above. The kinetic simulations were performed using a master equation solver to predict the temperature-dependency of the rate coefficients at the experimental pressure, as well as to predict the contribution of individual pathways. The major products predicted from the theoretical calculations were formaldehyde and formic acid, along with butyric acid from nBD and isobutyric acid from iBD reactions.

Comparison of Gas-Particle Partitioning of Glyoxal and Methylglyoxal in the Summertime Atmosphere at the Foot and Top of Mount Hua

Glyoxal and methylglyoxal are important volatile organic compounds in the atmosphere. The gas-particle partitioning of these carbonyl compounds makes significant contributions to O₃ formation. In this study, both the gas- and particle-phase glyoxal and methylglyoxal concentrations at the foot and top of Mount Hua were determined simultaneously. The results showed that the gaseous-phase glyoxal and methylglyoxal concentrations at the top were higher than those at the foot of the mountain. However, the concentrations for the particle phase showed the opposite trend. The average theoretical values of the gas-particle partitioning coefficients of the glyoxal and methylglyoxal concentrations (4.57 × 10^-10 and 9.63 × 10^-10 m³ μg^-1, respectively) were lower than the observed values (3.79 × 10^-3 and 6.79 × 10^-3 m³ μg^-1, respectively). The effective Henry's law constants (eff.K_H) of the glyoxal and methylglyoxal were in the order of 10⁸ to 10⁹ mol/kgH2O/atm, and they were lower at the foot than they were at the top. The particle/gas ratios (P/G ratios) of the glyoxal and methylglyoxal were 0.039 and 0.055, respectively, indicating more glyoxal and methylglyoxal existed in the gas phase. The factors influencing the partitioning coefficients of the glyoxal and methylglyoxal were positively correlated with the relative humidity (RH) and negatively correlated with the PM_2.5 value. Moreover, the partitioning coefficient of the glyoxal and methylglyoxal was more significant at the top than at the foot of Mount Hua.

Theoretical Study of the Gas-Phase Hydrolysis of Formaldehyde to Produce Methanediol and Its Implication to New Particle Formation

Aldehydes were speculated to be important precursor species in new particle formation (NPF). The direct involvement of formaldehyde (CH₂O) in sulfuric acid and water nucleation is negligible; however, whether its atmospheric hydrolysate, methanediol (CH₂(OH)₂), which contains two hydroxyl groups, participates in NPF is not known. This work investigates both CH₂O hydrolysis and NPF from sulfuric acid and CH₂(OH)₂ with quantum chemistry calculations and atmospheric cluster dynamics modeling. Kinetic calculation shows that reaction rates of the gas-phase hydrolysis of CH₂O catalyzed by sulfuric acid are 11-15 orders of magnitude faster than those of the naked path at 253-298 K. Based on structures and the calculated formation Gibbs free energies, the interaction between sulfuric acid/its dimer/its trimer and CH₂(OH)₂ is thermodynamically favorable, and CH₂(OH)₂ forms hydrogen bonds with sulfuric acid/its dimer/its trimer via two hydroxyl groups to stabilize clusters. Our further cluster kinetic calculations suggested that the particle formation rates of the system are higher than those of the binary system of sulfuric acid and water at ambient low sulfuric acid concentrations and low relative humidity. In addition, the formation rate is found to present a negative temperature dependence because evaporation rate constants contribute significantly to it. However, cluster growth is essentially limited by the weak formation of the largest clusters, which implies that other stabilizing vapors are required for stable cluster formation and growth.

Synergistic Effects of SO2 and NH3 Coexistence on SOA Formation from Gasoline Evaporative Emissions

Vehicular evaporative emissions make an increasing contribution to anthropogenic sources of volatile organic compounds (VOCs), thus contributing to secondary organic aerosol (SOA) formation. However, few studies have been conducted on SOA formation from vehicle evaporative VOCs under complex pollution conditions with the coexistence of NO_x, SO₂, and NH₃. In this study, the synergistic effects of SO₂ and NH₃ on SOA formation from gasoline evaporative VOCs with NO_x were examined using a 30 m³ smog chamber with the aid of a series of mass spectrometers. Compared with the systems involving SO₂ or NH₃ alone, SO₂ and NH₃ coexistence had a greater promotion effect on SOA formation, which was larger than the cumulative effect of the two promotions alone. Meanwhile, contrasting effects of SO₂ on the oxidation state (OSc) of SOA in the presence or absence of NH₃ were observed, and SO₂ could further increase the OSc with the coexistence of NH₃. The latter was attributed to the synergistic effects of SO₂ and NH₃ coexistence on SOA formation, wherein N-S-O adducts can be formed from the reaction of SO₂ with N-heterocycles generated in the presence of NH₃. Our study contributes to the understanding of SOA formation from vehicle evaporative VOCs under highly complex pollution conditions and its atmospheric implications.

Review of the influencing factors of secondary organic aerosol formation and aging mechanism based on photochemical smog chamber simulation methods

The formation and aging mechanism of secondary organic aerosol (SOA) and its influencing factors have attracted increasing attention in recent years because of their effects on climate change, atmospheric quality and human health. However, there are still large errors between air quality model simulation results and field observations. The currently undetected components during the formation and aging of SOA due to the limitation of current monitoring techniques and the interactions among multiple SOA formation influencing factors might be the main reasons for the differences. In this paper, we present a detailed review of the complex dynamic physical and chemical processes and the corresponding influencing factors involved in SOA formation and aging. And all these results were mainly based the studies of photochemical smog chamber simulation. Although the properties of precursor volatile organic compounds (VOCs), oxidants (such as OH radicals), and atmospheric environmental factors (such as NO_x, SO₂, NH₃, light intensity, temperature, humidity and seed aerosols) jointly influence the products and yield of SOA, the nucleation and vapor pressure of these products were found to be the most fundamental aspects when interpreting the dynamics of the SOA formation and aging process. The development of techniques for measuring intermediate species in SOA generation processes and the study of SOA generation and aging mechanism in complex systems should be important topics of future SOA research.

Probing Individual Particles Generated at the Freshwater-Seawater Interface through Combined Raman, Photothermal Infrared, and X-ray Spectroscopic Characterization

Sea spray aerosol (SSA) is one of the largest global sources of atmospheric aerosol, but little is known about SSA generated in coastal regions with salinity gradients near estuaries and river outflows. SSA particles are chemically complex with substantial particle-to-particle variability due to changes in water temperature, salinity, and biological activity. In previous studies, the ability to resolve the aerosol composition to the level of individual particles has proven necessary for the accurate parameterization of the direct and indirect aerosol effects; therefore, measurements of individual SSA particles are needed for the characterization of this large source of atmospheric aerosol. An integrated analytical measurement approach is required to probe the chemical composition of individual SSA particles. By combining complementary vibrational microspectroscopic (Raman and optical photothermal infrared, O-PTIR) measurements with elemental information from computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM-EDX), we gained unique insights into the individual particle chemical composition and morphology. Herein, we analyzed particles from four experiments on laboratory-based SSA production using coastal seawater collected in January 2018 from the Gulf of Maine. Individual salt particles were enriched in organics compared to that in natural seawater, both with and without added microalgal filtrate, with greater enrichment observed for smaller particle sizes, as evidenced by higher carbon/sodium ratios. Functional group analysis was carried out using the Raman and infrared spectra collected from individual SSA particles. Additionally, the Raman spectra were compared with a library of Raman spectra consisting of marine-derived organic compounds. Saccharides, followed by fatty acids, were the dominant components of the organic coatings surrounding the salt cores of these particles. This combined Raman, infrared, and X-ray spectroscopic approach will enable further understanding of the factors determining the individual particle composition, which is important for understanding the impacts of SSA produced within estuaries and river outflows, as well as areas of snow and ice melt.

Suppression of the phenolic SOA formation in the presence of electrolytic inorganic seed

Phenolic compounds are largely attributed to wildfire gases and rapidly react with atmospheric oxidants to form persistent phenoxy free radicals, which influence atmospheric chemistry and secondary organic aerosol (SOA) formation. In this study, phenol or o-cresol was photochemically oxidized under various conditions (NO_x levels, humidity, and seed conditions) in an outdoor photochemical reactor. Unexpectedly, SOA growth of both phenols was suppressed in the presence of salted aqueous aerosol compared to non-seed SOA. This discovery is different from the typical SOA formation of aromatic or biogenic hydrocarbons, which show noticeably higher SOA yields via organic aqueous reactions. Phenol, o-cresol, and their phenolic products (e.g., catechols) are absorbed in aqueous aerosol and form phenoxy radicals via heterogeneous reactions under sunlight. The resulting phenoxy radicals are redistributed between the gas and particle phases. Gaseous phenoxy radicals quickly react with ozone to form phenyl peroxide radicals and regenerated through a NO_x cycle to retard phenol oxidation and SOA formation. The explicit oxidation mechanisms of phenol or o-cresol in the absence of aqueous phase were derived including the Master Chemical Mechanism (MCM v3.3.1) and the path for peroxy radical adducts originating from the addition of an OH radical to phenols to form low volatility products (e.g., multi-hydroxy aromatics). The resulting gas mechanisms of phenol or o-cresol were, then, applied to the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model to predict SOA formation via multiphase partitioning of organics and aerosol-phase oligomerization. The model well simulated chamber-generated phenolic SOA in absence of wet-inorganic seed, but significantly overestimated SOA mass in presence of wet seed. This study suggests that heterogeneous chemistry to form phenoxy radicals needs to be included to improve SOA prediction from phenols. The suppression of atmospheric oxidation due to phenoxy radicals in wet inorganic aerosol can explain the low SOA formation during wildfire episodes.

Evaluation of Coriolis Micro Air Sampling to Detect Volatile and Semi-Volatile Organic Compounds

There are several analytical procedures available for the monitoring of volatile organic compounds (VOCs) in the air, which differ mainly on sampling procedures. The Coriolis micro air sampler is a tool normally designed for biological air sampling. In this paper, the Coriolis micro bio collector is used to evaluate its ability to sample organic contaminants sampling and detecting them when combined GC-MS. We also compare the use of the Coriolis micro with a standardized sampling method, which is the use of a lung box with a Nalophan^® bag. The results show that the Coriolis micro sampling method is suitable for the sampling of organic contaminants. Indeed, the Coriolis micro allows to sample and detect mainly semi-volatile molecules, while the lung box/Nalophan^® bags allow to sample more volatile molecules (highly volatile and volatile). These results were confirmed in the controlled air lab with a slight difference with the field. The simultaneous use of the both techniques allow to sample and detect a larger number of molecules with specific physicochemical properties to each sampling technique. In conclusion, the Coriolis micro can sample and detect volatile organic compounds present in air. We have shown that the development of alternative sampling methods and the use of non-target analysis are essential for a more comprehensive risk assessment. Moreover, the use of the Coriolis micro allows the detection of emergent molecules around the Thau lagoon.

Field observations and quantifications of atmospheric formaldehyde partitioning in gaseous and particulate phases

Formaldehyde (HCHO) can possibly be taken by atmospheric particles due to its moderate solubility. Although previous model studies have proposed that uptake by particles was a large sink for HCHO, direct observation of HCHO partitioning and estimation of HCHO uptake coefficient (γ) for tropospheric conditions are still limited. In this work, online measurements of gaseous HCHO (HCHO_g) and particulate HCHO (HCHO_p) were carried out simultaneously at an urban site in Beijing in winter and spring. The results indicated that the average concentrations of HCHO_p ranged from 0.15 to 0.4 μg m^-3, accounting for 1.2% to 10% of the total HCHO (i.e., HCHO_g + HCHO_p). The median values of estimated γ based on the measured data were in the range of about 1.09 ∗ 10^-5-2.42 ∗ 10^-4, with lower values during PM_2.5 pollution episodes. Besides, the pH and liquid water content of aerosols that are mainly determined by ambient relative humidity (RH) and inorganic salt composition were identified as the main influencing factors of γ. We propose that the HCHO uptake process was mainly driven by hydrone and hydrogen ions in particles.

Enhanced uptake of methacrolein at the acidic nanoparticle interface: Adsorption, heterogeneous reaction and impact for the secondary organic aerosol formation

Considering the moderate acidity of aerosols, the formation of secondary organic aerosols (SOA) through acid-catalyzed heterogeneous reactions has become a recent concern. However, the detailed information on the multiphase chemistry of organic compounds adsorbed onto acidic aerosols remains uncertain. In this work, we investigated the multiphase chemical processes between methacrolein (MACR) and sulfuric acid (SA) and their relationship with SOA formation. Results show that the aqueous nanoparticle interface, especially when it is an acidic nanoparticle interface, is a perfect area to adsorb and accommodate MACR. The occurrence percentage of MACR on the interface is more than 70%. With the increase of SA concentration, the first solvation shell changed from only water to the mixture of SA and water, which facilitates the heterogeneous hydration reaction of MACR. Compared with the neutral nanoparticle interface, the acidic nanoparticle interface exhibits a better ability to uptake and accommodate gaseous carbonyl species. Moreover, SA can catalyze the hydration reaction of MACR inside the aqueous media, and the resulting oligomers contribute to the formation and growth of SOA. The hydration reaction indirectly promotes the continuous adsorption of MACR at the acidic nanoparticle interface. The rate constant shows a positive altitude dependence, and acid-catalyzed reactions have an important impact on environmental chemistry, such as cloud SOA formation, within the range of about 2-6 km. This study reports a complete description of the heterogeneous interactions between unsaturated carbonyl species and acidic nanoparticles by using molecular dynamics and quantum chemistry methods, aiming to provide some insights for the further study on heterogeneous chemistry and its role in the formation of tropospheric SOA.

Non-ignorable contribution of anthropogenic source to aerosols in Arctic Ocean

Arctic Ocean (AO) atmospheric aerosols, which are a factor influencing regional and global climate, have been greatly influenced by an increase in anthropogenic sources. To identify the impact of anthropogenic sources on regional aerosols in the AO and middle and low latitudes (MLO), a single-particle aerosol mass spectrometer was used to count and size aerosols with diameters less than 2.5 μm (PM_2.5) and determine their chemical composition. The mean hourly count of PM_2.5 aerosols was 1639/h in the AO, which was 57.1% lower than that in the MLO. Na_MSA, sulfate, and Na_rich were three major components, which accounted for 74.3% of PM_2.5 aerosols in the AO. The size distribution of PM_2.5 aerosols was unimodal, peaking between 0.42 μm and 1.64 μm. A source apportionment method for single aerosol particles in the Arctic was established using positive matrix factorization (PMF) combined with backward air mass trajectory and principal component analysis (PCA). Three potential sources of aerosols were identified: marine sources; anthropogenic sources; and secondary formation. The largest contribution to aerosols in the AO was from marine sources, accounting for 50.6%. This source was 20.4% higher in the AO than that in the MLO. Secondary formation contributed 19.8% and 36.5% to aerosols in the AO and MLO, respectively. However, the contribution of anthropogenic sources to aerosols was 29.6% in the AO, and this was 3.7% lower than that in the MLO. Our study provides a useful method for identifying sources of aerosols in the Arctic, and the results showed that although marine sources were the largest contributors to aerosols in the AO, the contribution of anthropogenic sources could not be ignored.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.

Synergistic Uptake by Acidic Sulfate Particles of Gaseous Mixtures of Glyoxal and Pinanediol

The uptake of gaseous organic species by atmospheric particles can be affected by the reactive interactions among multiple co-condensing species, yet the underlying mechanisms remain poorly understand. Here, the uptake of unary and binary mixtures of glyoxal and pinanediol by neutral and acidic sulfate particles is investigated. These species are important products from the oxidation of volatile organic compounds (VOCs) under atmospheric conditions. The uptake to acidic aerosol particles greatly increased for a binary mixture of glyoxal and pinanediol compared to the unary counterparts. The strength of the synergism depended on the particle acidity and water content (i.e., relative humidity). The greater uptake was up to 2.5× to 8× at 10% relative humidity (RH) for glyoxal and pinanediol, respectively. At 50% RH, it was 2× and 1.2× for the two species. Possible mechanisms of acid-catalyzed cross reactions between the species are proposed to explain the synergistic uptake. The proposed mechanisms are applicable to a broader extent across atmospheric species having carbonyl and hydroxyl functionalities. The results thus suggest that synergistic uptake reactions can be expected to significantly influence the gas-particle partitioning of VOC oxidation products under atmospheric conditions and thus greatly affect their atmospheric transport and lifetime.

Computational Study of the Effect of Mineral Dust on Secondary Organic Aerosol Formation by Accretion Reactions of Closed-Shell Organic Compounds

The effect of dust aerosols on accretion reactions of water, formaldehyde, and formic acid was studied in the conditions of earth's troposphere at the DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97X-D/6-31++G** level of theory. A detailed analysis of the reaction mechanisms in the gas phase and on the surface of mineral dust, represented by mono- and trisilicic acid, revealed that mineral dust has the potential of decreasing reaction barrier heights. Specifically, at 0 K, mineral dust can lower the apparent energy barrier of the reaction of formaldehyde with formic acid to zero. However, when the entropic contributions to the reaction free energies were accounted for, mineral dust was found to selectively enhance the reaction of water with formaldehyde, while inhibiting the reaction of formaldehyde and formic acid, in the lower parts of the troposphere (with temperatures around 298 K). In the upper troposphere (with temperatures closer to 198 K), mineral dust catalyzes both reactions and also the reaction of methanol with formic acid. Despite the intrinsic potential of mineral dust, calculation of the catalytic enhancement parameter for a likely range of dust aerosol concentrations suggested that dust aerosols will not contribute to secondary organic aerosol formation via dimerization of closed-shell organic compounds. The main reason for this is the relatively low absolute concentration of tropospheric dust aerosol and its inefficiency in increasing the effective reaction rate coefficients.

Atmospheric chemistry of CH3CHO: the hydrolysis of CH3CHO catalyzed by H2SO4

We found the catalytic effect of H₂SO₄ on the hydrolysis of CH₃CHO in the atmosphere.

A density functional theory study of aldehydes and their atmospheric products participating in nucleation

The products of aldehydes from aldol condensation, hydration, and polymerization reactions can promote new particle formation by stabilizing sulfuric acid.

Formation of Low-Volatility Organic Compounds in the Atmosphere: Recent Advancements and Insights

Secondary organic aerosol (SOA) formation proceeds by bimolecular gas-phase oxidation reactions generating species that are sufficiently low in volatility to partition into the condensed phase. Advances in instrumentation have revealed that atmospheric SOA is less volatile and more oxidized than can be explained solely by these well-studied gas-phase oxidation pathways, supporting the role of additional chemical processes. These processes-autoxidation, accretion, and organic salt formation-can lead to exceedingly low-volatility species that recently have been identified in laboratory and field studies. Despite these new insights, the identities of the condensing species at the molecular level and the relative importance of the various formation processes remain poorly constrained. The thermodynamics of autoxidation, accretion, and organic salt formation can be described by equilibrium partitioning theory; a framework for which is presented here. This framework will facilitate the inclusion of such processes in model representations of SOA formation.

Effect of Pellet Boiler Exhaust on Secondary Organic Aerosol Formation from α-Pinene

Interactions between anthropogenic and biogenic emissions, and implications for aerosol production, have raised particular scientific interest. Despite active research in this area, real anthropogenic emission sources have not been exploited for anthropogenic-biogenic interaction studies until now. This work examines these interactions using α-pinene and pellet boiler emissions as a model test system. The impact of pellet boiler emissions on secondary organic aerosol (SOA) formation from α-pinene photo-oxidation was studied under atmospherically relevant conditions in an environmental chamber. The aim of this study was to identify which of the major pellet exhaust components (including high nitrogen oxide (NO_x), primary particles, or a combination of the two) affected SOA formation from α-pinene. Results demonstrated that high NO_x concentrations emitted by the pellet boiler reduced SOA yields from α-pinene, whereas the chemical properties of the primary particles emitted by the pellet boiler had no effect on observed SOA yields. The maximum SOA yield of α-pinene in the presence of pellet boiler exhaust (under high-NO_x conditions) was 18.7% and in the absence of pellet boiler exhaust (under low-NO_x conditions) was 34.1%. The reduced SOA yield under high-NO_x conditions was caused by changes in gas-phase chemistry that led to the formation of organonitrate compounds.

High molecular weight organic compounds (HMW-OCs) in severe winter haze: Direct observation and insights on the formation mechanism

High molecular weight organic compounds (HMW-OCs), formed as secondary organic aerosols (SOA), have been reported in many laboratory studies. However, little evidence of HMW-OCs formation, in particular during winter season in the real atmosphere, has been reported. In January 2013, Beijing faced historically severe haze pollution, in which the hourly PM_2.5 concentration reached as high as 974 μg m^-3. Four typical haze events (HE1 to HE4) were identified, and HE2 (Jan. 9-16) was the most serious of these. Based on the hourly observed chemical composition of PM_2.5 and the daily organic composition analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), we found that abundant ion peaks in m/z 200-850 appeared on heavy haze days, whereas these were negligible on a clear day, indicating the existence of HMW-OCs in the wintertime haze. A negative nonlinear correlation between HMW-OCs and O₃ suggested that gas oxidation was not likely to be the dominant mechanism for HMW-OCs formation. During the heavy haze events, the relative humidity and mass ratio of H₂O/PM_2.5 reached as high as 80% and 0.2, respectively. The high water content and its good positive correlation with HMW-OCs indicated that an aqueous-phase process may be a significant pathway in wintertime. The evidence that acidity was much higher during HE2 (0.37 μg m^-3) than on other days, as well as its strong correlation with HMW-OCs, indicated that acid-catalyzed reactions likely resulted in HMW-OCs formation during the heavy winter haze in Beijing.

The effects of acetaldehyde, glyoxal and acetic acid on the heterogeneous reaction of nitrogen dioxide on gamma-alumina

Heterogeneous reactions of nitrogen oxides on the surface of aluminium oxide result in the formation of adsorbed nitrite and nitrate. However, little is known about the effects of other species on these heterogeneous reactions and their products. In this study, diffuse reflectance infrared spectroscopy (DRIFTS) was used to analyze the process of the heterogeneous reaction of NO2 on the surface of aluminium oxide particles in the presence of pre-adsorbed organic species (acetaldehyde, glyoxal and acetic acid) at 298 K and reveal the influence of these organic species on the formation of adsorbed nitrite and nitrate. It was found that the pre-adsorption of organic species (acetaldehyde, glyoxal and acetic acid) on γ-Al2O3 could suppress the formation of nitrate to different extents. Under the same experimental conditions, the suppression of the formation of nitrate by the pre-adsorption of acetic acid is much stronger than that by pre-adsorption of acetaldehyde and glyoxal, indicating that the influence of acetic acid on the heterogeneous reaction of NO2 is different from that of acetaldehyde and glyoxal. Surface nitrite is formed and identified to be an intermediate product. For the heterogeneous reaction of NO2 on the surface of γ-Al2O3 with and without the pre-adsorption of acetaldehyde and glyoxal, it is firstly formed and then gradually disappears as the reaction proceeds, but for the reaction with the pre-adsorption of acetic acid, it is the final main product besides nitrate. This indicates that the pre-adsorption of acetic acid would promote the formation of nitrite, while the others would not change the trend of the formation of nitrite. The possible influence mechanisms of the pre-adsorption of acetaldehyde, glyoxal and acetic acid on the heterogeneous conversion of NO2 on γ-Al2O3 are proposed and atmospheric implications based on these results are discussed.

Uptake and release of gaseous species accompanying the reactions of isoprene photo-oxidation products with sulfate particles

Uptake and release of gaseous species was observed for the reactions of isoprene photo-oxidation products and sulfate particles.

Nanoparticle formation in a chemical storage room as a new incidental nanoaerosol source at a nanomaterial workplace

Chemical storage rooms located near engineered nanomaterials (ENMs) workplaces can be a significant source of unintentional nanoaerosol generation. A new incidental nanoparticle source was identified and characterized in a chemical storage room located at an ENMs workplace. Stationary and mobile measurements using on-line instruments and chemical analysis of volatile organic compounds (VOCs) were carried out to identify the source. The number of nanoaerosols emitted from the chemical storage room was found to be several orders of magnitude higher than that existing in the ENMs workplace. VOC analysis showed that the accumulated precursors and oxygenated VOCs in the chemical storage room could be attributed to incidental particle formation via gas-to-particle conversion. We stress the importance of identification of the incidental nanoaerosols to allow characterization of the nanoaerosols at ENMs workplaces, and to estimate additional nanoaerosols exposure, which was previously unknown. Hazardous chemical substances in the workplace have been regulated in many countries; however, most of the regulations are focused on gas-phase or liquid-phase substances. The present study emphasizes the importance of secondary pollutants in particulate form that can be generated from the gas or liquid phase of hazardous chemical substances.

Interactions between Heterogeneous Uptake and Adsorption of Sulfur Dioxide and Acetaldehyde on Hematite

Sulfur dioxide and organic aldehydes in the atmosphere are ubiquitous and often correlated with mineral dust aerosols. Heterogeneous uptake and adsorption of one of these species on mineral aerosols can potentially change the properties of the particles and further affect the subsequent heterogeneous reactions of the other species on the coating particles. In this study, the interactions between heterogeneous uptake and adsorption of sulfur dioxide and acetaldehyde on hematite are investigated by using in situ diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) at room temperature. It is found that the preadsorption of SO2 on α-Fe2O3 can significantly hinder the subsequent heterogeneous oxidation of CH3CHO to acetate, while the preadsorption of CH3CHO significantly suppresses the heterogeneous reaction of large amounts of SO2 on the surface of α-Fe2O3 and has a little influence on the uptake of small amount of SO2. The heterogeneous reactions of SO2 on α-Fe2O3 preadsorbed by CH3CHO change the existing acetate on the particle surface into chemisorbed acetic acid, for the enhancement of surface acidity after the uptake of SO2. During these processes, different surface hydroxyl groups showed different reactivities. Atmospheric implications of this study are discussed.

Thermodynamics of oligomer formation: implications for secondary organic aerosol formation and reactivity

Dimers and higher order oligomers, whether in the gas or particle phase, can affect important atmospheric processes such as new particle formation, and gas-particle partitioning. In this study, the thermodynamics of dimer formation from various oxidation products of α-pinene ozonolysis are investigated using a combination of Monte Carlo configuration sampling, semi-empirical and density functional theory (DFT) quantum mechanics, and continuum solvent modeling. Favorable dimer formation pathways are found to exist in both gas and condensed phases. The free energies of dimer formation are used to calculate equilibrium constants and expected dimer concentrations under a variety of conditions. In the gas phase, favorable pathways studied include formation of non-covalent dimers of terpenylic acid and/or cis-pinic acid and a covalently-bound peroxyhemiacetal. Under atmospherically relevant conditions, only terpenylic acid forms a dimer in sufficient quantities to contribute to new particle formation. Under conditions typically used in laboratory experiments, several dimer formation pathways may contribute to particle formation. In the condensed phase, non-covalent dimers of terpenylic acid and/or cis-pinic acid and covalently-bound dimers representing a peroxyhemiacetal and a hydrated aldol are favorably formed. Dimer formation is both solution and temperature dependent. A water-like solution appears to promote dimer formation over methanol- or acetonitrile-like solutions. Heating from 298 K to 373 K causes extensive decomposition back to monomers. Dimers that are not favorably formed in either the gas or condensed phase include hemi-acetal, ester, anhydride, and the di(α-hydroxy) ether.

Pattern of volatile aldehydes and aromatic hydrocarbons in the largest urban rainforest in the Americas

Atmospheric concentrations of aldehydes and monoaromatic hydrocarbons were determined in Tijuca Forest, the largest urban tropical forest in the Americas. The forest is a protected area, surrounded by the city of Rio de Janeiro. Data were also obtained in a commercial and a residential area for comparison. A total of 160 aldehyde samples and 60 BTEX (benzene, toluene, ethyl-benzene and xylenes) samples were collected from four locations between January and August of 2008. The aldehydes were collected using C18 resin cartridges coated with 2,4-dinitrophenylhydrazine and analyzed by high performance liquid chromatography (HPLC) with a diode array UV-Vis detector, while the BTEX samples were collected using tubes of coconut charcoal, which were then extracted with dichloromethane and analyzed by gas chromatography (GC). Within Tijuca Forest, formaldehyde and acetaldehyde levels were in the range of <detection limit - 5.09 ppbV and <detection limit - 4.08 ppbV, respectively. Formaldehyde concentrations strongly correlated with temperature and solar radiation. The different ratios for formaldehyde and acetaldehyde concentrations in the forest and in the urban sites clearly suggested that carbonyl levels within the forest might have an important contribution from biogenic sources. BTEX concentrations in the forest were very low, showing that the forest acted as a sink for many pollutants. Toluene/benzene ratios in the forest were also lower than in the city, which may be attributed to the faster photochemical oxidation of toluene. These observations were indicators of the low impact of the urban area on the studied forest.