Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy.

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-µm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Photoactivation of Chlorine and Its Catalytic Role in the Formation of Sulfate Aerosols.

We present a novel mechanism for the formation of photocatalytic oxidants in deliquescent NaCl particles, which can greatly promote the multiphase photo-oxidation of SO2 to produce sulfate. The photoexcitation of the [Cl--H3O+-O2] complex leads to the generation of Cl and OH radicals, which is the key reason for enhancing aqueous-phase oxidation and accelerating SO2 oxidation. The mass normalization rate of sulfate production from the multiphase photoreaction of SO2 on NaCl droplets could be estimated to be 0.80 × 10-4 µg·h-1 at 72% RH and 1.33 × 10-4 µg·h-1 at 81% RH, which is equivalent to the known O3 liquid-phase oxidation mechanism. Our findings highlight the significance of multiphase photo-oxidation of SO2 on NaCl particles as a non-negligible source of sulfate in coastal areas. Furthermore, this study underscores the importance of Cl- photochemistry in the atmosphere.

Formation Mechanism of Environmentally Persistent Free Radicals on Alkaline Earth Oxide Surfaces.

The formation of environmentally persistent free radicals (EPFRs) is usually related to transition-metal oxides in particulate matter (PM). However, recent studies suggest that alkaline-earth-metal oxides (AEMOs) in PM also influence EPFRs formation, but the exact mechanism remains unclear. Here, density functional theory calculations were performed to investigate the formation mechanism of EPFRs by C6H5OH on AEMO (MgO, CaO, and BaO) surfaces and compare it with that on transition-metal oxide (ZnO and CuO) surfaces. Results indicate that EPFRs can be rapidly formed on AEMOs by dissociative adsorption of C6H5OH, accompanied by electrons transfer. As the alkalinity of AEMOs increases, both adsorption energy and the number of electron transfers gradually increase. Also, the stability of the formed EPFRs is mainly attributed to the electrostatic and van der Waals interactions between the phenoxy radical and surfaces. Notably, the formation mechanism of EPFRs on AEMOs is similar to that on ZnO but differs from that on CuO, as suggested through geometric structure and charge distribution analyses. This study not only elucidates the formation mechanisms of EPFRs on AEMOs but also provides theoretical insights into addressing EPFRs pollution.

Rapid sulfuric acid-dimethylamine nucleation enhanced by nitric acid in polluted regions.

Recent research [Wang et al., Nature 581, 184-189 (2020)] indicates nitric acid (NA) can participate in sulfuric acid (SA)-ammonia (NH3) nucleation in the clean and cold upper free troposphere, whereas NA exhibits no obvious effects at the boundary layer with relatively high temperatures. Herein, considering that an SA-dimethylamine (DMA) nucleation mechanism was detected in megacities [Yao et al., Science 361, 278-281 (2018)], the roles of NA in SA-DMA nucleation are investigated. Different from SA-NH3 nucleation, we found that NA can enhance SA-DMA-based particle formation rates in the polluted atmospheric boundary layer, such as Beijing in winter, with the enhancement up to 80-fold. Moreover, we found that NA can promote the number concentrations of nucleation clusters (up to 27-fold) and contribute 76% of cluster formation pathways at 280 K. The enhancements on particle formation by NA are critical for particulate pollution in the polluted boundary layer with relatively high NA and DMA concentrations.

Chemical Implications of Rapid Reactive Absorption of I2O4 at the Air-Water Interface.

Marine aerosol formation involving iodine-bearing species significantly affects the global climate and radiation balance. Although recent studies outline the critical role of iodine oxide in nucleation, much less is known about its contribution to aerosol growth. This paper presents molecular-level evidence that the air-water interfacial reaction of I2O4 mediated by potent atmospheric chemicals, such as sulfuric acid (H2SO4) and amines [e.g., dimethylamine (DMA) and trimethylamine (TMA)], can occur rapidly on a picosecond time scale by Born-Oppenheimer molecular dynamics simulations. The interfacial water bridges the reactants while facilitating the DMA-mediated proton transfer and stabilizing the ionic products of H2SO4-involved reactions. The identified heterogeneous mechanisms exhibit the dual contribution to aerosol growth: (i) the ionic products (e.g., IO3-, DMAH+, TMAH+, and HSO4-) formed by reactive adsorption possess less volatility than the reactants and (ii) these ions, such as alkylammonium salts (e.g., DMAH+), are also highly hydrophilic, further facilitating hygroscopic growth. This investigation enhances not only our understanding of heterogeneous iodine chemistry but also the impact of iodine oxide on aerosol growth. Also, these findings can bridge the gap between the abundance of I2O4 in the laboratory and its absence in field-collected aerosols and provide an explanation for the missing source of IO3-, HSO4-, and DMAH+ in marine aerosols.

Methanesulfonic acid and iodous acid nucleation: a novel mechanism for marine aerosols.

By seeding clouds, new particle formation (NPF) has a substantial impact on radiation balance, bio-geochemical cycles and global climate. Over oceans, both methanesulfonic acid (CH3S(O)2OH, MSA) and iodous acid (HIO2) have been reported to be closely associated with NPF events; however, much less is known about whether they can jointly nucleate to form nanoclusters. Hence, quantum chemical calculations and Atmospheric Cluster Dynamics Code (ACDC) simulations were performed to investigate the novel mechanism of MSA-HIO2 binary nucleation. The results indicate that MSA and HIO2 can form stable clusters via multiple interactions including hydrogen bonds, halogen bonds, and electrostatic forces between ion pairs after proton transfer, which are more diverse than those in MSA-iodic acid (HIO3) and MSA-dimethylamine (DMA) clusters. Interestingly, HIO2 can be protonated by MSA exhibiting base-like behavior, but it differs from base nucleation precursors by self-nucleation rather than solely binding to MSA. Due to the greater stability of MSA-HIO2 clusters, the formation rate of MSA-HIO2 clusters can be even higher than that of MSA-DMA clusters, suggesting that MSA-HIO2 nucleation is a non-negligible source of marine NPF. This work proposes a novel mechanism of MSA-HIO2 binary nucleation for marine aerosols and provides deeper insights into the distinctive nucleation characteristics of HIO2, which can help in constructing a more comprehensive sulfur- and iodine-bearing nucleation model for marine NPF.

Iodous acid - a more efficient nucleation precursor than iodic acid.

Iodous acid (HIO2), a vital iodine oxyacid, potentially plays an important role in the formation of new particles in marine areas (He et al., Science, 2021, 371, 589-595). However, the nucleation mechanism of HIO2 is still poorly understood. Herein, the self-nucleation of HIO2 under different atmospheric conditions is investigated by a combination of quantum chemical calculations and the Atmospheric Cluster Dynamics Code (ACDC) simulations. The results indicate that HIO2 can form relatively stable molecular clusters through hydrogen bonds and halogen bonds, and the self-nucleation of HIO2 proceeds by sequential addition of HIO2 or HIO2-based small clusters. Besides, in order to better illustrate the role of HIO2 in new particle formation (NPF) in marine areas, we compare its nucleation properties with those of iodic acid (HIO3), a significant iodine-containing nucleation precursor in marine regions. We find that the cluster formation rate of the self-nucleation of HIO2 is higher than that of the self-nucleation of HIO3 although [HIO2] is lower than [HIO3], which indicates that the HIO2 molecule is a more efficient nucleation precursor than the HIO3 molecule. Therefore, the self-nucleation of HIO2 could become one of the most important sources for NPF in marine areas, which could provide potential theoretical evidence for explaining the intensive NPF events observed in these areas.

Unexpected quenching effect on new particle formation from the atmospheric reaction of methanol with SO3.

Despite the high abundance in the atmosphere, alcohols in general and methanol in particular are believed to play a small role in atmospheric new particle formation (NPF) largely due to the weak binding abilities of alcohols with the major nucleation precursors, e.g., sulfuric acid (SA) and dimethylamine (DMA). Herein, we identify a catalytic reaction that was previously overlooked, namely, the reaction between methanol and SO3, catalyzed by SA, DMA, or water. We found that alcohols can have unexpected quenching effects on the NPF process, particularly in dry and highly polluted regions with high concentrations of alcohols. Specifically, the catalytic reaction between methanol and SO3 can convert methanol into a less-volatile species--methyl hydrogen sulfate (MHS). The latter was initially thought to be a good nucleation agent for NPF. However, our simulation results suggest that the formation of MHS consumes an appreciable amount of atmospheric SO3, disfavoring further reactions of SO3 with H2O. Indeed, we found that MHS formation can cause a reduction of SA concentration up to 87%, whereas the nucleation ability of MHS toward new particles is not as good as that of SA. Hence, a high abundance of methanol in the atmosphere can lower the particle nucleation rate by as much as two orders of magnitude. Such a quenching effect suggests that the recently identified catalytic reactions between alcohols and SO3 need to be considered in atmospheric modeling in order to predict SA concentration from SO2, while also account for their potentially negative effect on NPF.

Gas-phase catalytic hydration of I2O5 in the polluted coastal regions: Reaction mechanisms and atmospheric implications.

Marine aerosols play an important role in the global aerosol system. In polluted coastal regions, ultra-fine particles have been recognized to be related to iodine-containing species and is more serious due to the impact of atmospheric pollutants. Many previous studies have identified iodine pentoxide (I2O5, IP) to be the key species in new particles formation (NPF) in marine regions, but the role of IP in the polluted coastal atmosphere is far to be fully understood. Considering the high humidity and concentrations of pollutants in the polluted coastal regions, the gas-phase hydration of IP catalyzed by sulfuric acid (SA), nitric acid (NA), dimethylamine (DMA), and ammonia (A) have been investigated at DLPNO-CCSD(T)//ωB97X-D/aug-cc-pVTZ + aug-cc-pVTZ-PP with ECP28MDF (for iodine) level of theory. The results show that the hydration of IP involves a significant energy barrier of 22.33 kcal/mol, while the pollutants SA, NA, DMA, and A all could catalyze the hydration of IP. Especially, with SA and DMA as catalysts, the hydration reactions of IP present extremely low barriers and high rate constants. It is suggested that IP is unstable under the catalysis of SA and DMA to generate iodic acid, which is the key component in NPF in marine regions. Thus, the catalytic hydration of IP is very likely to trigger the formation of iodine-containing particles. Our research provides a clear picture of the catalytic hydration of IP as well as theoretical guidance for NPF in the polluted coastal atmosphere.

Tetrahedral Cyclopentadienylmetal Carbonyl Clusters of Manganese and Chromium: A Theoretical Study.

Tetranuclear Cp4M4(CO)4 clusters have been synthesized for iron and vanadium but not for the intermediate first-row transition metals manganese and chromium. All of the low-energy structures of these "missing" Cp4M4(CO)4 (M = Mn, Cr) species are shown by density functional theory to consist of a central M4 tetrahedron with each of the four faces capped by a µ3-CO group. The individual low-energy structures differ in their spin states and in their formal metal-metal bond orders along the six edges of their central M4 tetrahedra. The two low-energy Cp4Mn4(µ3-CO)4 structures are a triplet structure with all Mn-Mn single bonds and a singlet structure with one Mn≡Mn triple bond and five Mn-Mn single bonds along the six tetrahedral edges. Related low-energy Cp4Cr4(µ3-CO)4 structures include a quintet structure with all Cr-Cr single bonds and a singlet structure with two Cr≡Cr triple bonds and four Cr-Cr single bonds. However, the potential energy surface of the Cp4Cr4(CO)4 system is complicated by three other structures of comparable energies including two triplet structures and one quintet structure with various combinations of single, double, and triple chromium-chromium bonds in the central Cr4 tetrahedron.

The potential mechanism of atmospheric new particle formation involving amino acids with multiple functional groups.

Amino acids are recognized as significant components of atmospheric aerosols. However, their potential role in atmospheric new particle formation (NPF) is poorly understood, especially aspartic acid (ASP), one of the most abundant amino acids in the atmosphere. It has not only two advantageous carboxylic acid groups but also one amino group, both of which are both effective groups enhancing NPF. Herein, the participation mechanism of ASP in the formation of new particle involving sulfuric acid (SA)-ammonia (A)-based system has been studied using the Density Functional Theory (DFT) combined with the Atmospheric Clusters Dynamic Code (ACDC). The results show that the addition of ASP molecules in the SA-A-based clusters provides a promotion on the interaction between SA and A molecules. Moreover, ACDC simulations indicate that ASP could present an obvious enhancement effect on SA-A-based cluster formation rates. Meanwhile, the enhancement strength R presents a positive dependence on [ASP] and a negative dependence on [SA] and [A]. Besides, the enhancement effect of ASP is compared with that of malonic acid (MOA) with two carboxylic acid groups (Chemosphere, 2018, 203, 26-33), and ASP presents a more obvious enhancement effect than MOA. The mechanism of NPF indicates that ASP could contribute to cluster formation as a "participator" which is different from the "catalytic" role of MOA at 238 K. These new insights are helpful to understand the mechanism of NPF involving organic compounds with multiple functional groups, especially the abundant amino acids, such as the ASP, in the urban/suburban areas with intensive human activities and industrial productions and therefore the abundant sources of amino acids. Furthermore, the NPF of the SA-A-based system involving amino acid should be considered when assessing the environmental risk of amino acid.

Catalytic sulfate formation mechanism influenced by important constituents of cloud water via the reaction of SO2 oxidized by hypobromic acid in marine areas.

Comprehensive investigations of the possible formation pathways of sulfate, the main composition of atmospheric aerosol in marine areas, continue to challenge atmospheric chemists. As one of the most important oxidation routes of S(iv) contributing to sulfate formation, the reaction process of S(iv) oxidized by hypobromic acid, which is ubiquitous with the gas-phase mixing ratios of ∼310 ppt and has a well-known oxidative capacity, has attracted wide attention. However, little information is available about the detailed reaction mechanism. Especially, due to the abundant species in cloud water, the potential effect of these compositions on these reaction processes and the corresponding effect mechanism are also uncertain. Using high-level quantum chemical calculations, we theoretically elucidate the two-step mechanism of Br+ transfer proposed by experiment through the verification of the key BrSO3- intermediate formation and subsequent hydrolysis reaction or the uncovered reaction of BrSO3- intermediate with OH-. Further, the novel and more competitive mechanisms (OH+ or O atom transfer pathways) that have not been considered in previous studies, leading to sulfate formation directly, have been found. Furthermore, it should be mentioned that we revealed the effect mechanism of constituents catalyzed in cloud water, especially the important H2O-catalyzed mechanism. In addition, all the above pathways follow this catalytic mechanism. This finding indicates a linkage between the complex nature of the atmospheric constituents and related atmospheric reaction, as well as the enhanced occurrence of atmospheric secondary sulfate formation in the atmosphere. Hence, this exploration of sulfate formation related to hypobromic acid could provide a better understanding about the sources of sulfate in marine areas.

Molecular-Scale Mechanism of Sequential Reaction of Oxalic Acid with SO3: Potential Participator in Atmospheric Aerosol Nucleation.

Recent research has shown the almost barrierless cycloaddition reaction of the carboxylic acid with one SO3 to form products with group of -OSO3H, which can form stable clusters with the nucleation precursors through hydrogen bonds (Mackenzie et al., Science 2015, 349, 58). Oxalic acid (OA), the simplest and prevalent dicarboxylic acid, was selected as an example to clarify the possibility to react with two SO3 sequentially and the nucleation potential of products. The results indicate that OA can sequentially react with two SO3 through low reaction barriers to form the primary product (oxalic sulfuric anhydride (OSA)) and the secondary product (oxalic disulfuric anhydride (ODSA)). Interactions between atmospheric nucleation precursors and OSA, ODSA, or OA are in the order of ODSA > OSA > OA through evaluating the stability of generated clusters by the topological, thermodynamics, and kinetic analysis, which implies generated products could be nucleation stabilizers with nucleation potential positively correlating with the number of -OSO3H. This reaction mechanism contributes to a comprehensive understanding of the reactivity of dicarboxylic acid in the polluted environment as well as the role of products in organosulfur chemistry and, to some extent, help to explain the missing sources of new particle formation.

Glyoxylic Sulfuric Anhydride from the Gas-Phase Reaction between Glyoxylic Acid and SO3: A Potential Nucleation Precursor.

Oxocarboxylic acids, one of the most important organic species, are detected in aerosols in various environments. Recent studies suggest that the gas-phase reactions between carboxylic acids and SO3 could form carboxylic sulfuric anhydrides, which might participate in nucleation. Here, glyoxylic acid (GA), the most abundant oxocarboxylic acid in the atmosphere, has been selected as an example to study the reactions between oxocarboxylic acids and SO3 and the nucleation potentials of products. The reaction between GA and SO3 that generates glyoxylic sulfuric anhydride (GSA) and the hydrolysis of GSA are investigated using computational methods. The results show that the reaction is almost barrierless, and GSA is stable against water. Additionally, the clusters of GSA and common nucleation species (sulfuric acid and ammonia) are more stable than the analogous clusters of GA, because they have more hydrogen bonds and proton transfers. It suggests that GA tends to transfer itself to a much better nucleation precursor, GSA, through a reaction with SO3, and GSA can drive nucleation and contribute to new particle formation (NPF). This mechanism might be general for all oxocarboxylic acids and could help to deeply understand the roles of oxocarboxylic acids in NPF.

Expression and regulation of miR-449a and AREG in cerebral ischemic injury.

Rodent focal ischemia models are widely used to mimic and examine human strokes. To the best of our knowledge, no investigation has systematically examined the expression changes of microRNA (miR)-449a and Amphiregulin (AREG) as well as their biological relationship during middle cerebral artery occlusion (MCAO) and oxygen and glucose deprivation/reperfusion (OGD/R). The present study examined the histological and behavioral outcomes of MCAO and the function of miR-449a and AREG in cerebral ischemic injury. Rats were subjected to 2 h MCAO, which was followed by reperfusion. miR-449a and AREG were examined in the injury tissues of MCAO rats and the OGD/R cell line by reverse transcription-quantitative polymerase chain reaction. Protein expressions of AREG in the injury tissues of MCAO rats was measured using an immunohistochemistry and the protein expression levels of AREG, epidermal growth factor receptor (EGFR), phosphatidylinositol 3-kinase (PI3K)/ protein kinase B (Akt) and the phosphorylation level of Akt (p-Akt) were analyzed by western blotting. Cell apoptosis was examined following the knock down and subsequent overexpression of AREG in a human OGD/R neuronal cell line by small interfering RNAs (siRNAs) and plasmid transfection. Luciferase reporter assays were used to validate the target of miR-449a. The expression changes and regulatory mechanisms of miR-449a and AREG in an ischemia/reperfusion (I/R) injury model were examined in vivo and in vitro. The neurological deficit score, brain edema volume, cerebral infarct area, and the number of apoptosis cells in ischemic rats were all markedly elevated, than that in the control rats. The expression of miR-449a was decreased and AREG was increased in the MCAO rats and human OGD/R neuronal cell line. miR-449a inhibition or AREG overexpression in OGD/R cells resulted in a significant decrease in apoptotic cells, and AREG was revealed to be one of the direct targets of miR-449a. Molecular recovery was observed following transfection with miR-449a mimics and AREG knockdown in an OGD/R model in vitro. The present study demonstrated that miR-449a was downregulated while AREG was upregulated in cerebral ischemic injury, and the recovery of neurological function can be obtained following the overexpression of miR-449a and the knockdown of AREG in an I/R injury model. miR-449a functions in ischemic stroke via directly targeting AREG. These findings suggest a novel mechanism involving in cerebral I/R injury model and may aid investigators in gaining a deeper understanding of strokes in a clinical setting.

Mechanistic Insight into the Reaction of Organic Acids with SO3 at the Air-Water Interface.

The gas-phase reaction of organic acids with SO3 has been recognized as essential in promoting aerosol-particle formation. However, at the air-water interface, this reaction is much less understood. We performed systematic Born-Oppenheimer molecular dynamics (BOMD) simulations to study the reaction of various organic acids with SO3 on a water droplet. The results show that with the involvement of interfacial water molecules, organic acids can react with SO3 and form the ion pair of sulfuric-carboxylic anhydride and hydronium. This mechanism is in contrast to the gas-phase reaction mechanisms in which the organic acid either serves as a catalyst for the reaction between SO3 and H2 O or reacts with SO3 directly. The distinct reaction at the water surface has important atmospheric implications, for example, promoting water condensation, uptaking atmospheric condesation species, and incorporating "SO4 2- " into organic species in aerosol particles. Therefore, this reaction, typically occurring within a few picoseconds, provides another pathway towards aerosol formation.

Nitric Acid-Amine Chemistry in the Gas Phase and at the Air-Water Interface.

Gas-phase simulations of nitric acid-amine chemistry suggest that the fundamental acid-base interaction between HNO3 and NH3 results in a variety of HNO3-NH3-based complexes, such as (HNO3)3·(NH3)2, (HNO3)3·(NH3)3, and (HNO3)4·(NH3)3, that can be formed. The formation of these complexes in the gas phase follow different growth mechanisms under different relative humidity conditions. On the other hand, at the air-water interface, Born-Oppenheimer molecular dynamics simulations suggest that the formation of the fundamental NO3-··(R1)(R2)NH2+ [for NH3, R1 = R2 = H; CH3NH2, R1 = H, R2 = CH3; and (CH3)2NH, R1 = R2 = CH3] ion pairs require the formation of the HNO3··(R1)(R2)NH complexes in the gas-phase prior to their adsorption on the water surface. Ion-pair formation at the water surface involves proton transfer from HNO3 to (R1)(R2)NH and occurs within a few femtoseconds of the simulation. The NO3-··(R1)(R2)NH2+ ion pairs preferentially remain at the interface over the picosecond time scale, where they are stabilized via hydrogen bonding with surface water molecules. This offers a novel chemical framework for understanding gas-to-particle partitioning in the atmosphere. These results not only improve our understanding of the formation of nitrate particulates in polluted urban environments, but also provide useful guidelines for understanding particle formation in forested or coastal environments, in which organic acids and organosulfates are present in significant quantities and their exact role in particle formation remains elusive.

Self-Catalytic Reaction of SO3 and NH3 To Produce Sulfamic Acid and Its Implication to Atmospheric Particle Formation.

Sulfur trioxide (SO3) is one of the most active chemical species in the atmosphere, and its atmospheric fate has profound implications to air quality and human health. The dominant gas-phase loss pathway for SO3 is generally believed to be the reaction with water molecules, resulting in sulfuric acid. The latter is viewed as a critical component in the new particle formation (NPF). Herein, a new and competitive loss pathway for SO3 in the presence of abundant gas-phase ammonia (NH3) species is identified. Specifically, the reaction between SO3 and NH3, which produces sulfamic acid, can be self-catalyzed by the reactant (NH3). In dry and heavily polluted areas with relatively high concentrations of NH3, the effective rate constant for the bimolecular SO3-NH3 reaction can be sufficiently fast through this new loss pathway for SO3 to become competitive with the conventional loss pathway for SO3 with water. Furthermore, this study shows that the final product of the reaction, namely, sulfamic acid, can enhance the fastest possible rate of NPF from sulfuric acid and dimethylamine (DMA) by about a factor of 2. An alternative source of stabilizer for acid-base clustering in the atmosphere is suggested, and this new mechanism for NPF has potential to improve atmospheric modeling in highly polluted regions.

The role of nitric acid in atmospheric new particle formation.

Nitric acid, an air pollutant with strong acidity and oxidizability, can be found in considerable quantities in the gas and aerosol phase. Understanding the role of nitric acid in atmospheric new particle formation is essential to study the complicated nucleation mechanism. Using density functional theory combined with the Atmospheric Clusters Dynamic Code (ACDC), the role of nitric acid in the formation of new particles has been investigated under different atmospheric conditions (different precursor concentrations and temperatures). The results show that nitric acid can form clusters with sulfuric acid and ammonia by hydrogen bond or even proton-transfer interactions. The concentrations of clusters involving nitric acid can be comparable with those of sulfuric acid-ammonia-based clusters, considering the thermodynamic stability combined with the realistic atmospheric concentrations of precursors. Within the atmospheric concentration range, nitric acid can enhance the formation rates of sulfuric acid-ammonia clusters, especially at low temperature, low sulfuric acid concentration and high ammonia concentration. In addition, the new particle formation mechanism indicates that nitric acid can contribute to the cluster formation and the role of nitric acid in the cluster formation pathway is as a "bridge" connecting the smaller and larger clusters.

Clustering mechanism of oxocarboxylic acids involving hydration reaction: Implications for the atmospheric models.

The formation of atmospheric aerosol particles from condensable gases is a dominant source of particulate matter in the boundary layer, but the mechanism is still ambiguous. During the clustering process, precursors with different reactivities can induce various chemical reactions in addition to the formation of hydrogen bonds. However, the clustering mechanism involving chemical reactions is rarely considered in most of the nucleation process models. Oxocarboxylic acids are common compositions of secondary organic aerosol, but the role of oxocarboxylic acids in secondary organic aerosol formation is still not fully understood. In this paper, glyoxylic acid, the simplest and the most abundant atmospheric oxocarboxylic acid, has been selected as a representative example of oxocarboxylic acids in order to study the clustering mechanism involving hydration reactions using density functional theory combined with the Atmospheric Clusters Dynamic Code. The hydration reaction of glyoxylic acid can occur either in the gas phase or during the clustering process. Under atmospheric conditions, the total conversion ratio of glyoxylic acid to its hydration reaction product (2,2-dihydroxyacetic acid) in both gas phase and clusters can be up to 85%, and the product can further participate in the clustering process. The differences in cluster structures and properties induced by the hydration reaction lead to significant differences in cluster formation rates and pathways at relatively low temperatures.