Phase Particles Research Articles

Hydrophilic and Hydrophobic: Modified GeO2 Aerogels by Ambient Pressure Drying.

An ever-increasing number of applications of oxide aerogels places a high demand on wettability-tuning techniques. This work explores the possibility to cheaply prepare GeO2 aerogels with controlled wettability by an ambient pressure drying (APD) method. GeO2 aerogels are prepared via two synthetic routes. Surface modification is carried out by soaking the gels in a silylating agent solution; type and concentration of the modifier are optimized to achieve a large surface area. The aerogels have been characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, nitrogen adsorption and contact angle measurements. The effect of surface modification on the phase composition and particle size of the aerogels is described. In summary, the work provides a new cheap production method for the preparation of both hydrophobic and hydrophilic GeO2 aerogels with contact angle varying from 30° to 141° and with surface area of 90-140 m2/g, which facilitates the expansion of their diverse applications. GeO2 aerogel synthesis by APD is reported for the first time.

Investigation of microstructure, mechanical and oxidation properties of WC-Co-Ni-Fe-Al multicomponent hardmetals

In this paper, the influences of Al element originating from AlN additives on the mechanical properties and oxidation behavior of WC-Co-Ni-Fe hardmetals were studied systematically. During the sintering process, Al element diffused into the WC hard grains and led to the formation of (Ni,Co,Fe)3Al particles in the metal binder phase of hardmetals. A small amount of Al element enhanced the hardness of WC-Co-Ni-Fe hardmetal via the solution strengthening and dispersion strengthening. However, the addition of Al element decreased the fracture toughness of hardmetals. The hardness and fracture toughness of multicomponent hardmetals with the addition of 0.5 wt% AlN were 90.4HRA and 10.6 MPa m1/2, respectively. Additionally, the introduction of Al also significantly improved the oxidation resistance of hardmetals due to the existence of Al in WO3 and (Ni,Co,Fe)WO4. With the addition of 2.0 wt% AlN, the weight gain of WC-Co-Ni-Fe-Al multicomponent hardmetal after 5 h of oxidation at 700 °C was 66.82 % of that of the Al-free hardmetal.

Preparation of fine-grained/ultrafine-grained Nb521 alloy with superior mechanical property by friction stir processing

High strength-ductility synergy is difficult to achieve in Nb alloys. Although high strength has been achieved through severe plastic deformation (SPD) technology, led to low ductility in alloys. In this work, FSP technology was applied to treat Nb–5W–2Mo–1Zr-0.1C (Nb521) alloys in preparation of fine-grained (FG)/ultrafine-grained (UFG) Nb521 with excellent strength and ductility. The microstructure evolution and mechanical property improvement mechanism were systematically studied for Nb521 alloy through various characterization pathways. The research results indicated that UFG Nb521 alloy with a grain size of 0.63 ± 0.41 μm can be prepared using low shoulder plunge depth FSP (LPD-FSP), which is the first report of UFG Nb521 alloy. The main reason for the formation of onion rings structure in SZ is the periodic wear of the stirring tool, and the onion rings structure does not cause mechanical damage. The texture formed by Nb521 alloy under different processing parameters is off-axis shear texture, which matches the ideal shear texture of D2 (112‾)[111] after rotation. In addition, this study also elaborated on the refinement mechanism of the second phase particles (Nb, Zr) C in Nb521 alloy during FSP. This study also indicated that the increase in yield strength of FSP samples at room temperature is mainly determined by grain refinement. These findings provided new ideas for the development of high-performance niobium alloys.

Study on dynamic recrystallization and precipitation behavior of hot compressed Al-20Zn-0.5Mg-0.5Sc alloy

The dynamic recrystallization (DRX) behavior of the Al-20Zn-0.5Mg-0.5Sc alloy was investigated under specific conditions using a Gleeble-3500 thermal simulator. The resultant grain size, recrystallization degree and precipitated phases were studied. The results indicate that the sensitivity of DRX grain size is highly affected by temperature, feebly influenced by strain and intermediately affected by strain rate. The DRX grains are grown by an average of 3.49 μm with increasing temperature. The DRX behavior is sensitive to temperature, strain and strain rate, resulting in changes in average grain size, percentage of HAGBs and degree of recrystallization under different parameters. At 400 ℃ / 40 % / 0.1 s−1, the average grain size is refined to 12.90 μm, and the corresponding HAGBs and recrystallization degree are approximately 46.2 % and 34.3 %, respectively. The Al3Sc, Zn and η (MgZn2) phases are precipitated in the hot-compressed alloy. The Al3Sc phase promotes the precipitation of the Zn and η phases. The size, amount and morphology of Al3Sc, Zn and η phases at different parameters have an important influence on the movement of dislocations and nucleation of grains, which leads to changes in DRX grain size and the degree of recrystallization. The fine precipitated phase particles Al3Sc and η inhibit the growth of recrystallized grain by pinning grain boundaries and exerting a blocking force on dislocations. Dynamic precipitation is the dominant mechanism for the changes in DRX grain size and degree of recrystallization. To achieve the refined grains and homogeneous microstructures, the optimum parameters are recommended as 400 ℃, 0.1 s−1 and 50 %.

Effect of cooling rates and heat treatment time on Si phase of AlSi10Mg alloy manufactured by selective laser melting: Microstructure evolution and strengthening mechanism

As a processing technique to change the organization and properties of materials, heat treatment was widely used to strengthen metallic materials. This study examines the impact of varying cooling rates and heat treatment durations on the mechanical properties and microstructure of AlSi10Mg alloys produced through Selective laser melting (SLM). The heat treatment process was Solution Heat Treatment (SHT, 525 °C), and the heat treatment time includes 1 h, 2 h, and 3 h. Cooling methods consist of Refrigerant Cooling (RC), Water Cooling (WC), Air Cooling (AC), Oil Cooling (OC), and Furnace Cooling (FC). As the cooling rate decreases and the heat treatment time increases, the size of the formed Si phase particles varies from small to large, and the distribution deviates from inhomogeneous to homogeneous. In addition, the number of precipitated particles increases and then decreases. To investigate the impact of cooling rates and heat treatment time on the mechanical properties of AlSi10Mg alloys, quasi-static tensile and compression tests were performed. On the other hand, the tensile strength of the heat-treatment samples decreases while the ductility increases, and the Vickers hardness of the samples also decreases with decreasing cooling rate and increasing heat treatment time. The results of the study showed that the cooling rate and heat treatment time of the material had a greater effect on its tensile properties than on its compressive properties. The samples treated at 525 °C - 2 h - AC exhibited the highest tensile and compressive properties. Finally, the analysis of the strengthening mechanism of AlSi10Mg alloys through the presence of Si phase particles of varying sizes was conducted using transmission electron microscopy.

Simulation and experimental study of tool passivation based on magnetic elastic abrasive particles

The presence of micro-cracks and serrations on the tool often leads to its premature failure. However, these microscopic defects can be mitigated through passivation treatment of the tool. Magnetoelastic abrasive particles are new composite particles formed by using abrasive phase particles, magnetic media phase particles and polymer matrix according to a certain ratio, the abrasive particles have both viscoelasticity and magnetic conductivity. The introduction of magnetoelastic abrasive grains into the dual-disk tool passivation equipment can effectively improve the surface quality of the tool and increase the passivation efficiency. At the outset, a model for tool passivation was crafted employing the ABAQUS simulation software. This model aimed to evaluate the effects of diverse passivation variables on the stress levels, the shape of the impact, and the depth of the impact experienced by the tool. Subsequently, by applying statistical theory combined with fitting function techniques, a mathematical model was constructed to determine the passivation volume of the tool subjected to multiple magnetic elastic abrasive particles. This model was then used to examine the impact count and passivation volume associated with each passivation factor. Finally, a tool passivation experiment was conducted using a dual-disk magnetic tool passivation device. The results showed that the smaller the impact velocity of magnetic elastic abrasive particles, the stress and impact depth of the cutting tool are decreasing, and the deformation of the cutting edge is caused by compressive stress. The error of impact depth calculated by simulation and mathematical model is within 20 %, which proves that the mathematical model can better calculate the impact depth of magnetic elastic abrasive particles on the cutting edge of the tool.

The influence of Zn addition on the microstructure and mechanical and corrosion properties of warm rolled Al[sbnd]Mg alloys containing Er and Zr

The effects of a minor Zn addition on the mechanical and corrosion properties and microstructure of a high Mg content AlMg alloy containing Er and Zr in the warm-rolled state were studied using tensile test, nitric acid mass loss test (NAMLT), exfoliation corrosion susceptibility test (ASSET), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The tensile test results showed that the 0.58 wt% Zn addition to the Al-Mg-Er-Zr alloy increased the yield strength from 297 to 351 MPa and tensile strength from 416 to 445 MPa, but decreased the elongation from 13.3 % to 12.5 %. The NAMLT and ASSET results showed that the two warm rolled alloys were initially in the stabilization state, but the Al-Mg-Er-Zr alloy without Zn added became sensitized severely after the accelerated sensitization annealing (ASA) at 100 °C. The Zn addition improved the intergranular corrosion (IGC) resistance and exfoliation corrosion (EC) resistance significantly. The TEM results showed that, for the Al-Mg-Er-Zr alloy, there were Al3(Er,Zr) phase particles in the matrix and β (Al3Mg2) phase particles separated from each other at the grain boundary. After the ASA treatment, more β phase particles were precipitated and covered the grain boundary completely. For the Al-Mg-Zn-Er- Zr alloy, another nanoscale T (Al32(Mg, Zn)49) phase was precipitated in the matrix, and there were no grain boundary phase particles observed at the grain boundary, because the precipitation of T phase consumed the supersaturated Mg in the matrix, thus suppressing the formation of grain boundary phase particles during the ASA treatment and resulting in a good corrosion resistance. The strengthening effect of the Zn addition was mainly due to the formation of T phase particles during the warm rolling process.

Synergistic enhancement of strength and ductility of Mg-12.8Gd-3.8Y-1.9Zn-0.5Zr alloy by low-temperature solid solution treatment during forging process

Forging and low-temperature solid solution treatment to regulate Mg-12.8Gd-3.8Y-1.9Zn-0.5Zr (wt%) alloy were investigated in this paper, which exhibited a heterogeneous microstructure. The alloy demonstrated a yield strength (YS) of 266.68 MPa, an ultimate tensile strength (UTS) of 337.01 MPa, and elongation (EL) of 8.98 %. It was shown that the strength and ductility of the Mg-12.8Gd-3.8Y-1.9Zn-0.5Zr (wt%) alloy were synergistically enhanced. This enhancement can be attributed to the alteration of the second phase particles through low-temperature solid solution treatment, as well as the hardening effect induced by hetero-deformation. The low-temperature solid solution treatment allowed the large-sized brittle Mg5RE phase to be dissolved, while the deformation-induced dislocation tangle in the coarse grains (CGs) promotes the co-segregation of Gd and Zn elements, resulting in the precipitation-strengthened γ′ phase. The presence of dynamically precipitated phases within the dynamic recrystallized (DRXed) zone enhanced the strain regulation ability in different regions and inhibited crack propagation. Furthermore, the hetero-deformation induced (HDI) hardening effect, arising from the interaction between CGs and fine grains (FGs) in the heterogeneous microstructure, effectively prevented necking and improved ductility.

The effect of COVID-19 epidemic and sandstorm on distribution of short-, medium-, and long-chain chlorinated paraffins in outdoor atmosphere of Xi'an, Northwest China

The short-, medium- and long-chain chlorinated paraffins (SCCPs, MCCPs, and LCCPs) were measured in outdoor fine particulate matter (PM2.5) and gas-phases in Xi'an in spring, autumn and winter of 2021, which containing two special events during this time, the sandstorms happened in spring and the COVID-19 epidemic in winter. The CPs concentrations in PM2.5 were seasonal variated, with spring (mean: 17.5 ng/m3) > autumn (mean: 8.6 ng/m3) > winter (mean: 5.8 ng/m3). The CPs concentrations in gas phases (mean:12.6 ng/m3) were lower than in PM2.5 in spring, and it showed shorter carbon chain and lower chlorinated CPs dominated in SCCPs in gas phase (C10, Cl5,6) compared with in PM2.5 (C11, Cl6,7). The absorption was the main mechanism for the partitioning of CP in particle and gaseous phase. The shorter carbon chain CPs (C10,11, C14) with lower chlorinated (Cl6,7) CPs dominated in SCCPs and MCCPs, while the longer carbon chain (C25 or C29) with Cl6,12 dominated in the LCCPs. So, more researches about the LCCPs should be concerned. Additionally, the CPs concentrations of all samples during the sandstorms in spring were increased 30.4%–136.5% and the proportion of SCCPs was obviously elevated. CPs concentrations obviously decreased during the COVID-19 epidemic in winter, while SCCPs, MCCPs attributed comparable in autumn and winter. Potential source analysis showed that during the sandstorms in spring, atmospheric pollutants were mainly transported over a long distance from northern and northeast of Shaanxi Province, while it was mainly from local emission during the COVID-19 epidemic in winter. The health risk assessment of CPs displayed that there had no or very low inhalation exposure risks of CPs for all age groups, but the younger age groups (younger than 6 years old) should be played more attention for their not fully development of their immune and respiratory systems.

Structure and mechanical properties of low-density AlCrFeTiX (X = Co, Ni, Cu) high-entropy alloys produced by spark plasma sintering

In this study, low-density (< 6 g/cm3) AlCrFeTiX (x = Co, Ni, Cu) high-entropy alloys were produced by mechanical alloying and spark plasma sintering (SPS), and their structures and mechanical properties after SPS and annealing at 1000 °C for 24 h were reported. Both after SPS and annealing, the AlCrFeTiX (x = Co, Ni, Cu) alloys consisted of an L21 matrix phase with embedded bcc and C14 Laves phase particles. All the alloys had a nano-sized structure after SPS that retained in the AlCrFeTiCo and AlCrFeTiNi alloys after annealing. In the AlCrFeTiCu alloy, the annealing led to a coarsening, with an increase in the size of structural constituents above 1 μm. Compression tests showed that the AlCrFeTiX (x = Co, Ni, Cu) alloys after SPS were brittle at 25 °C, but the AlCrFeTiCo alloy exhibited the highest peak strength of 3792 MPa. At 600 °C, the AlCrFeTiCo alloy after SPS also demonstrated the best performance, with the yield strength, peak strength, and plastic strain of 1960 MPa, 2121 MPa, and 1.8 %, respectively. The annealing did not eliminate the room-temperature brittleness, but improved the mechanical properties at 600 °C. The AlCrFeTiCo alloy showed a 15 %-increase in yield strength (2264 MPa) and more than 2.5 times higher compressive plasticity (4.7 %). The AlCrFeTiNi alloy became more ductile (1 %) and stronger (2200 MPa). For the AlCrFeTiCu alloy, the annealing had a negative effect that resulted in a halved plasticity at 600 °C. The AlCrFeTiCo and AlCrFeTiNi alloys after annealing showed record-high specific yield strength values at 600 °C, which were 383 and 371 MPa*cm3/g, respectively. With these values, the AlCrFeTiCo and AlCrFeTiNi alloys outperformed all the low-density medium-/high-entropy alloys available in literature to date obtained both by SPS or conventional casting. The structure formation and mechanical properties, as well as the response of these features to annealing, were extensively discussed.

Attaining exceptional wear resistance in an in-situ ceramic phase reinforced NbMoWTa refractory high entropy alloy composite by Spark plasma sintering

The refractory high-entropy alloys (RHEAs) exhibit great potential as structural components for aerospace equipment. However, their lack of wear resistance and increased coefficient of friction at room temperature (RT) impose limitations on their practical applications. Therefore, further enhancements are required to improve their friction and wear properties under RT. In this context, the development of NbMoWTa(h-BN)x RHEA ceramic composites in this work offers a viable solution to address this issue. Experimental results demonstrate that the addition of h-BN leads to the in-situ generation of (Nb,Ta)N/(Nb,Ta)2N and (Nb,Ta)B2 ceramic phases, significantly enhancing the hardness and wear resistance of the composites. The wear rate of NbMoWTa(h-BN)0.5 reaching as low as 1.32 × 10−8 mm3/Nm, which is four orders of magnitude lower than that of the RHEA. The NbMoWTa RHEA exhibits significant adhesive wear, which can be effectively mitigated in composites through the uniform dispersion of ceramic phase particles with lower mean free path. The abrasive particles primarily interact with the hard strengthening phase, effectively inhibiting plastic deformation in their vicinity. Consequently, the reduced mean free path between the ceramic phases limits the likelihood of metal matrix removal. Subsequently, aided by the presence of ceramic phases, the spontaneous formation of protective third bodies further inhibit surface material removal and ultimately ensures exceptional wear resistance.

Atmospheric Bases-Enhanced Iodic Acid Nucleation: Altitude-Dependent Characteristics and Molecular Mechanisms.

Iodic acid (IA), the key driver of marine aerosols, is widely detected within the gas and particle phases in the marine boundary layer (MBL) and even the free troposphere (FT). Although atmospheric bases like dimethylamine (DMA) and ammonia (NH3) can enhance IA particles formation, their different efficiencies and spatial distributions make the dominant base-stabilization mechanisms of forming IA particles unclear. Herein, we investigated the IA-DMA-NH3 nucleation system through quantum chemical calculations at the DLPNO-CCSD(T)/aug-cc-pVTZ(-PP)//ωB97X-D/6-311++G(3df,3pd) + aug-cc-pVTZ-PP level of theory and cluster dynamics simulations. We provide molecular-level evidence that DMA and NH3 can jointly stabilize the IA clusters. The formation rates of IA clusters initially decline before rising from the MBL to the FT, owing to variations in mechanism. In the MBL, IA-DMA nucleation predominates, while the contribution of IA-DMA-NH3 synergistic nucleation cannot be overlooked in polar and NH3-polluted regions. In the lower FT, IA-DMA-NH3 nucleation prevails, whereas in the upper FT, IA-NH3 nucleation dominates. The efficiency of IA-DMA-NH3 nucleation is comparable to that of IA-iodous acid nucleation in the MBL and sulfuric acid-NH3 nucleation in the FT. Hence, the IA-DMA-NH3 mechanism holds promise for revealing the missing sources of tropospheric IA particles.

Research on optimizing focused ultrasonic parameters for Surfactant-Free nanoemulsion with prolonged stability

Stable-state emulsions with no phase separation and dispersed-particle aggregation can be utilized in various fields, such as cosmetics, pharmaceuticals, food, and drug delivery. However, the physicochemical properties and stability of emulsions are significantly affected by factors such as concentration, mixing method, droplet size, and temperature. Surfactants (emulsifiers), which are used to form stable emulsions, adversely affect the human body and environment and influence the properties of emulsions, thereby limiting their development. This study manufactured stable emulsions without a surfactant using ultrasonic equipment. The oil particle size distributions, zeta potentials, microscopic observations, and emulsion stabilities of six emulsions (with an oil content of 1 %) prepared using different frequencies (250–1000 kHz) and output powers (50–150 W) were analyzed, immediately after preparation at 25 °C and 3 d thereafter. Finally, it was possible to manufacture a stable emulsion without particle size change or phase separation with a particle size in the 100 nm range and a surface charge value of −40 mV or more under conditions of 400 kHz and 150 W. This study proposed a method (with the optimum conditions) for manufacturing surfactant-free emulsions by analyzing the stability of emulsions manufactured under various frequencies and output-power conditions. The proposed method could open new frontiers in emulsion development and applications.

Analysis of thermal non‐equilibrium model on the Ree–Eyring nanofluid flow influenced by an inclined magnetic field

AbstractThis research seeks to explore the behavior of Ree–Eyring nanofluid over an extended surface influenced by an inclined magnetic field within a permeable medium. The local thermal non‐equilibrium between the particle, liquid, and solid phases is represented by a three‐temperature model. The problem is addressed numerically using bvp4c code in MATLAB software. The findings are displayed in the format of tables and graphs. The study shows that higher values of the interface heat transfer parameter led to a decrease and increase in the fluid phase and solid phase Nusselt number, respectively. The velocity and concentration distributions decrease with increasing porosity coefficient. Nevertheless, the fluid phase temperature distribution shows an opposing trend. Furthermore, increasing the non‐Newtonian fluid parameter leads to a raise in the surface drag coefficient.

Resolving intermediates during the growth of aluminum deuteroxide (Hydroxide) polymorphs in high chemical potential solutions

Aluminum hydroxide polymorphs are of widespread importance yet their kinetics of nucleation and growth remain beyond the reach of current models. Here we attempt to unveil the reaction processes underlying the polymorphs formation at high chemical potential. We examine their formation in-situ from supersaturated alkaline sodium aluminate solutions using deuteration and time-resolved neutron pair distribution function analyses, which indicate the formation of individual Al(OD)3 layers as an intermediate particle phase. These layers ultimately stack to form gibbsite- or bayerite-like layered heterostructures. Ex-situ characterization of the recovered precipitates using 27Al magic angle spinning nuclear magnetic resonance spectroscopy, Raman, X-ray diffraction, and scanning electron microscopy, suggests the presence of additional intermediate states, an amorphous compound bearing both tetrahededrally- and penta-coordinated Al3+. These observations reveal the complex pathways to form Al(OD)3 monolayers via either transient pentacoordinate species or amorphous-to-ordered transitions. The subsequent crystallization of admixed gibbsite/bayerite is followed by an Al(OD)3 monolayer attachment process.

Modeling Indoor Inorganic Aerosol Concentrations During the ATHLETIC Campaign with IMAGES.

In 2018, the ATHLETIC campaign was conducted at the University of Colorado Dal Ward Athletic Center and characterized dynamic indoor air composition in a gym environment. Among other parameters, inorganic particle and gas-phase species were alternatingly measured in the gym's supply duct and weight room. The Indoor Model of Aerosols, Gases, Emissions, and Surfaces (IMAGES) uses the inorganic aerosol thermodynamic equilibrium model, ISORROPIA, to estimate the partitioning of inorganic aerosols and corresponding gases. In this study herein, measurements from the ATHLETIC campaign were used to evaluate IMAGES' performance. Ammonia emission rates, nitric acid deposition, and particle deposition velocities were related to observed occupancy, which informed these rates in IMAGES runs. Initially, modeled indoor inorganic aerosol concentrations were not in good agreement with measurements. A parametric investigation revealed that lowering the temperature or raising the relative humidity used in the ISORROPIA model drove the semivolatile species more toward the particle phase, substantially improving modeled-measured agreement. One speculated reason for these solutions is that aerosol water was enhanced by increasing the RH or decreasing the temperature. Another is that thermodynamic equilibrium was not established in this indoor setting or that the thermodynamic parametrizations in ISORROPIA are less accurate for typical indoor settings. This result suggests that applying ISORROPIA indoors requires further careful experimental validation.

Silver-modified polydopamine-coated silica core-shell nanospheres as functional additives for lubricants and shear thickening fluids

The limited effectiveness of conventional nano-additives in carrier liquids has prompted research into multi-capable, sustainable, and intelligent application systems. Herein, multifunctional SiO2-PDA@Ag nanoparticles were synthesized by dotting silver (Ag) nanoparticles on polydopamine (PDA)-coated SiO2 nanospheres (SiO2-PDA). The base oil (PAO) containing these synthetic nanoparticles showed excellent lubricity under high contact pressure, and the mechanism was analyzed. The addition of SiO2-PDA@Ag modified with Ag nanoparticles showed better tribological performances compared to PAO and base oils with starting nanomaterials. A concentration of 3 wt% SiO2-PDA@Ag provided the most effective performance. Furthermore, multi-phase STFs were developed by incorporating SiO2-PDA@Ag particles into SiO2-based STFs, significantly enhancing the shear thickening effect. Compared to the pure SiO2-based STF, the peak viscosity of the multi-phase STF increased by about 89.36 %. Notably, the multi-phase STF exhibited higher stress under squeeze, which facilitates the development of high-performance STF devices in extrusion mode. This enhancement is attributed to the nanocore-shell structure of SiO2-PDA@Ag, which effectively prevents the agglomeration of dispersed phase particles in organic carrier medium. The results of this work provide a promising additive nanoparticle in the field of smart materials and lubricants.

Effect of Aging Process on Strengthening Phase Particles of 6056 Aluminum Alloy for Bolts

Effect of Aging Process on Strengthening Phase Particles of 6056 Aluminum Alloy for Bolts

Characteristics and sources of polycyclic aromatic hydrocarbons in dew in urban ecosystems in Northeast China

The concentrations of polycyclic aromatic hydrocarbons (PAHs) in the atmospheres above major cities in Northeast China are relatively high. Dew is an effective way to settle and remove particulate and gaseous matter in the formation process. Notably, 16-PAH concentration dew analysis was conducted in four major cities (Harbin, Changchun, Shenyang and Dalian) from April to November 2023. In the study area, 2–3-ring PAHs dominated in dew in both the dissolved and particle phases, followed by 4-ring and 5–6-ring PAHs. Phe was the most common individual PAH in dew. Because PAHs in wet deposition are closely related to those in the atmosphere, the concentration of PAHs in dew was much lower than that in snow. The proportions of the gaseous-phase scavenging ratio (Wg) to the total scavenging ratio (Wt) in Harbin, Changchun, Shenyang and Dalian increased with increasing temperature, reaching 22.98%, 29.02%, 30.71% and 33.22%, respectively, which indicated that the ability of dew to remove PAHs in the particle phase was greater than that to remove PAHs in the gas phase. The concentration of dissolved-phase PAHs in dew was lower than that of particle-phase PAHs. The total PAH fluxes were 43.25, 33.35, 32.16, and 25.63 ng/m2·d in Harbin, Changchun, Shenyang and Dalian, respectively, decreasing from north to south. The dew amount was much smaller than the rainfall and snow amount (by two orders of magnitude), and the PAH flux in dew was much lower than that in rain and snow. Isomeric ratio assessment coupled with principal component analysis (PCA) indicated that the source of PAHs in dew was a combination of vehicle emissions and biomass and coal combustion. PAHs in dew mainly originated from local sources rather than from long-range atmospheric transport. This study revealed the effects of PAHs in dew on the air quality and the removal of PAHs by dew.

Effects of Ammonia Mitigation on Secondary Organic Aerosol and Ammonium Nitrate Particle Formation in Photochemical Reacted Gasoline Vehicle Exhausts

Gaseous air pollutants emitted primarily by anthropogenic sources form secondary products through photochemical reactions, complicating the regulatory analysis of anthropogenic emissions in the atmosphere. We used an environmental chassis dynamometer and a photochemical smog chamber to conduct a parameter sensitivity experiment to investigate the formation of secondary products from a gasoline passenger car. To simulate the mitigation of ammonia emissions from gasoline vehicle exhausts assuming future emission controls and to allow photochemical oxidation and aging of the vehicle exhaust, ammonia was selectively removed by a series of five denuders installed between the vehicle and photochemical smog chamber. Overall, there were no differences in the formation of secondary organic aerosols and ozone with or without ammonia mitigation. However, the potential for ammonium nitrate particle formation was significantly reduced with ammonia mitigation. In addition, ammonia mitigation resulted in increased aerosol acidity due to nitric acid in the gas phase not being neutralized by ammonia and condensing onto the liquid particle phase, indicating a potentially important secondary effect associated with ammonia mitigation. Thus, we provide new insights into the effects of ammonia mitigation on secondary emissions from gasoline vehicle exhaust and into a potentially useful experimental approach for determining primary and secondary emissions.