Models for estimating nanoparticle transmission efficiency through an adverse axial electric field

An adverse electric field is often encountered or utilized when classifying charged nanoparticles or ions according to their electrical mobility. For instance, the classified charged particles usually have to be transported through an adverse electric field against the aerosol sample flow before exiting the outlet of a differential mobility analyzer (DMA). Recently, we reported the transmission of charged nanoparticles through the DMA adverse axial electric field and its improvements. Herein, the simplified analytical model used in that study and a new simplified numerical model to evaluate particle transmission efficiency through the adverse axial electric field are introduced in detail. In addition to the DMA sample outlet, these models are also tested for the electrical mobility filter (EMF) for segregating charged nanoparticles, especially under the unfavorable conditions when the assumptions for these models are violated. The modeled results are compared to a Monte Carlo method based on single-particle tracking and the experimentally determined transmission efficiencies. For the typical geometries and test conditions of a half-mini DMA sample outlet and the EMF, the mean absolute differences between these models and the Monte Carlo method are less than 1%. However, the accuracy of these models is guaranteed only when their assumptions are satisfied, i.e., when the adverse electric field is longer than 4 times the tube radius so that the field lines are axially parallel and the tube length before the adverse electric field is at least half of the entrance length for the air flow. In addition, the simplified analytical model may deviate from the true transmission efficiency when the adverse electric field is longer than 10% of the entire tube. In such cases that the assumptions for both the simplified analytical model and the simplified numerical model are violated, the Monte Carlo method can be used instead.An adverse electric field is often encountered or utilized when classifying charged nanoparticles or ions according to their electrical mobility. For instance, the classified charged particles usually have to be transported through an adverse electric field against the aerosol sample flow before exiting the outlet of a differential mobility analyzer (DMA). Recently, we reported the transmission of charged nanoparticles through the DMA adverse axial electric field and its improvements. Herein, the simplified analytical model used in that study and a new simplified numerical model to evaluate particle transmission efficiency through the adverse axial electric field are introduced in detail. In addition to the DMA sample outlet, these models are also tested for the electrical mobility filter (EMF) for segregating charged nanoparticles, especially under the unfavorable conditions when the assumptions for these models are violated. The modeled results are compared to a Monte Carlo method based on single-particle tracking and the experimentally determined transmission efficiencies. For the typical geometries and test conditions of a half-mini DMA sample outlet and the EMF, the mean absolute differences between these models and the Monte Carlo method are less than 1%. However, the accuracy of these models is guaranteed only when their assumptions are satisfied, i.e., when the adverse electric field is longer than 4 times the tube radius so that the field lines are axially parallel and the tube length before the adverse electric field is at least half of the entrance length for the air flow. In addition, the simplified analytical model may deviate from the true transmission efficiency when the adverse electric field is longer than 10% of the entire tube. In such cases that the assumptions for both the simplified analytical model and the simplified numerical model are violated, the Monte Carlo method can be used instead. Copyright (c) 2019 American Association for Aerosol ResearchPeer reviewe

Exploring Students’ Intercultural Communicative Competence Cultivation in Junior High School English Teaching

The study intended to investigate the current situation of junior high school students’ intercultural communicative competence and explore some factors impacting the cultivation of students’ intercultural communicative competence in English teaching. The study analyzed three factors impacting students’ intercultural communicative competence: language barriers, devoid of intercultural communicative knowledge and ethnocentrism. Accordingly the study proposed some strategies to enhance the cultivation of students’ intercultural communicative competence: firstly, teachers should design more intercultural background knowledge in English teaching. Secondly, students should actively participate in intercultural communicative communication and continuously enhance their intercultural communicative awareness; thirdly, schools should provide intercultural training for teachers to improve their intercultural communication literacy

Investigating the effectiveness of condensation sink based on heterogeneous nucleation theory

New Particle Formation (NPF) is regularly observed to occur in heavily polluted Chinese megacities. However, in these NPF events, the survival probability of small clusters into larger aerosol particles is higher than theoretically predicted. One explanation for this could be that the loss rate of clusters due to scavenging by pre-existing particles, which is described by condensation sink, is lower than expected. In this study, we describe the loss of clusters due to condensation sink by using heterogeneous nucleation theory, and investigate if ineffectiveness of heterogeneous nucleation can result in a significantly lowered effective condensation sink. We find that in principle it is possible that due to properties of the system there is no heterogeneous nucleation, and this can significantly influence the magnitude of effective condensation sink and thus increase the survival probability of clusters.Peer reviewe

Electrical Mobility Spectrometer Using a Diethylene Glycol Condensation Particle Counter for Measurement of Aerosol Size Distributions Down to 1 nm. Aerosol Sci.

We report a new scanning mobility particle spectrometer (SMPS) for measuring number size distributions of particles down to ∼1 nm mobility diameter. This SMPS includes an aerosol charger, a TSI 3085 nano differential mobility analyzer (nanoDMA), an ultrafine condensation particle counter (UCPC) using diethylene glycol (DEG) as the working fluid, and a conventional butanol CPC (the "booster") to detect the small droplets leaving the DEG UCPC. The response of the DEG UCPC to negatively charged sodium chloride particles with mobility diameters ranging from 1-6 nm was measured. The sensitivity of the DEG UCPC to particle composition was also studied by comparing its response to positively charged 1.47 and 1.70 nm tetra-alkyl ammonium ions, sodium chloride, and silver particles. A high resolution differential mobility analyzer was used to generate the test particles. These results show that the response of this UCPC to sub-2 nm particles is sensitive to particle composition. The applicability of the new SMPS for atmospheric measurement was demonstrated during the Nucleation and Cloud Condensation Nuclei (NCCN) field campaign (Atlanta, Georgia, summer 2009). We operated the instrument at saturator and condenser temperatures that allowed the efficient detection of sodium chloride particles but not of air ions having the same mobility. We found that particles as small as 1 nm were detected during nucleation events but not at other times. Factors affecting size distribution measurements, including aerosol charging in the 1-10 nm size range, are discussed. For the charger used in this study, bipolar charging was found to be more effective for sub-2 nm particles than unipolar charging. No ion induced nucleation inside the charger was observed during the NCCN campaign

Formation and growth of sub-3 nm particles in megacities : impact of background aerosols

New particle formation (NPF) occurs frequently in various atmospheric environments and contributes majorly to the aerosol number budget. In megacities, the high concentrations of gaseous precursors and background aerosols add complexity to this process. Based on long-term measurements (373 days) in urban Beijing, we examine the formation and growth of sub-3 nm particles under the effects of background aerosols, as indicated by the condensation sink (CS) or the Fuchs surface area. The median CS and the median PM2.5 mass concentration for the days with NPF events were 0.03 s(-1) and 34 mu g m(-3), respectively. The high loss rates of both molecular clusters and sub-3 nm particles to background aerosols reduce their atmospheric residence time and suppress their survival. As the key clusters for H2SO4-base nucleation, sulfuric acid dimer and trimer concentrations in Beijing decrease significantly when CS increases and the scavenging becomes stronger. The occurrence of NPF events and the formation of sub-3 nm particles in Beijing is governed by CS. 95% of the observed NPF days occurred with CS values below 0.03 s(-1). During NPF events, high concentrations of sub-3 nm particles were formed and they mostly ranged from 10(3) to 10(5) cm(-3) with a median value of 6.2 x 10(3) cm(-3). Driven by the fast H2SO4-base nucleation, the daily maximum formation rate of 1.5 nm particles in Beijing has a mean value of 77 cm(-3) s(-1) and is much higher than that in clean environments. However, the mean growth rate of sub-3 nm particles in Beijing was only 2.6 nm h(-1), not significantly different from that in clean environments. The relatively low growth rate and the high level of scavenging by background aerosols result in low survival of newly formed particles. The analyses also reinforce prior results on the need to correct conventional methods to adequately quantify the formation and growth rates when analyzing data from megacities with strong coagulation scavenging due to background aerosols. The conventional balance formula underestimates the formation rate of 1.5 nm particles, while the conventional appearance time method overestimates the growth rate of sub-3 nm particles. These findings highlight the governing role of background aerosols in urban NPF.Peer reviewe

Transfer Functions and Penetrations of Five Differential Mobility Analyzers for Sub-2 nm Particle Classification

The transfer functions and penetrations of five differential mobility analyzers (DMAs) for sub-2 nm particle classification were evaluated in this study. These DMAs include the TSI nanoDMA, the Caltech radial DMA (RDMA) and nanoRDMA, the Grimm nanoDMA, and the Karlsruhe-Vienna DMA. Measurements were done using tetra-alkyl ammonium ion standards with mobility diameters of 1.16, 1.47, and 1.70 nm. These monomobile ions were generated by electrospray followed by high resolution mobility classification. Measurements were focused at an aerosol-to-sheath flow ratio of 0.1. A data inversion routine was developed to obtain the true transfer function for each test DMA, and these measured transfer functions were compared with theory. DMA penetration efficiencies were also measured. An approximate model for diffusional deposition, based on the modified Gormley and Kennedy equation using an effective length, is given for each test DMA. These results quantitatively characterize the performance of the test DMAs in classifying sub-2 nm particles and can be readily used for DMA data inversion

The effectiveness of the coagulation sink of 3–10 nm atmospheric particles

As a major source of ultrafine particles, new particle formation (NPF) occurs frequently in various environments. However, the survival of new particles and the frequent occurrence of NPF events in polluted environments have long been perplexing, since new particles are expected to be scavenged by high coagulation sinks Towards solving these problems, we establish an experimental method and directly measure the effectiveness of the size-dependent coagulation sink of monodisperse 3-10 nm particles in well-controlled chamber experiments. Based on the chamber experiments and long-term atmospheric measurements from Beijing, we then discuss the survival of new particles in polluted environments. In the chamber experiments, the measured coagulation sink of 3-10 nm particles increases significantly with a decreasing particle size, whereas it is not sensitive to the compositions of test particles. Comparison between the measured coagulation coefficient with theoretical predictions shows that almost every coagulation leads to the scavenging of one particle, and the coagulation sink exceeds the hard-sphere kinetic limit due to van der Waals attractive force. For urban Beijing, the effectiveness of the coagulation sink and a moderate or high (e.g., > 3 nm h(-1)) growth rate of new particles can explain the occurrence of measured NPF events; the moderate growth rate further implies that, in addition to gaseous sulfuric acid, other gaseous precursors also contribute to the growth of new particles.Peer reviewe

Interactions between aerosol organic components and liquid water content during haze episodes in Beijing

Aerosol liquid water (ALW) is ubiquitous in ambient aerosol and plays an important role in the formation of both aerosol organics and inorganics. To investigate the interactions between ALW and aerosol organics during haze formation and evolution, ALW was modelled based on long-term measurement of submicron aerosol composition in different seasons in Beijing. ALW contributed by aerosol inorganics (ALW(inorg)) was modelled by ISORROPIA II, and ALW contributed by organics (ALW(org)) was estimated with kappa-Kohler theory, where the real-time hygroscopicity parameter of the organics (kappa(org)) was calculated from the real-time organic oxygen-to-carbon ratio (O/C). Overall particle hygroscopicity (kappa(total)) was computed by weighting component hygroscopicity parameters based on their volume fractions in the mixture. We found that ALW(org), which is often neglected in traditional ALW modelling, contributes a significant fraction (18 %-32 %) to the total ALW in Beijing. The ALW(org) fraction is largest on the cleanest days when both the organic fraction and kappa(org) are relatively high. The large variation in O/C, from 0.2 to 1.3, indicates the wide variety of organic components. This emphasizes the necessity of using real-time kappa(org), instead of fixed kappa(org), to calculate ALW(org) in Beijing. The significant variation in K org (calculated from O/C), together with highly variable organic or inorganic volume fractions, leads to a wide range of kappa(total) (between 0.20 and 0.45), which has a great impact on water uptake. The variation in organic O/C, or derived K org , was found to be influenced by temperature (T), ALW, and aerosol mass concentrations, among which T and ALW both have promoting effects on O/C. During high-ALW haze episodes, although the organic fraction decreases rapidly, O/C and derived K org increase with the increase in ALW, suggesting the formation of more soluble organics via heterogeneous uptake or aqueous processes. A positive feedback loop is thus formed: during high-ALW episodes, increasing kappa(org), together with decreasing particle organic fraction (or increasing particle inorganic fraction), increases kappa(total), and thus further promotes the ability of particles to uptake water.Peer reviewe

Survival probabilities of atmospheric particles : comparison based on theory,cluster population simulations, and observations in Beijing

Atmospheric new particle formation (NPF) events are regularly observed in urban Beijing, despite high concentrations of background particles which, based on theory, should inhibit NPF due to high values of coagulation sink (CoagS). The survival probability, which depends on both CoagS and particle growth rate (GR), is a key parameter in determining the occurrence of NPF events as it describes the fraction of newly formed particles that survive from a smaller diameter to a larger diameter. In this study, we investigate and compare survival probabilities from 1.5 to 3 nm (J(3)/J(1.5)), from 3 to 6 nm (J(6)/J(3)), and from 6 to 10 nm (J(10)/J(6)) based on analytical formulae, cluster population simulations, and atmospheric observations from Beijing. We find that survival probabilities based on the cluster population simulations and one of the analytical formulae are in a good agreement. However, at low ratios between the background condensation sink (CS) and GR, and at high concentrations of sub-3 nm clusters, cluster-cluster collisions efficiently lower survival probabilities in the cluster population simulations. Due to the large concentrations of clusters and small particles required to considerably affect the survival probabilities, we consider it unlikely that cluster-cluster collisions significantly affect atmospheric survival probabilities. The values of J(10)/J(6) observed in Beijing show high variability, most likely due to influences of primary particle emissions, but are on average in relatively good agreement with the values based on the simulations and the analytical formulae. The observed values of J(6)/J(3) are mostly lower than those predicted based on the simulations and the analytical formulae, which could be explained by uncertainties in CS and GR. The observed values of J(3)/J(1.5) at high CS / GR are much higher than predicted based on the simulations and the analytical formulae. We argue that uncertainties in GR or CS are unlikely to solely explain the observed values of J(3)/J(1.5) under high CS conditions. Thus, further work is needed to better understand the factors influencing survival probabilities of sub-3 nm atmospheric particles in polluted environments.Peer reviewe