Impacts of coagulation on the appearance time method for new particle growth rate evaluation and their corrections

The growth rate of atmospheric new particles is a key parameter that determines their survival probability of becoming cloud condensation nuclei and hence their impact on the climate. There have been several methods to estimate the new particle growth rate. However, due to the impact of coagulation and measurement uncertainties, it is still challenging to estimate the initial growth rate of new particles, especially in polluted environments with high background aerosol concentrations. In this study, we explore the influences of coagulation on the appearance time method to estimate the growth rate of sub-3 nm particles. The principle of the appearance time method and the impacts of coagulation on the retrieved growth rate are clarified via derivations. New formulae in both discrete and continuous spaces are proposed to correct for the impacts of coagulation. Aerosol dynamic models are used to test the new formulae. New particle formation in urban Beijing is used to illustrate the importance of considering the impacts of coagulation on the sub-3 nm particle growth rate and its calculation. We show that the conventional appearance time method needs to be corrected when the impacts of coagulation sink, coagulation source, and particle coagulation growth are non-negligible compared to the condensation growth. Under the simulation conditions with a constant concentration of non-volatile vapors, the corrected growth rate agrees with the theoretical growth rates. However, the uncorrected parameters, e.g., vapor evaporation and the variation in vapor concentration, may impact the growth rate obtained with the appearance time method. Under the simulation conditions with a varying vapor concentration, the average bias in the corrected 1.5-3 nm particle growth rate ranges from 6 %-44 %, and the maximum bias in the size-dependent growth rate is 150 %. During the test new particle formation event in urban Beijing, the corrected condensation growth rate of sub-3 nm particles was in accordance with the growth rate contributed by sulfuric acid condensation, whereas the conventional appearance time method overestimated the condensation growth rate of 1.5 nm particles by 80 %.Peer reviewe

Acid-Base Clusters during Atmospheric New Particle Formation in Urban Beijing

Molecular clustering is the initial step of atmospheric new particle formation (NPF) that generates numerous secondary particles. Using two online mass spectrometers with and without a chemical ionization inlet, we characterized the neutral clusters and the naturally charged ion clusters during NPF periods in urban Beijing. In ion clusters, we observed pure sulfuric acid (SA) clusters, SA-amine clusters, SA-ammonia (NH3) clusters, and SA-amine-NH3 clusters. However, only SA clusters and SA-amine clusters were observed in the neutral form. Meanwhile, oxygenated organic molecule (OOM) clusters charged by a nitrate ion and a bisulfate ion were observed in ion clusters. Acid-base clusters correlate well with the occurrence of sub-3 nm particles, whereas OOM clusters do not. Moreover, with the increasing cluster size, amine fractions in ion acid-base clusters decrease, while NH3 fractions increase. This variation results from the reduced stability differences between SA-amine clusters and SA-NH3 clusters, which is supported by both quantum chemistry calculations and chamber experiments. The lower average number of dimethylamine (DMA) molecules in atmospheric ion clusters than the saturated value from controlled SA-DMA nucleation experiments suggests that there is insufficient DMA in urban Beijing to fully stabilize large SA clusters, and therefore, other basic molecules such as NH3 play an important role.Peer reviewe

Sulfuric acid-amine nucleation in urban Beijing

New particle formation (NPF) is one of the major sources of atmospheric ultrafine particles. Due to the high aerosol and trace gas concentrations, the mechanism and governing factors for NPF in the polluted atmospheric boundary layer may be quite different from those in clean environments, which is however less understood. Herein, based on long-term atmospheric measurements from January 2018 to March 2019 in Beijing, the nucleation mechanism and the influences of H2SO4 concentration, amine concentrations, and aerosol concentration on NPF are quantified. The collision of H2SO4-amine clusters is found to be the dominating mechanism to initialize NPF in urban Beijing. The coagulation scavenging due to the high aerosol concentration is a governing factor as it limits the concentration of H2SO4-amine clusters and new particle formation rates. The formation of H2SO4-amine clusters in Beijing is sometimes limited by low amine concentrations. Summarizing the synergistic effects of H2SO4 concentration, amine concentrations, and aerosol concentration, we elucidate the governing factors for H2SO4-amine nucleation for various conditions.Peer reviewe

Seasonal Characteristics of New Particle Formation and Growth in Urban Beijing

Understanding the atmospheric new particle formation (NPF) process within the global range is important for revealing the budget of atmospheric aerosols and their impacts. We investigated the seasonal characteristics of NPF in the urban environment of Beijing. Aerosol size distributions down to similar to 1 nm and H2SO4 concentration were measured during 2018-2019. The observed formation rate of 1.5 nm particles (J(1.5)) is significantly higher than those in the clean environment, e.g., Hyytiala, whereas the growth rate is not significantly different. Both J(1.5) and NPF frequency in urban Beijing show a clear seasonal variation with maxima in winter and minima in summer, while the observed growth rates are generally within the same range around the year. We show that ambient temperature is a governing factor driving the seasonal variation of J(1.5). In contrast, the condensation sink and the daily maximum H2SO4 concentration show no significant seasonal variation during the NPF periods. In all four seasons, condensation of H2SO4 and (H2SO4)(n)(amine)(n) clusters contributes significantly to the growth rates in the sub-3 nm size range, whereas it is less important for the observed growth rates of particles above 3 nm. Therefore, other species are always needed for the growth of larger particles.Peer reviewe

Is reducing new particle formation a plausible solution to mitigate particulate air pollution in Beijing and other Chinese megacities?

Atmospheric gas-to-particle conversion is a crucial or even dominant contributor to haze formation in Chinese megacities in terms of aerosol number, surface area and mass. Based on our comprehensive observations in Beijing during 15 January 2018-31 March 2019, we are able to show that 80-90% of the aerosol mass (PM2.5) was formed via atmospheric reactions during the haze days and over 65% of the number concentration of haze particles resulted from new particle formation (NPF). Furthermore, the haze formation was faster when the subsequent growth of newly formed particles was enhanced. Our findings suggest that in practice almost all present-day haze episodes originate from NPF, mainly since the direct emission of primary particles in Beijing has considerably decreased during recent years. We also show that reducing the subsequent growth rate of freshly formed particles by a factor of 3-5 would delay the buildup of haze episodes by 1-3 days. Actually, this delay would decrease the length of each haze episode, so that the number of annual haze days could be approximately halved. Such improvement in air quality can be achieved with targeted reduction of gas-phase precursors for NPF, mainly dimethyl amine and ammonia, and further reductions of SO2 emissions. Furthermore, reduction of anthropogenic organic and inorganic precursor emissions would slow down the growth rate of newly-formed particles and consequently reduce the haze formation.Peer reviewe

Continuous and comprehensive atmospheric observations in Beijing : a station to understand the complex urban atmospheric environment

Peer reviewe

The effect of COVID-19 restrictions on atmospheric new particle formation in Beijing

During the COVID-19 lockdown, the dramatic reduction of anthropogenic emissions provided a unique opportunity to investigate the effects of reduced anthropogenic activity and primary emissions on atmospheric chemical processes and the consequent formation of secondary pollutants. Here, we utilize comprehensive observations to examine the response of atmospheric new particle formation (NPF) to the changes in the atmospheric chemical cocktail. We find that the main clustering process was unaffected by the drastically reduced traffic emissions, and the formation rate of 1.5 nm particles remained unaltered. However, particle survival probability was enhanced due to an increased particle growth rate (GR) during the lockdown period, explaining the enhanced NPF activity in earlier studies. For GR at 1.5-3 nm, sulfuric acid (SA) was the main contributor at high temperatures, whilst there were unaccounted contributing vapors at low temperatures. For GR at 3-7 and 7-15 nm, oxygenated organic molecules (OOMs) played a major role. Surprisingly, OOM composition and volatility were insensitive to the large change of atmospheric NOx concentration; instead the associated high particle growth rates and high OOM concentration during the lockdown period were mostly caused by the enhanced atmospheric oxidative capacity. Overall, our findings suggest a limited role of traffic emissions in NPF.Peer reviewe

Numerical Simulation of Flow Field Optimizing the Rotating Segregation Purification of Silicon for SoG-Si

In order to improve the preparation efficiency of high-purity silicon, a new method of rotary segregation purification has been developed to prepare polysilicon. Numerical simulation based on ANSYS19.0 software and water model experiments were used to study the distribution of flow field and optimize the impurity removal process. A numerical simulation model suitable for rotating interface is established. The simulation result is in good agreement with the water model experiment. A vortex flow is found in the middle of the mold when the mold insertion depth is 90 mm. The vortex is conducive to the thinning of the impurity enrichment layer at the solid-liquid interface. With the mold insertion depth increases from 90 to 170 mm, two vortex flows appear in the middle and bottom of the mold, respectively. Moreover, setting the mold rotation rate at 100 rpm can contribute to a more stable flow field and a higher melt flow velocity. When the diffusion layer thickness is less than 0.1 mm, the impurity segregation coefficient can approach close to its equilibrium segregation coefficient, indicating that impurity segregation be effectively enhanced by increasing the rotation rate of mold, strengthening the effect of solidification rate and increasing the rotational speed. Industrial tests were carried out at the 100 kg level. The result shows that the rotary segregation method and equipment can achieve the removal of very low impurity in silicon (99.999 pct), and SoG-Si (99.9999 pct) was obtained. This method provides a new way for silicon purification, and it is believed that better results can be obtained through continuous improvement

Numerical Simulation of Flow Field Optimizing the Rotating Segregation Purification of Silicon for SoG-Si

In order to improve the preparation efficiency of high-purity silicon, a new method of rotary segregation purification has been developed to prepare polysilicon. Numerical simulation based on ANSYS19.0 software and water model experiments were used to study the distribution of flow field and optimize the impurity removal process. A numerical simulation model suitable for rotating interface is established. The simulation result is in good agreement with the water model experiment. A vortex flow is found in the middle of the mold when the mold insertion depth is 90 mm. The vortex is conducive to the thinning of the impurity enrichment layer at the solid-liquid interface. With the mold insertion depth increases from 90 to 170 mm, two vortex flows appear in the middle and bottom of the mold, respectively. Moreover, setting the mold rotation rate at 100 rpm can contribute to a more stable flow field and a higher melt flow velocity. When the diffusion layer thickness is less than 0.1 mm, the impurity segregation coefficient can approach close to its equilibrium segregation coefficient, indicating that impurity segregation be effectively enhanced by increasing the rotation rate of mold, strengthening the effect of solidification rate and increasing the rotational speed. Industrial tests were carried out at the 100 kg level. The result shows that the rotary segregation method and equipment can achieve the removal of very low impurity in silicon (99.999 pct), and SoG-Si (99.9999 pct) was obtained. This method provides a new way for silicon purification, and it is believed that better results can be obtained through continuous improvement

Numerical Simulation of Flow Field Optimizing the Rotating Segregation Purification of Silicon for SoG-Si

In order to improve the preparation efficiency of high-purity silicon, a new method of rotary segregation purification has been developed to prepare polysilicon. Numerical simulation based on ANSYS19.0 software and water model experiments were used to study the distribution of flow field and optimize the impurity removal process. A numerical simulation model suitable for rotating interface is established. The simulation result is in good agreement with the water model experiment. A vortex flow is found in the middle of the mold when the mold insertion depth is 90 mm. The vortex is conducive to the thinning of the impurity enrichment layer at the solid-liquid interface. With the mold insertion depth increases from 90 to 170 mm, two vortex flows appear in the middle and bottom of the mold, respectively. Moreover, setting the mold rotation rate at 100 rpm can contribute to a more stable flow field and a higher melt flow velocity. When the diffusion layer thickness is less than 0.1 mm, the impurity segregation coefficient can approach close to its equilibrium segregation coefficient, indicating that impurity segregation be effectively enhanced by increasing the rotation rate of mold, strengthening the effect of solidification rate and increasing the rotational speed. Industrial tests were carried out at the 100 kg level. The result shows that the rotary segregation method and equipment can achieve the removal of very low impurity in silicon (99.999 pct), and SoG-Si (99.9999 pct) was obtained. This method provides a new way for silicon purification, and it is believed that better results can be obtained through continuous improvement. (C) The Minerals, Metals & Materials Society and ASM International 202