Organic Solvent Nanofiltration of Water-in-Oil Pickering Emulsions—What Influences Permeability?

Pickering emulsions (PEs) have received increasing interest for their application in catalytic multiphase reactions. Organic solvent nanofiltration of PEs was shown to be a promising procedure for efficient and effective catalyst recycling. In this work, a systematic parameter study to identify the main influencing parameters on PE filtration was conducted for a large variety of PE compositions for the first time. In addition to temperature, only the type of organic solvent significantly influenced the filtration performance, which could be mathematically modeled via a combination of the solution–diffusion and the resistance in the series model. Particle type and concentration, dispersed phase fraction and the presence of reaction (by-)products did not show any significant impact on the permeability. The stirrer speed only became important when emulsions stabilized by particles without the tendency to form 3D network structures were filtered in long-term filtration experiments. These results pave the way towards the application of PE membrane filtration for catalyst recovery in continuous liquid/liquid multiphase reactions and enable broad operation windows. As the mechanical separation of PEs was shown to be a very robust process, the emulsion composition can now be tuned to meet the needs of the reaction without any (significant) loss in filtration performance.

Spatial and temporal scales of variability for indoor air constituents

Historically air constituents have been assumed to be well mixed in indoor environments, with single point measurements and box modeling representing a room or a house. Here we demonstrate that this fundamental assumption needs to be revisited through advanced model simulations and extensive measurements of bleach cleaning. We show that inorganic chlorinated products, such as hypochlorous acid and chloramines generated via multiphase reactions, exhibit spatial and vertical concentration gradients in a room, with short-lived ⋅OH radicals confined to sunlit zones, close to windows. Spatial and temporal scales of indoor constituents are modulated by rates of chemical reactions, surface interactions and building ventilation, providing critical insights for better assessments of human exposure to hazardous pollutants, as well as the transport of indoor chemicals outdoors.

Single-particle Raman spectroscopy for studying physical and chemical processes of atmospheric particles

Atmospheric particles experience various physical and chemical processes and change their properties during their lifetime. Most studies on atmospheric particles, both in laboratory and field measurements, rely on analyzing an ensemble of particles. Because of different mixing state of individual particles, only average properties can be obtained from studies using ensembles of particles. To better understand the fate and environmental impacts of atmospheric particles, investigations on their properties and processes at a single-particle level are valuable. Among a wealth of analytic techniques, single-particle Raman spectroscopy provides an unambiguous characterization of individual particles under atmospheric pressure in a non-destructive and in-situ manner. This paper comprehensively reviews the application of such a technique in the studies of atmospheric particles, including particle hygroscopicity, phase transition and separation, and solute-water interactions, particle pH, and multiphase reactions. Investigations on enhanced Raman spectroscopy and bioaerosols on a single-particle basis are also reviewed. For each application, we describe the principle and representative examples of studies. Finally, we present our views on future directions on both technique development and further applications of single-particle Raman spectroscopy in studying atmospheric particles.

Let the Biocatalyst Flow

Industrial biocatalysis has been identified as one of the key enabling technologies that, together with the transition to continuous processing, offers prospects for the development of cost-efficient manufacturing with high-quality products and low waste generation. This feature article highlights the role of miniaturized flow reactors with free enzymes and cells in the success of this endeavor with recent examples of their use in single or multiphase reactions. Microfluidics-based droplets enable ultrahigh-throughput screening and rapid biocatalytic process development. The use of unique microreactor configurations ensures highly efficient contacting of multiphase systems, resulting in process intensification and avoiding problems encountered in conventional batch processing. Further integration of downstream units offers the possibility of biocatalyst recycling, contributing to the cost-efficiency of the process. The use of environmentally friendly solvents supports effective reaction engineering, and thus paves the way for these highly selective catalysts to drive sustainable production.

Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN): integrated analysis and intensive winter campaign 2018

High-RH-favored multiphase reactions can significantly change the chemical composition of fine particles and thereby modify their physicochemical properties.

Characterization of submicron particles by time-of-flight aerosol chemical speciation monitor (ToF-ACSM) during wintertime: aerosol composition, sources, and chemical processes in Guangzhou, China

Particulate matter (PM) pollution in China is an emerging environmental issue which policy makers and the public have increasingly paid attention to. In order to investigate the characteristics, sources, and chemical processes of PM pollution in Guangzhou, field measurements were conducted from 20 November 2017 to 5 January 2018, with a time-of-flight aerosol chemical speciation monitor (ToF-ACSM) and other collocated instruments. Mass concentrations of non-refractory submicron particulate matter (NR-PM1) measured by the ToF-ACSM correlated well with those of PM2.5 or PM1.1 measured by filter-based methods. The organic mass fraction increased from 45 % to 53 % when the air switched from non-pollution periods to pollution episodes (EPs), indicating significant roles of organic aerosols (OAs) during the whole study. Based on the mass spectra measured by the ToF-ACSM, positive matrix factorization (PMF) with the multilinear engine (ME-2) algorithm was performed to deconvolve OA into four factors, including hydrocarbon-like OA (HOA, 12 %), cooking OA (COA, 18 %), semi-volatile oxygenated OA (SVOOA, 30 %), and low-volatility oxygenated OA (LVOOA, 40 %). Furthermore, we found that SVOOA and nitrate were significantly contributed from local traffic emissions while sulfate and LVOOA were mostly attributed to regional pollutants. Comparisons between this work and other previous studies in China show that secondary organic aerosol (SOA) fraction in total OA increases spatially across China from the north to the south. Two distinctly opposite trends for NR-PM1 formation were observed during non-pollution periods and pollution EPs. The ratio of secondary PM (SPM = SVOOA + LVOOA + sulfate + nitrate + ammonium) to primary PM (PPM = HOA + COA + chloride), together with peroxy radicals RO2∗ and ozone, increased with increasing NR-PM1 concentration during non-pollution periods, while an opposite trend of these three quantities was observed during pollution EPs. Furthermore, oxidation degrees of both OA and SOA were investigated using the f44∕f43 space and the results show that at least two OOA factors are needed to cover a large range of f44 and f43 in Guangzhou. Comparisons between our results and other laboratory studies imply that volatile organic compounds (VOCs) from traffic emissions, in particular from diesel combustion and aromatic compounds, are the most likely SOA precursors in Guangzhou. Peroxy radical RO2∗ was used as a tracer for SOA formed through gas-phase oxidation. For non-pollution periods, SOA concentration was reasonably correlated with RO2∗ concentration during both daytime and nighttime, suggesting that gas-phase oxidation was primarily responsible for SOA formation. However, there was no correlation between SOA and RO2∗ in pollution EPs, suggesting a dramatically changed mechanism for SOA formation. This conclusion can also be supported by different features of SOA in a van Krevelen diagram between non-pollution periods and pollution EPs. Furthermore, for pollution EPs, when NR-PM1 mass concentration was divided into six segments, in each segment except for the lowest one SOA concentration was correlated moderately with RO2∗ concentration, suggesting that gas-phase oxidation still plays important roles in SOA formation. The intercepts of the above linear regressions, which likely correspond to the extent of other mechanisms (i.e., heterogeneous and multiphase reactions), increase with increasing NR-PM1 mass concentration. Our results suggest that while gas-phase oxidation contributes predominantly to SOA formation during non-pollution periods, other mechanisms such as heterogeneous and multiphase reactions play more important roles in SOA formation during pollution EPs than gas-phase oxidation.

Characterization of submicron particles by Time-of-Flight Aerosol Chemical Speciation Monitor (ToF-ACSM) during wintertime: aerosol composition, sources and chemical processes in Guangzhou, China

Particulate matter (PM) pollution in China is an emerging environmental issue which policy makers and public have increasingly paid attention to. In order to investigate the characteristics, sources, and chemical processes of PM pollution in Guangzhou, a field measurement was conducted from 20 November 2017 to 5 January 2018, with a Time-of-Flight Aerosol Chemical Speciation Monitor (ToF-ACSM) and other collocated instruments. Mass concentrations of non-refractory submicron particulate matters (NR-PM1) measured by the ToF-ACSM were correlated well with those of PM2.5 or PM1.1 measured by filter-based methods. The organic mass fraction increased from 45 % to 53 % when the air switched from non-pollution periods to pollution episodes, indicating significant roles of organic aerosols (OA) during the whole study. Based on the mass spectra measured by the TOF-ACSM, Positive Matrix Factorization (PMF) with multilinear engine (ME-2) algorithm was performed to deconvolve OA into four factors, including hydrocarbon-like OA (HOA, 12 %), cooking OA (COA, 18 %), semi-volatile oxygenated OA (SVOOA, 30 %), and low-volatility oxygenated OA (LVOOA, 40 %). Furthermore, we found that SVOOA and nitrate were significantly contributed from local traffic emissions while sulfate and LVOOA were mostly attributed to regional pollutants. Comparisons between this work and other previous studies in China show that SOA fraction in total OA increases spatially across China from the North to the South. Two distinctly opposite trends for NR-PM1 formation were observed during non-pollution period and pollution EPs. The ratio of secondary PM (SPM = SVOOA + LVOOA + sulfate + nitrate + ammonium) to primary PM (PPM = HOA + COA + chloride), together with peroxy radicals RO2* and ozone, increased with increasing NR-PM1 concentration during non-pollution period, while an opposite trend of these three quantities was observed during pollution EPs. Furthermore, oxidation degrees of both OA and SOA were investigated using the f44/f43 space and the results show that at least two OOA factors are needed to cover a large range of f44 and f43 in Guangzhou. Comparisons between our results and other laboratory studies imply that volatile organic compounds (VOCs) from traffic emissions in particular from diesel combustion and aromatic compounds are most possible SOA precursors in Guangzhou. Peroxy radical RO2* was used as a tracer for SOA formed through gas phase oxidation. For non-pollution period, SOA concentration was reasonably correlated with RO2* concentration during both daytime and nighttime, suggesting that gas phase oxidation was primarily responsible for SOA formation. For pollution EPs, when NR-PM1 mass concentration was divided into six segments, in each segment except for the lowest one, SOA concentration was correlated moderately with RO2* concentration, suggesting that gas phase oxidation still plays important roles in SOA formation. In addition, the slopes of linear regressions for the above correlations increased with increasing NR-PM1 mass concentration, probably representing enhanced gas-to-particle partitioning under high NR-PM1 concentration. The intercepts of the above linear regressions, which correspond to the extent of other mechanisms (i.e., heterogeneous and multiphase reactions), increased with increasing NR-PM1 mass concentration. Our results suggest that while gas phase oxidation contributes predominantly to SOA formation during non-pollution periods, other mechanisms such as heterogeneous and multiphase reactions play more important roles in SOA formation during pollution EPs than gas phase oxidation.