Hygroscopic Particles Research Articles

Innovative Air-Preconditioning Method for Accurate Particulate Matter Sensing in Humid Environments.

Smart cities rely on a network of sensors to gather real-time data on various environmental factors, including air quality. This paper addresses the challenges of improving the accuracy of low-cost particulate matter sensors (LCPMSs) which can be compromised by environmental conditions, such as high humidity, which is common in many urban areas. Such weather conditions often lead to the overestimation of particle counts due to hygroscopic particle growth, resulting in a potential public concern, although most of the detected particles consist of just water. The paper presents an innovative design for an indicative air-quality measuring station that integrates the particulate matter sensor with a preconditioning subsystem designed to mitigate the impact of humidity. The preconditioning subsystem works by heating the incoming air, effectively reducing the relative humidity and preventing the hygroscopic growth of particles before they reach the sensor. To validate the effectiveness of this approach, parallel measurements were conducted using both preconditioned and non-preconditioned sensors over a period of 19 weeks. The data were analyzed to compare the performance of the sensors in terms of accuracy for PM1, PM2.5, and PM10 particles. The results demonstrated a significant improvement in measurement accuracy for the preconditioned sensor, especially in environments with high relative humidity. When the conditions were too severe and both sensors started measuring incorrect values, the preconditioned sensor-measured values were closer to the actual values. Also, the period of measuring incorrect values was shorter with the preconditioned sensor. The results suggest that the implementation of air preconditioning subsystems in LCPMSs deployed in smart cities can provide a cost-effective solution to overcome humidity-related inaccuracies, thereby improving the overall quality of measured air pollution data.

Multiple-charging effects on the CCN activity and hygroscopicity of surrogate black carbon particles

Accurate measurements of cloud condensation nuclei (CCN) activity and hygroscopicity of black carbon (BC)-containing particles are particularly important because of the positive climate forcing from these particles. Such measurements are typically conducted on particles selected by a Differential Mobility Analyzer (DMA), which in addition to singly charged particles transmits multiply charged larger particles that have the same electrical mobility. These larger particles activate at lower supersaturations than the singly charged particles, biasing measurements and resulting in overestimation of CCN activity and hygroscopicity parameter (κ). Here, we measure the CCN activity and determine κ for different BC surrogates with electrical mobility diameters from 100 to 200 nm selected 1) only by electrical mobility with a DMA, and 2) by both electrical mobility and mass using a DMA and a Centrifugal Particle Mass Analyzer (CPMA), thus allowing selection of only singly charged particles. We demonstrate the use of the DMA-CPMA system in resolving biases caused by multiply charged particles, and we show that the effect of multiple charging on the CCN activity of the BC particles is strongly influenced by morphology dispersion, i.e., the variability due to the range of morphologies of particles that have the same electrical mobility and mass. Our findings show that electrical mobility-based methods alone are unlikely to lead to accurate results in measurements of CCN activation and hygroscopicity of BC particles, even for those with a more compact morphology.

The hygroscopicity of nano-plastic particles and implications for cloud formation and climate

The hygroscopicity of nano-plastic particles and implications for cloud formation and climate

Measurement report: Hygroscopicity of size-selected aerosol particles in the heavily polluted urban atmosphere of Delhi: impacts of chloride aerosol

Abstract. Recent research has revealed the crucial role of wintertime, episodic high chloride (H-Cl) emissions in the Delhi region, which significantly impact aerosol hygroscopicity and aerosol-bound liquid water, thus contributing to the initiation of Delhi fog episodes. However, these findings have primarily relied on modeled aerosol hygroscopicity, necessitating validation through direct hygroscopicity measurements. This study presents the measurements of non-refractory bulk aerosol composition of PM1 from an Aerodyne aerosol chemical speciation monitor and for first-time size-resolved hygroscopic growth factors (nucleation, Aitken, and accumulated mode particles) along with their associated hygroscopicity parameters at 90 % relative humidity using a hygroscopic tandem differential mobility analyzer at the Delhi Aerosol Supersite. Our observations demonstrate that the hygroscopicity parameter for aerosol particles varies from 0.00 to 0.11 (with an average of 0.03 ± 0.02) for 20 nm particles, 0.05 to 0.22 (0.11 ± 0.03) for 50 nm particles, 0.05 to 0.30 (0.14 ± 0.04) for 100 nm particles, 0.05 to 0.41 (0.18 ± 0.06) for 150 nm particles, and 0.05 to 0.56 (0.22 ± 0.07) for 200 nm particles. Surprisingly, our findings demonstrate that the period with H-Cl emissions displays notably greater hygroscopicity (0.35 ± 0.06) in comparison to spans marked by high biomass burning (0.18 ± 0.04) and high hydrocarbon-like organic aerosol (0.17 ± 0.05) and relatively cleaner periods (0.27 ± 0.07). This research presents initial observational proof that ammonium chloride is the main factor behind aerosol hygroscopic growth and aerosol-bound liquid water content in Delhi. The finding emphasizes ammonium chloride's role in aerosol–water interaction and related haze/fog development. Moreover, the high chloride levels in aerosols seem to prevent the adverse impact of high organic aerosol concentrations on cloud condensation nuclei activity.

Effects of volatile organic compounds and new particle formation on real-time hygroscopicity of PM2.5 particles in Seosan, Republic of Korea

The hygroscopicity of PM2.5 particles plays an important role in PM2.5 haze in Northeast Asian countries by influencing particle growth and chemical composition. New particle formation (NPF) and atmospheric volatile organic compounds (VOCs) are factors that influence particle hygroscopicity. However, the lack of real-time hygroscopicity measurements has deterred the understanding of their effects on particle hygroscopicity. In this study, two intensive monitoring campaigns were conducted during the summer of 2021 and spring of 2022 using real-time aerosol instruments, including a humidified tandem differential mobility analyzer (HTDMA), in Seosan, Republic of Korea. The hygroscopicity parameter κ was calculated from the real-time HTDMA measurement data (κGf). The diurnal variations in κGf exhibited strong inverse linear correlations with the total concentration of VOCs (CTVOC) during the two campaigns. The higher atmospheric CTVOC in summer increased the growth rate of the particle diameter from 10 to 40 nm (6 nm/h) compared with that in spring (2.7 nm/h), resulting in a faster change in κGf for 40-nm particles in summer than in spring because of the increase in organic matter in the chemical compositions of particles. In addition, NPF events introduced additional tiny fresh particles into the atmosphere, which reduced the κGf of 40-nm particles and increased the intensity of the less hygroscopic peaks (κGf < 0.1) of κ-probability density functions (κ-PDF) in NPF days. However, 100-nm particles exhibited fewer changes in κGf than 40-nm particles, resulting in additional dominant hygroscopic peaks (κ ∼ 0.2) of κ-PDFs in both NPF and non-NPF days. When κGf values measured in Seosan were compared with those in other Northeast Asian countries in the literature, the κ values for 40-nm particles were lower than those (κ > 0.2) measured in Beijing and Guangzhou, but those for 100-nm particles were close to those measured in the two cities.

Marine Fuel Regulations and Engine Emissions: Impacts on Physicochemical Properties, Cloud Activity and Emission Factors

AbstractMarine regulations aim to reduce sulfur and nitrogen exhaust emissions from maritime shipping. Here, two compliance pathways for reducing sulfur dioxide emissions, fuel sulfur content reduction and exhaust wet scrubbing, are studied for their effects on physicochemical properties and cloud forming abilities of engine exhaust particles. A test‐bed diesel engine was utilized to study fresh exhaust emissions from combustion of non‐compliant, high sulfur content fuel with (WS) and without (HiS) the usage of a wet scrubber as well as a regulatory compliant, low sulfur content fuel (LoS). Particle number emissions are decreased by ≈99% when switching to LoS due to absence of 20–30 nm sulfate rich particles. While number emissions for WS are also decreased, a shift in the sulfate mode toward larger sizes was found to increase particle mass emission factors by at least 31%. Changes in the mixing state induced by the compliance measures are reflected in the hygroscopicity of the exhaust particles. Fuel sulfur reduction decreased cloud condensation nuclei emissions by at least 97% due to emissions of primarily hydrophobic soot particles. Wet scrubbing increased those emissions, mainly driven by changes in particle size distributions. Our results indicate that both compliance alternatives have no obvious impact on the ice forming abilities of 200 nm exhaust particles. These detailed results are relevant for atmospheric processes and might be useful input parameters for cloud‐resolving models to investigate ship aerosol‐cloud interactions and to quantify the impact of shipping on radiative budgets from local to global scales.

A study on the influence of inorganic ions, organic carbon and microstructure on the hygroscopic property of soot

Abstract. Soot is a crucial component of aerosols in the atmosphere. Understanding the hygroscopicity of soot particles is important for studying their role as cloud condensation nuclei (CCN) as well as their chemical behavior and atmospheric lifetime. However, there is still a lack of comprehensive understanding regarding the factors that determine the hygroscopic properties of soot. In this work, the hygroscopic behavior of soot particles generated from different types of fuel combustion and aged with SO2 for varying durations was measured by a vapor sorption analyzer. Various characterizations of soot were conducted to understand the key factors that influence the hygroscopic properties of soot. It was found that water-soluble substances in soot facilitate the completion of monolayer water adsorption at low relative humidity and increase the number of water adsorption layers at high relative humidity. On the other hand, soot prepared from fuel burning typically lacks water-soluble inorganic ions, and their hygroscopicity is primarily influenced by organic carbon (OC) and microstructure. Furthermore, the hygroscopicity of soot can be enhanced by the formation of sulfate due to heterogeneous oxidation of SO2. These finding sheds light on the critical factors that affect soot hygroscopicity during water adsorption and allows for estimating the interaction between water molecules and soot particles in a humid atmosphere.

Application of physisorption analyzer in measuring the hygroscopicity of atmospheric aerosols

The hygroscopicity represents a vital physicochemical attribute of aerosols, governing their phase state, acidity, viscosity, chemical reactivity, and other properties, thereby influencing various atmospheric processes, including visibility, radiative forcing, and the formation of cloud condensation nuclei. In this study, the hygroscopicity of common inorganic aerosol particles (NH4)2SO4 and NaCl in the atmosphere was determined using a physisorption analyzer under 298 K conditions, exploring its application prospects in measuring atmospheric aerosol hygroscopicity. By measuring the adsorption of particles to water vapor at different relative humidity (RH) levels, the variation curve of the aerosol particle hygroscopic mass growth factor (MGF) and deliquescence relative humidity (DRH) were obtained. Comparisons with vacuum FTIR methods, the Extended Aerosol Inorganic Model (EAIM), and relevant literature data showed good agreement. This research paves the way for future applications of physisorption analyzer in the study of atmospheric aerosols.

The Impacts of Dust Storms With Different Transport Pathways on Aerosol Chemical Compositions and Optical Hygroscopicity of Fine Particles in the Yangtze River Delta

AbstractAerosol physicochemical properties during two dust storms (DS1: March 30–31, and DS2: May 7–8) are measured in 2021 in Nanjing, aiming to investigate the impacts of dust storms on aerosol chemical compositions and optical hygroscopicity in the Yangtze River Delta (YRD). During DS1, the dust air masses are transported in the lower atmosphere and pass through the inlands and sea areas before reaching the YRD region. During DS2, the dust air masses are transported in the upper atmosphere and pass through the inlands only. Both of them are accompanied by an increase in black carbon (BC) mass fraction and a decrease in nitrate mass fraction in fine particles (PM2.5, particles with diameters less than 2.5 μm) near the surface. However, the impacts on the mass fractions of organics or sulfate are adverse between DS1 and DS2. During dust‐influence periods the enlarged mass ratios of sulfate to nitrate (greater than 4) promote the occurrence of deliquescent behavior of ambient aerosols although dust aerosols can significantly suppress aerosol optical hygroscopicity. To improve model simulations of aerosol optical hygroscopicity, it is necessary to use a segment parameterization to describe aerosol light scattering enhancement factor during dust‐influence periods. The closure study of optical hygroscopicity parameters with two different methods reveals the impact of aerosol scattering Ångström exponent on the estimation of optical hygroscopicity parameter. The results highlight that the impact of dust storms on aerosol chemical compositions and optical hygroscopicity are different for DS events with different transport pathways.

Interaction of water mists with bibulous filter media and hygroscopic particles in pressure drop jump of fibrous filter media

This study aims to quantitatively evaluate the impact of humid airflows (unsaturated and saturated airflows with water mists), water absorption by filters (wood cellulose and polyester fibre filter media) and hygroscopic loading dust (KCl and Al2O3 particles) on the pressure drop jump. Additionally, proposed methods to alleviate this tendency are known as the “wet blockage” of filters in practice. The experimental results demonstrated that the water content of the airflows mainly contributed to the pressure drop jump in the filters. Increasing the water content from 4.2 g/m3 to 11.3 g/m3 resulted in a fourfold decrease in the water-loading capacity of the tested filter medium. However, increasing the water absorption of the filters by a factor of three times caused a reduction in the water-loading capacity of approximately 42 mg/cm2 based on the self-defined parameter tc. Furthermore, the non-hygroscopic particles delayed the pressure drop jump, which is associated with the dust-holding capacity of filters.

Theoretical derivation of aerosol lidar ratio using Mie theory for CALIOP-CALIPSO and OPAC aerosol models

Abstract. The extinction-to-backscattering ratio, popularly known as lidar (light detection and ranging) ratio of atmospheric aerosols is an important optical property, which is essential to retrieve the extinction profiles of atmospheric aerosols. Lidar satellite observations can provide the global coverage of atmospheric aerosols along with their vertical extent. NASA's Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite is the only space-based platform available, so far, that provides the vertical profiles of extinction due to atmospheric aerosols. A physics-based theoretical approach is presented in the present paper that estimates lidar ratio values for CALIPSO aerosol models, which can be used as inputs to determine the extinction profiles of aerosols using CALIPSO data. The developed methodology was also qualified by comparing it with the lidar ratio values derived using AERONET (AErosol RObotic NETwork) datasets. Lidar ratio values for CALIPSO aerosol models were estimated in the range of 38.72 to 85.98 sr at 532 nm, whereas at 1064 nm lidar ratio varied between 20.11 to 71.11 sr depending upon the aerosol type and their size distributions. Aerosols are compositions of various particles; thus, the presence of water vapour in the atmosphere can affect the optical properties of the aerosols. Thus, the effect of relative humidity on lidar ratio was studied using Optical Properties of Aerosols and Clouds (OPAC) aerosol models, which are the standard aerosol models against the cluster-classified AERONET and CALIPSO aerosol models. Water-soluble particles contribute substantially in clean continental, clean marine, tropical marine and desert aerosol models and are hygroscopic in nature. Hygroscopic sulfate particles dominate the Antarctic aerosols during summertime. In the presence of relative humidity between 0 %–80 %, the lidar ratio values were observed to decrease from 53.59 to 47.13, from 53.66 to 47.15, from 53.70 to 47.16, and from 55.32 to 48.78 sr at 532 nm for clean continental, clean marine, tropical marine, and desert aerosols, respectively, whereas lidar ratio gradually increased from 47.13 to 51, from 47.15 to 51, from 47.16 to 51, and from 48.78 to 51.68 sr, respectively, for these aerosol models when relative humidity was between 80 %–99 %, due to constituent hygroscopic particles. In the case of Antarctic aerosols, the lidar ratio was observed to increase from 57.73 to 97.64 sr due to hygroscopic sulfate particles that backscattered heavily in the presence of water vapour at 532 nm. The soot particles dominate the polluted continental and polluted marine particles, causing an increase in lidar ratio over its corresponding clean counterpart. Similar results were observed at 1064 nm for OPAC aerosol models.

Development of a physics-based method for calibration of low-cost particulate matter sensors and comparison with machine learning models

Low cost particulate matter sensors are receiving significant attention as they can be used in large number for spatial and temporal measurement of PM mass and number concentration. However, the data reliability is questionable as these sensors are affected by numerous parameters such as temperature, relative humidity, and hygroscopicity of particles. To ensure accurate and reliable measurement of particulate matter concentrations, performance evaluation and calibration of LCS co-located with reference instrument at site is essential. In this study, the performance of the low-cost sensor (APT Maxima) was evaluated with the reference instrument SASS (Speciation Air Sampling System) sampler for developing suitable calibration factor that include impact of the meteorological parameter such as relative humidity and temperature, and hygroscopicity. In this study, we have demonstrated a systematic physics-based method for calibration of LCS based on κ-Köhler theory and Mie theory. For comparison with statistical models, linear regression and machine learning algorithm were also applied. In physics-based model, calculated total light scattered intensity shows good linearity with reference PM2.5 measurements. Physics based model performed better for both the sites as compared to MLR, kNN, RF, and GB ML algorithms with R2, RMSE, and MAE values of 0.72,18.21, and 13.36 for Bhopal site, and 0.91, 7.84, and 5.76 for Kashmir site, respectively. Study indicates that physics-based approach for LCS calibration is suitable and can be transferable to different sites.

Significant impact of water-soluble organic matter on hygroscopicity of fine particles under low relative humidity condition

Uncertainties in estimating the hygroscopicity of bulk aerosols under conditions of low relative humidity (RH) or below the deliquescent RH (DRH) of aerosols remain to be significant, mainly due to the presence of water-soluble organic matter (WSOM). To quantify the contributions of WSOM to aerosol hygroscopicity and associated uncertainties, a field campaign was conducted to measure the hygroscopic growth curve (f(RH)) of bulk aerosols online, dominant chemical compositions in PM2.5 online and offline, and size distributions of the dominant chemical compositions offline during the dry and wet seasons of 2019–2020 in urban Guangzhou of south China. Based on the measured f(RH), the hygroscopicity parameter (κ) of bulk aerosols (κ-f(RH)) exhibits a logarithmic increase with increasing RH until RH reaches 69 %. Beyond this threshold, κ-f(RH) increases very slowly with further increase of RH, reaching 0.32 ± 0.04 during the dry season and 0.31 ± 0.05 during the wet season. The κ of WSOM (κ-WSOM) was further estimated to be 0.22 ± 0.03 and 0.13 ± 0.04 in the dry and wet seasons, respectively, when RH > 69 %. WSOM significantly affects κ-f(RH) by retarding the deliquescence process of aerosols and altering the mass ratio of water-soluble inorganic salts (WSIS) to WSOM within the size range of 0.4–0.9 μm, especially under low RH conditions (<60 %). The κ-f(RH) under low RH conditions was revised based on the logarithmic regression equation between RH and the ratio of measured κ-f(RH) to estimated κ-f(RH>69%). f(RH) of WSIS and WSOM were then corrected using the revised κ-f(RH) under low RH conditions, which showed 22–31 % lower values than those produced by the IMPROVE formulas.

Substantially positive contributions of new particle formation to cloud condensation nuclei under low supersaturation in China based on numerical model improvements

Abstract. New particle formation (NPF) and subsequent particle growth are important sources of condensation nuclei (CN) and cloud condensation nuclei (CCN). While many observations have shown positive contributions of NPF to CCN at low supersaturation, negative NPF contributions were often simulated in polluted environments. Using the observations in a coastal city of Qingdao, Beijing, and Gucheng in north China, we thoroughly evaluate the simulated number concentrations of CN and CCN using an NPF-explicit parameterization embedded in the WRF-Chem model. For CN, the initial simulation shows large biases of particle number concentrations at 10–40 and 40–100 nm. By adjusting the process of gas–particle partitioning, including the mass accommodation coefficient (MAC) of sulfuric acid, the phase changes in primary organic aerosol emissions, and the condensational amount of nitric acid, the improvement of the particle growth process yields substantially reduced overestimation of CN. Regarding CCN, secondary organic aerosol (SOA) formed from the oxidation of semi-volatile and intermediate-volatility organic compounds (S/IVOCs) is called SI-SOA, the yield of which is an important contributor. At default settings, the SI-SOA yield is too high without considering the differences in precursor oxidation rates. Lowering the SI-SOA yield under linear H2SO4 nucleation scheme results in much-improved CCN simulations compared to observations. On the basis of the bias-corrected model, we find substantially positive contributions of NPF to CCN at low supersaturation (∼ 0.2 %) over broad areas of China, primarily due to competing effects of increasing particle hygroscopicity, a result of reductions in SI-SOA amount, surpassing that of particle size decreases. The bias-corrected model is robustly applicable to other schemes, such as the quadratic H2SO4 nucleation scheme, in terms of CN and CCN, though the dependence of CCN on SI-SOA yield is diminished likely due to changes in particle composition. This study highlights potentially much larger NPF contributions to CCN on a regional and even global basis.

Evaluation of hygroscopic cloud seeding in warm-rain processes by a hybrid microphysics scheme using a Weather Research and Forecasting (WRF) model: a real case study

Abstract. To evaluate the hygroscopic cloud seeding in reality, this study develops a hybrid microphysics scheme using a Weather Research and Forecasting (WRF) model, WDM6-NCU (WDM6 modified by National Central University), which involves 43 bins of seeded cloud condensation nuclei (CCN) in the WDM6 bulk method scheme. This scheme can describe the size distribution of seeded CCN and explain the process of the CCN imbedding and cloud and raindrop formation in detail. Furthermore, based on the observational CCN size distribution applied in the modelling, a series of tests on cloud seeding were conducted during the seeding periods of 21–22 October 2020 with stratocumulus clouds. The model simulation results reveal that seeding in in-cloud regions with an appropriate CCN size distribution can yield greater rainfall and that spreading the seeding agents over an area of 40–60 km2 is the most efficient strategy to create a sufficient precipitation rate. With regard to the microphysical processes, the main process that causes the enhancement of precipitation is the strengthening of the accretion process of raindrops. In addition, hygroscopic particles larger than 0.4 µm primarily contribute to cloud-seeding effects. The study results could be used as references for model development and warm-cloud-seeding operations.

Characteristics of Scanning Flow Condensation Particle Counter (SFCPC): A rapid approach for retrieving hygroscopicity and chemical composition of sub-10 nm aerosol particles

Characteristics of Scanning Flow Condensation Particle Counter (SFCPC): A rapid approach for retrieving hygroscopicity and chemical composition of sub-10 nm aerosol particles

Rapid growth and high cloud-forming potential of anthropogenic sulfate aerosol in a thermal power plant plume during COVID lockdown in India

The COVID lockdown presented an interesting opportunity to study the anthropogenic emissions from different sectors under relatively cleaner conditions in India. The complex interplays of power production, industry, and transport could be dissected due to the significantly reduced influence of the latter two emission sources. Here, based on measurements of cloud condensation nuclei (CCN) activity and chemical composition of atmospheric aerosols during the lockdown, we report an episodic event resulting from distinct meteorological conditions. This event was marked by rapid growth and high hygroscopicity of new aerosol particles formed in the SO2 plume from a large coal-fired power plant in Southern India. These sulfate-rich particles had high CCN activity and number concentration, indicating high cloud-forming potential. Examining the sensitivity of CCN properties under relatively clean conditions provides important new clues to delineate the contributions of different anthropogenic emission sectors and further to understand their perturbations of past and future climate forcing.

Characterization of dust-related new particle formation events based on long-term measurement in the North China Plain

Abstract. Mineral dust is a major natural atmospheric aerosol that impacts the Earth's radiation balance. The significant scavenging process of fine particles by the strong wind during the dust episodes provided a relatively clean environment that was favourable for new particle formation (NPF) occurrence. In this study, the NPF occurred following the dust episodes (dust-related NPF), and other cases under clean and polluted conditions were classified based on the long-term particle number size distribution (PNSD) in urban Beijing in spring from 2017 to 2021. It was found that the observed formation (Jobs) and growth rate (GR) of dust-related NPF events were approximately 50 % and 30 % lower than the values of other NPF days, respectively. A typical severe dust storm that originated from Mongolia and swept over northern China on 15–16 March 2021 was analysed, to illustrate how the dust storm influences the NPF event. The maximum hourly mean PM10 mass concentration reached 8000 µg m−3 during this dust storm. The occurrence of an NPF event after a dust storm was facilitated due to the low condensation sink (∼ 0.005 s−1) caused by the strong dilution process of pre-existing particles. However, a downward trend of particle hygroscopicity was found during dust storm and NPF event as compared with the polluted episode, resulting in an increasing trend of the critical diameter at different supersaturations (ss), where aerosols are activated as cloud condensation nuclei (CCN), although the NPF event occurred when dust faded. The critical diameter was elevated by approximately 6 %–10 % (ss = 0.2 % and 0.7 %) during the dust storm, resulting in a lower CCN activation ratio, especially at low supersaturation. Modifications of the nucleation and growth process, as well as the particle-size distribution and hygroscopicity of the dust particles, provide valuable information that reveals the underlying climate and air quality effects of Asian mineral dust.

The effect of relative humidity on deposition pattern in inertial impactors: The role of particle elasticity and surface attraction

The role of relative humidity on particle bounce in an inertial impactor was investigated through a program of experiments and particle trajectory simulations. Inertial impactors are devices used to obtain particle size distributions by passing particle-laden air through a nozzle and collecting particles having sufficient inertia on a flat surface placed directly below the nozzle. Herein monodisperse hygroscopic particles impacted the flat hydrophobic surface of a single stage inertial impactor while varying the relative humidity of the flow. The results show that a circular deposition pattern occurs when the relative humidity is high. When the relative humidity is low secondary deposits beyond the circular deposit also occur. Particle trajectory simulations were performed where particle/surface interactions were quantified via the Hamaker constant (A) and the loss of kinetic energy via the coefficient of restitution (e). In a first-of-its-kind approach, (A, e) were iteratively adjusted until prominent features of the particle deposits in the experiments and simulations agreed, thereby providing values of (A,e) for each relative humidity. The results show that the observed deposition behavior is due to (i) the increase with relative humidity of the repulsion between the hygroscopic particles and the hydrophobic surface, (ii) multiple bounces along the particle trajectory, and (iii) kinetic energy loss at each particle bounce.

Chip-Based Dark-filed Microscopy for Sensing the Dynamical Changes of Single Aerosol Nanoparticles

The hygroscopicity of aerosol nanoparticles with the high number concentration in atmosphere significantly affects atmospheric chemistry and visibility, global climate, and human health. Therefore, it is critical to analyze the hygroscopicity of aerosol particles in an atmospheric ambient, where the aerosol's chemical compositions and physical states determine its capability of water uptake. In this work, with the advantages of non-destructive detection, highly sensitivity, and wide-field imaging, a planar photonic chip-based dark-field microscope is developed for real-time imaging and sensing the hygroscopic growth dynamics of single aerosol nanoparticles (diameter around 100 nm) in a label-free manner. The experimental results of single-component and multi-components are well consistent with the calculation results based on extended aerosol inorganic model. This photonic-chip based dark-field microscope can be extended to investigate the physicochemical reactions of various aerosols in real atmospheric environment.