Computer-controlled Raman microspectroscopy (CC-Raman): A method for the rapid characterization of individual atmospheric aerosol particles

The ability of an atmospheric aerosol particle to impact climate by acting as a cloud condensation nucleus (CCN) or an ice nucleus (IN), as well as scatter and absorb solar radiation is determined by its physicochemical properties at the single particle level, specifically size, morphology, and chemical composition. The identification of the secondary species present in individual aerosol particles is important as aging, which leads to the formation of these species, can modify the climate relevant behavior of particles. Raman microspectroscopy has a great deal of promise for identifying secondary species and their mixing with primary components, as it can provide detailed information on functional groups present, morphology, and internal structure. However, as with many other detailed spectroscopic techniques, manual analysis by Raman microspectroscopy can be slow, limiting single particle statistics and the number of samples that can be analyzed. Herein, the application of computer-controlled Raman (CC-Raman) for detailed physicochemical analysis that increases throughput and minimizes user bias is described. CC-Raman applies automated mapping to increase analysis speed allowing for up to 100 particles to be analyzed in an hour. CC-Raman is applied to both laboratory and ambient samples to demonstrate its utility for the analysis of both primary and, most importantly, secondary components (sulfate, nitrate, ammonium, and organic material). Reproducibility and precision are compared to computer controlled-scanning electron microscopy (CCSEM). The greater sample throughput shows the potential for CC-Raman to improve particle statistics and advance our understanding of aerosol particle composition and mixing state, and, thus, climate-relevant properties.

© 2017 American Association for Aerosol Research     

Cloud condensation nuclei activity and droplet formation of primary and secondary organic aerosol mixtures

Understanding the mixing behavior of anthropogenic primary and biogenic secondary organic aerosol (POA and SOA) is important for characterizing their interactions with water vapor. The following work expands upon previous studies and investigates cloud condensation nuclei (CCN) activity and droplet kinetics of α-pinene SOA formed in an environmental chamber and mixed with diesel or motor oil-diesel fuel POA. The changes in the aerosol mixing are similar to previously published work but this study provides new CCN activity and droplet information. The CCN activity of the unmixed aerosol systems are measured separately; κ = 0.15, 0.11, 0.022 for α-pinene SOA, diesel POA and motor oil-diesel fuel POA, respectively. In the α-pinene SOA + diesel POA mixture, the CCN activity, characterized by κ-hygroscopicity, decreases from κ = 0.15 to 0.06 after an initial injection of the POA but increases to κ = 0.12. The increase in CCN activity occurs after particle collision (coagulation and wall-loss) rates dominate aerosol processes in the chamber. The α-pinene SOA + motor oil-fuel POA does not readily mix and the CCN activity of the complex system increases with time (from κ = 0.022 to 0.10). An empirical equation using unit mass resolution (UMR) AMS data of two different ion fragments reasonably predicts CCN activity of the POA and SOA mixtures. CCN measurement may be a promising tool to gain additional insight into the complex mixtures of organic aerosol and subsequent interactions with water vapor.

Copyright © 2018 American Association for Aerosol Research     

Advanced aerosol optical tweezers chamber design to facilitate phase-separation and equilibration timescale experiments on complex droplets

The phase-separation of mixed aerosol particles and the resulting morphology plays an important role in determining the interactions of liquid aerosols with their gas-phase environment. We present the application of a new aerosol optical tweezers chamber for delivering a uniformly mixed aerosol flow to the trapped droplet's position for performing experiments that determine the phase-separation and resulting properties of complex mixed droplets. This facilitates stable trapping when adding additional phases through aerosol coagulation, and reproducible measurements of the droplet's equilibration timescale. We demonstrate the trapping of pure organic carbon droplets, which allows us to study the morphology of droplets containing pure hydrocarbon phases to which a second phase is added by coagulation. A series of experiments using simple compounds are presented to establish our ability to use the cavity enhanced Raman spectra to distinguish between homogeneous single-phase, and phase-separated core–shell or partially engulfed morphologies. The core–shell morphology is distinguished by the pattern of the whispering gallery modes (WGMs) in the Raman spectra where the WGMs are influenced by refraction through both phases. A core–shell optimization algorithm was developed to provide a more accurate and detailed analysis of the WGMs than is possible using the homogeneous Mie scattering solution. The unique analytical capabilities of the aerosol optical tweezers provide a new approach for advancing our understanding of the chemical and physical evolution of complex atmospheric particulate matter, and the important environmental impacts of aerosols on atmospheric chemistry, air quality, human health, and climate change.

Copyright © 2016 American Association for Aerosol Research     

1-octanol-water partitioning as a classifier of water soluble organic matters: Implication for solubility distribution

Water-soluble organic matters (WSOMs) play an important role in determining magnitudes of climatic and environmental impacts of organic aerosol particles because of their contributions to hygroscopic growth and cloud formation. These processes are dependent on water solubility as well as distribution of this property in a particle, yet no method has been available to quantify such characteristics. In this study, we developed a theoretical framework to classify WSOM by 1-octanol-water partitioning that has a strong correlation with water solubility. 1-octanol-water partitioning coefficient also has a strong correlation with a traditional solid phase extraction method, facilitating interpretation of data from the technique. The theoretical analysis demonstrated that the distributions of WSOM classified by 1-octanol-water partitioning depend on (1) the volume ratio of 1-octanol and aqueous phases, and (2) extraction steps. The method was tested by using organic aerosol particles generated by smoldering of a mosquito coil, which serves as a surrogate for biomass burning particles. The WSOM extracted from the mosquito coil burning particles was classified by 1-octanol-water partitioning at different volume ratios. These solutions, including both the 1-octanol and aqueous phases, were nebulized to generate particles for measurements using an online aerosol mass spectrometer. The mass spectra indicated that highly oxygenated species tend to be highly soluble, while high molecular weight compounds are less soluble. Linear combinations of these mass spectra allowed the estimation of the mass fractions of WSOM partitioned to 1-octanol and aqueous phases, thereby facilitating the evaluation of the mass fractions of cloud condensation nuclei (CCN) active materials.

© 2017 American Association for Aerosol Research     

Development of polydisperse aerosol generation and measurement procedures for wind tunnel evaluation of size-selective aerosol samplers

Accurate development and evaluation of inlets for representatively collecting ambient particulate matter typically involves the use of monodisperse particles in aerosol wind tunnels. However, the resource requirements of using monodisperse aerosols for inlet evaluation creates the need for more rapid and less-expensive techniques to enable determination of size-selective performance in aerosol wind tunnels. The goal of recent wind tunnel research at the U.S. EPA was to develop and validate the use of polydisperse aerosols, which provide more rapid, less resource-intensive test results, which still meet data quality requirements necessary for developing and evaluating ambient aerosol inlets. This goal was successfully achieved through comprehensive efforts regarding polydisperse aerosol generation, dispersion, collection, extraction, and analysis over a wide range of aerodynamic particle sizes. Using proper experimental techniques, a sampler’s complete size-selective efficiency curve can be estimated with polydisperse aerosols in a single test, as opposed to the use of monodisperse aerosols, which require conducting multiple tests using several different particle sizes. While this polydisperse aerosol technique is not proposed as a regulatory substitute for use of monodisperse aerosols, the use of polydisperse aerosols is advantageous during an inlet’s development where variables of sampling flow rate and inlet geometry are often iteratively evaluated before a final inlet design can be successfully achieved. Complete Standard Operating Procedures for the generation, collection, and analysis of polydisperse calibration aerosols are available from EPA as downloadable files. The described experimental methods will be of value to other researchers during the development of ambient sampling inlets and size-selective evaluation of the inlets in aerosol wind tunnels.

© 2018 American Association for Aerosol Research     

Application of centrifugal filter to aerosol size distribution measurement

In the present work, the centrifugal filter proposed by the authors was applied to classify aerosol particles followed by the detection of total mass or number concentrations so as to measure the size distribution of aerosol particles. The structure and operating condition of the centrifugal filter were optimized in order to attain sharp separation curves with various cut-off sizes between 0.3 and 10 μm. The aerosol penetrating the centrifugal filter at various rotation speeds was measured with a photometer to determine the total mass concentration. The virtue of this system is that the cut-off size is varied just by scanning the rotation speed of filter and that it can be applied to the measurement of high concentration aerosols without dilution by choosing an appropriate filter medium. As a result, the centrifugal filter was successfully applied to measure the size distribution of solid particles in size ranging from 0.3 to 10 μm.

Copyright © 2017 American Association for Aerosol Research     

Seasonal Variability of Particle-Associated Organic Compounds Near a Heavily Traveled Secondary Road

Size-resolved aerosol samples were collected both upwind and downwind of a large secondary road in the winter and spring of 2007 to assess contributions of on-road emissions to ambient aerosols. The aerosol samples were extracted and analyzed for a wide variety of organic compounds including polycyclic aromatic hydrocarbons (PAHs), alkanes, sugars, and organic acids. The results showed a strong seasonal pattern where the concentrations of most compounds were higher in winter than in spring. Some of the biogenic sugars were the exception, which might be the result of a “spring blooming season.” The surprising result was that the upwind site located in a residential neighborhood had very similar concentrations of most organic compounds compared to the near-roadway site. Possible reasons for the lack of differences in organic chemical concentrations between the near-road and control sites include: a large urban background concentration of aerosols superimposed on any local source; shifting wind directions that make the “downwind” site upwind during the night; and additional local sources in the residential neighborhood such as wood burning in winter.

Copyright 2014 American Association for Aerosol Research     

Comparative performance of a thermal denuder and a catalytic stripper in sampling laboratory and marine exhaust aerosols

The performance of a thermal denuder (thermodenuder—TD) and a fresh catalytic stripper (CS) was assessed by sampling laboratory aerosol, produced by different combinations of sulfuric acid, octacosane, and soot particles, and marine exhaust aerosol produced by a medium-speed marine engine using high sulfur fuels. The intention was to study the efficiency in separating non-volatile particles. No particles could be detected downstream of either device when challenged with neat octacosane particles at high concentration. Both laboratory and marine exhaust aerosol measurements showed that sub-23 nm semi-volatile particles are formed downstream of the thermodenuder when upstream sulfuric acid approached 100 ppbv. Charge measurements revealed that these are formed by re-nucleation rather than incomplete evaporation of upstream aerosol. Sufficient dilution to control upstream sulfates concentration and moderate TD operation temperature (250°C) are both required to eliminate their formation. Use of the CS following an evaporation tube seemed to eliminate the risk for particle re-nucleation, even at a ten-fold higher concentration of semi-volatiles than in case of the TD. Particles detected downstream of the CS due to incomplete evaporation of sulfuric acid and octacosane aerosol, did not exceed 0.01% of upstream concentration. Despite the superior performance of CS in separating non-volatile particles, the TD may still be useful in cases where increased sensitivity over the traditional evaporation tube method is needed and where high sulfur exhaust concentration may fast deplete the catalytic stripper adsorption capacity.

Copyright © 2018 American Association for Aerosol Research     

The First Combined Thermal Desorption Aerosol Gas Chromatograph—Aerosol Mass Spectrometer (TAG-AMS)

To address the critical need for improving the chemical characterization of the organic composition of ambient particulate matter, we introduce a combined thermal desorption aerosol gas chromatograph—aerosol mass spectrometer (TAG-AMS). The TAG system provides in-situ speciation of organic chemicals in ambient aerosol particles with hourly time resolution for marker compounds indicative of sources and transformation processes. However, by itself the TAG cannot separate by particle size and it typically speciates and quantifies only a fraction of the organic aerosol (OA) mass. The AMS is a real-time, in-situ instrument that provides quantitative size distributions and mass loadings for ambient fine OA and major inorganic fractions; however, by itself the AMS has limited ability for identification of individual organic compounds due to the electron impact ionization detection scheme used without prior molecular separation.

The combined TAG-AMS system provides real-time detection by AMS followed by semicontinuous analysis of the TAG sample that was acquired during AMS operation, achieving simultaneous and complementary measurements of quantitative organic mass loading and detailed organic speciation. We have employed a high-resolution time-of-flight mass spectrometer (HR-ToF-MS) to enable elemental-level determination of OA oxidation state as measured on the AMS, and to allow improved compound identification and separation of unresolved complex mixtures (UCM) measured on the TAG. The TAG-AMS interface has been developed as an upgrade for existing AMS systems. Such measurements will improve the identification of organic constituents of ambient aerosol and contribute to the ability of atmospheric chemistry models to predict ambient aerosol composition and loadings.

Copyright 2014 American Association for Aerosol Research     

A novel high-volume Photochemical Emission Aging flow tube Reactor (PEAR)

Aerosols emitted from various anthropogenic and natural sources undergo constant physicochemical transformations in the atmosphere, altering their impacts on health and climate. This article presents the design and characteristics of a novel Photochemical Emission Aging flow tube Reactor (PEAR). The PEAR was designed to provide sufficient aerosol mass and flow for simultaneous measurement of the physicochemical properties of aged aerosols and emission exposure studies (in vivo and in vitro). The performance of the PEAR was evaluated by using common precursors of secondary aerosols as well as combustion emissions from a wood stove and a gasoline engine. The PEAR was found to provide a near laminar flow profile, negligible particle losses for particle sizes above 40?nm, and a narrow residence time distribution. These characteristics enable resolution of temporal emission patterns from dynamic emission sources such as small-scale wood combustion. The formation of secondary organic aerosols (SOA) in the PEAR was found to be similar to SOA formation in a smog chamber when toluene and logwood combustion emissions were used as aerosol sources. The aerosol mass spectra obtained from the PEAR and smog-chamber were highly similar when wood combustion was used as the emission source. In conclusion, the PEAR was found to plausibly simulate the photochemical aging of organic aerosols with high flow rates, needed for studies to investigate the effects of aged aerosols on human health. The method also enables to study the aging of different emission phases in high time resolution, and with different OH-radical exposures up to conditions representing long-range transported aerosols.

Copyright © 2019 American Association for Aerosol Research     

Methodology for quantifying the volatile mixing state of an aerosol

Mixing state refers to the relative proportions of chemical species in an aerosol, and the way these species are combined; either as a population where each particle consists of a single species (‘externally mixed’) or where all particles individually consist of two or more species (‘internally mixed’) or the case where some particles are pure and some particles consist of multiple species. The mixing state affects optical and hygroscopic properties, and quantifying it is therefore important for studying an aerosol's climate impact. In this article, we describe a method to quantify the volatile mixing state of an aerosol using a differential mobility analyzer, centrifugal particle mass analyzer, catalytic denuder, and condensation particle counter by measuring the mass distributions of the volatile and non-volatile components of an aerosol and determining how the material is mixed within and between particles as a function of mobility diameter. The method is demonstrated using two aerosol samples from a miniCAST soot generator, one with a high elemental carbon (EC) content, and one with a high organic carbon (OC) content. The measurements are presented in terms of the mass distribution of the volatile and non-volatile material, as well as measures of diversity and mixing state parameter. It was found that the high-EC soot nearly consisted of only pure particles where 86% of the total mass was non-volatile. The high-OC soot consisted of either pure volatile particles or particles that contained a mixture of volatile and non-volatile material where 8% of the total mass was pure volatile particles and 70% was non-volatile material (with the remaining 22% being volatile material condensed on non-volatile particles).

© 2016 American Association for Aerosol Research     

Computational simulation of the dynamics of secondary organic aerosol formation in an environmental chamber

A key atmospheric process that is studied in laboratory chambers is the oxidation of volatile organic compounds to form low volatility products that condense on existing atmospheric particles (or nucleate) to form organic aerosol, so-called secondary organic aerosol. The laboratory chamber operates as a chemical reactor, in which a number of chemical and physical processes take place: gas-phase chemistry, transport of vapor oxidation products to suspended particles followed by uptake into the particles, deposition of vapors on the walls of the chamber, deposition of particles on the walls of the chamber, and coagulation of suspended particles. Understanding the complex interplay among these simultaneous physicochemical processes is necessary in order to interpret the results of chamber experiments. Here we develop and utilize a comprehensive computational model for dynamics of vapors and particles in a laboratory chamber and analyze chamber behavior over a range of physicochemical conditions.

Copyright © 2018 American Association for Aerosol Research     

Performance of Two Fluorescence-Based Real-Time Bioaerosol Detectors: BioScout vs. UVAPS

A 405 nm diode laser-based on-line bioaerosol detector, BioScout, was tested and compared with the Ultraviolet Aerodynamic Particle Sizer (UVAPS). Both instruments are based on laser-induced fluorescence of particles. Only a fraction of microbial particles produce enough fluorescence light to be detected by the instruments. This fluorescent particle fraction (FPF) is aerosol and instrument specific. The FPF values for common bacterial and fungal spores and biochemical particles were experimentally determined for both instruments. The BioScout exhibited higher FPF values for all the test aerosols except coenzyme NADH. The difference was higher for smaller particles. The FPF values of fungal spores and bacteria varied between 0.34 to 0.77 and 0.13 to 0.54 for the BioScout and the UVAPS, respectively. The results indicate that the 405 nm diode laser is a useful excitation source for fluorescence-based real-time detection of microbial aerosols. The FPF results of this study can be utilized to estimate the actual concentrations of bacterial and fungal spores in fluorescence-based ambient measurements.

Copyright 2014 American Association for Aerosol Research     

A New Laser Induced Incandescence–Mass Spectrometric Analyzer (LII-MS) for Online Measurement of Aerosol Composition Classified by Black Carbon Mixing State

We developed a laser induced incandescence–mass spectrometric analyzer (LII-MS) for online measurements quantifying the aerosol chemical compositions with respect to the mixing state of black carbon (BC). The LII-MS is developed as a tandem series comprising an LII chamber to detect and vaporize BC-containing particles and a particle trap laser desorption mass spectrometer (PT-LDMS: Takegawa et al. 2012). The PT-LDMS collects aerosol particles transferred from the LII chamber and quantifies the chemical compositions. A newly designed collection probe, coupled with the sheath-air inlet nozzle of the LII chamber, enables a high throughput of aerosol particles without significant dilution. Total aerosol particles can be analyzed in the PT-LDMS by turning off the laser (MS mode), and the aerosol particles externally mixed with BC can be analyzed by turning on the laser (LII-MS mode). The difference in the PT-LDMS signals between the MS and LII-MS modes yields the chemical composition of materials internally mixed with BC. Performance of the developed instrument was evaluated in the laboratory by generating BC particles internally-mixed with oleic acid (OL) and BC particles externally mixed with ammonium sulfate particles. Preliminary results from ambient measurements are also presented and discussed.

Copyright 2014 American Association for Aerosol Research     

The Use of Nonlinear Acoustics as an Energy-Efficient Technique for Aerosol Removal

This article is a feasibility study on using nonlinear acoustic effects, acoustic streaming and acoustic radiation pressure, for aerosol removal in an air duct. Unlike previous research, which used acoustics solely to cause aerosol agglomeration prior to aerosol removal in traditional duct collection systems, this article considers the acoustic streaming effect, which is significant but was previously neglected. Monodispersed polystyrene spheres with diameters ranging from 0.3 to 6 μm were tested. The proposed system removed 12–20% of the submicron aerosols and 25–32% of the micron aerosols when the airflow rate was approximately 90 L/min. Acoustic streaming introduces stagnation points on the surface of the air duct and removes the aerosols by deposition. Acoustic radiation pressure causes aerosols to form agglomerates. This enhances inertial impaction and/or gravitational sedimentation, which further enhances the removal efficiency of micron aerosols. The particle-removal efficiency is proportional to the duration that the aerosols are exposed to the acoustic field. The pressure drop due to the nonlinear acoustic effects is negligible; thus, power consumption is minimal. This system has the potential to be developed into an energy-efficient technique for aerosol removal.

Copyright 2014 American Association for Aerosol Research     

Aerosol Generation by a Spray-Drying Technique Under Coulomb Explosion and Rapid Evaporation for the Preparation of Aerosol Particles for Inhalation Tests

In this study, nanosized (<100 nm) aerosol particles with high mass concentrations for inhalation tests were generated by a spray-drying technique with combining Coulomb explosion and rapid evaporation of the droplets. Under typical spray-drying conditions, aerosol particles with average diameter of 50–150 nm were prepared from a suspension of NiO nanoparticles with a primary diameter of 15–30 nm. Under the Coulomb explosion method, the sprayed droplets were charged by being mixed with unipolar ions to break up the droplets, which resulted in the generation of smaller aerosol particles with diameters of 15–30 nm and high number concentrations. Under the rapid evaporation method, the droplets were heated immediately after being sprayed to avoid inertial impaction on the flow path due to shrinkage of the droplet, which increased the mass concentration of the aerosol particles. The combination of the Coulomb explosion and rapid evaporation of droplets resulted in the generation of aerosol particles with sizes less than 100 nm and mass concentrations greater than 1 mg/m³; these values are often necessary for inhalation tests. The aerosols generated under the combined method exhibited good long-term stability for inhalation tests. The techniques developed in this study were also applied to other metal oxide nanoparticle materials and to fibrous multiwalled carbon nanotubes.

Copyright 2014 American Association for Aerosol Research     

Toward the Determination of Joint Volatility-Hygroscopicity Distributions: Development and Response Characterization for Single-Component Aerosol

This work presents the development and characterization of a thermodenuder for the study and interpretation of aerosol volatility. Thermodenuder measurements are further combined with a continuous-flow streamwise thermal gradient CCN counter to obtain the corresponding aerosol hygroscopicity. The thermodenuder response function is characterized with monodisperse aerosol of variable volatility and hygroscopicity. The measurements are then interpreted with a comprehensive instrument model embedded within an optimization framework to retrieve aerosol properties with constrained uncertainty. Special attention is given to the interpretation of the size distribution of the thermodenuded aerosol, deconvoluting the effects of impurities and multiple charging, and to simplifications on the treatment of thermodenuder geometry, temperature, the cooling section, and the effects of curvature and accommodation coefficient on inferred particle volatility. Retrieved vapor pressures are consistent with published literature and shown to be most sensitive to uncertainty in the accommodation coefficient.

Copyright 2014 American Association for Aerosol Research     

Intercomparison of a portable and two stationary mobility particle sizers for nanoscale aerosol measurements

During occupational exposure studies, the use of conventional scanning mobility particle sizers (SMPS) provides high quality data but may convey transport and application limitations. New instruments aiming to overcome these limitations are being currently developed. The purpose of the present study was to compare the performance of the novel portable NanoScan SMPS TSI 3910 with that of two stationary SMPS instruments and one ultrafine condensation particle counter (UCPC) in a controlled atmosphere and for different particle types and concentrations.

The results show that NanoScan tends to overestimate particle number concentrations with regard to the UCPC, particularly for agglomerated particles (ZnO, spark generated soot and diesel soot particles) with relative differences >20%. The best agreements between the internal reference values and measured number concentrations were obtained when measuring compact and spherical particles (NaCl and DEHS particles). With regard to particle diameter (modal size), results from NanoScan were comparable < [± 20%] to those measured by SMPSs for most of the aerosols measured.

The findings of this study show that mobility particle sizers using unipolar and bipolar charging may be affected differently by particle size, morphologies, particle composition and concentration. While the sizing accuracy of the NanoScan SMPS was mostly within ±25%, it may miscount total particle number concentration by more than 50% (especially for agglomerated particles), thus making it unsuitable for occupational exposure assessments where high degree of accuracy is required (e.g., in tier 3). However, can be a useful instrument to obtain an estimate of the aerosol size distribution in indoor and workplace air, e.g., in tier 2.     

Influence of surfactants on growth of individual aqueous coarse mode aerosol particles

Understanding the links between aerosol and cloud and radiative properties remains a large uncertainty in predicting Earth's changing energy budget. Surfactants are observed in ambient atmospheric aerosol particles, and their effect on cloud droplet growth is a mechanism that was, until recently, neglected in model calculations of particle activation and droplet growth. In this study, coarse mode aqueous aerosol particles were created containing the surfactant Igepal CA-630 and NaCl. The evaporation and condensation of these individual aqueous particles were investigated using an aerosol optical trap combined with Raman spectroscopy. For a relative humidity (RH) change from 70% to 80%, droplets containing both Igepal and NaCl at atmospheric concentrations exhibited on average more than 4% larger changes in droplet radii, compared to droplets containing NaCl only. This indicates enhanced water uptake in the presence of surfactants, but this result is unexpected based on the standard calculation of the effect of surfactants, using surface tension reduction and/or hygroscopicity changes, for particles of this size. One implication of these results is that in periods with increasing RH, surfactant-containing aqueous particles may grow larger than similarly sized aqueous NaCl particles without surfactants, thus shifting atmospheric particle size distributions, influencing particle growth, and affecting aerosol loading, visibility, and radiative forcing.

Copyright © 2018 American Association for Aerosol Research     

Numerical analysis of the dynamics of aerosol inertial collection and aggregation on raindrops

A three-dimensional stochastic model is developed for predicting atmospheric aerosol collection and aggregation on the surface of a falling raindrop at its terminal velocity. Potential flow and viscous flow are assumed as the flow fields in the vicinity of the large and the small raindrops, respectively. The results show that hydrophobic coarse mode aerosols collected by either small raindrops (d_c < 100 μm) or large drops (d_c > 100 μm) form aggregations on the surfaces of drops, and accumulation mode aerosols tend to be captured by the aggregations or hydrophobic coarse particles which have been collected by the drops, and this may significantly enhance the capability of the raindrop for fine aerosol collection. When the aggregation effect is considered in the calculation, fine aerosol efficiency can be promoted by one to two orders of magnitude. Therefore, fine particle collision efficiency by raindrops is underestimated by employing the classical dynamic theory which neglects the particle aggregation effect. However, the collection efficiency of coarse particles remains almost constant with the increase in the amount of particles collected by large drops, while there is only a slight increase in efficiency by small raindrops upon increasing in particle concentration. This implies that the traditional limiting trajectory method can still be used for the calculation of coarse particle collection efficiencies by either small or large raindrops.

Copyright © 2018 American Association for Aerosol Research