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The chemical speciation of nanoparticles is technically challenging because of the minute mass of the particles. There is a constant need for more sensitive collection methods and chemical analyses. In this study, we demonstrated the applicability of a surface enhanced Raman scattering (SERS) technique on the rapid and sensitive chemical analysis of individual nanoparticles. SERS technique provides a significant enhancement of the scattering efficiency over traditional Raman spectroscopy. The novelty of the proposed technique is that the SERS substrate is used directly as the sampling substrate of a condensational growth tube (CGT) sampler, which can activate nanoparticles into water droplets and ensure simultaneous inertial sampling and SERS pretreatment. First, we investigated applicability of the method on mono-dispersed (20 nm, 50 nm, or 100 nm) ammonium sulfate (AS) and levoglucosan (LG) particles as model aerosols. The method was then applied to ambient nanoparticles. The successful detection of peaks corresponding to sulfate ν(SO42-) and organics ν(C-H) indicates that our proposed method to combine a CGT sampler and SERS showed a sensitivity high enough to provide deep insights into the chemical speciation of atmospheric nanoparticles as small as 20 nm in diameter. [ABSTRACT FROM AUTHOR]

Heterogeneous reactions between atmospheric aerosol particles and gaseous pollutants, such as those forming brown carbon (BrC), represent an important mechanism. These reactions alter the particle chemical compositions and aerosol-climate interactions. While most studies assume homogeneous particle compositions, organic coatings can be formed on solid or highly viscous particles due to heterogeneous reactions but the underlying mechanism is relatively less examined. We used electrospray surface-enhanced Raman spectroscopy (ES-SERS) to directly probe the formation of BrC coatings on methylaminium sulfate, nitrate, and chloride particles from heterogeneous reactions with gas-phase glyoxal. To create BrC coatings on particle surfaces, heterogeneous reactions were performed under low relative humidity (RH) conditions (i.e., 10 or 30% RH). The reacted particles fluoresced when irradiated at 532 nm in normal Raman analysis, indirectly suggesting the presence of light-absorbing species in them. Further ES-SERS analyses showed Raman bands of 1,3-dimethylimidazole, one of the major known products of reactions of glyoxal with methylaminium, from all the reacted particles at 30% RH. However, only methylaminium sulfate particles showed the formation of BrC coatings at 10% RH. We speculate that methylaminium sulfate particles may have more surface adsorbed water (SAW) than the other particle samples to initiate the formation of BrC coatings detectable by ES-SERS. The present study highlights the surface sensitivity of ES-SERS as well as the potential importance of SAW in heterogeneous reactions of atmospheric particles with gaseous pollutants. Copyright © 2019 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Reactive uptake by ammonium (NH4+) salts is one of the major pathways for the gas-to-particle partitioning of alkyl amines. Recent studies using particles of individual ammonium salts and mixtures with hydrophilic organics have revealed that the degree of amine uptake depends on the phase state of ammonium salts, the particulate water contents and particle viscosity. The role of hydrophobic organic compounds, another important category of particulate organics commonly detected in the ambient atmosphere, in amine uptake remains unknown. Here we report the uptake of dimethylamine (DMA) by ammonium sulfate (AS) particles coated with fresh or ozone-aged bulk oleic acid (OA) at 60%, 30%, and <5% relative humidities (RHs) using an electrodynamic balance coupled with Raman spectroscopy. OA and DMA were selected to represent hydrophobic organics and alkyl amines, respectively. Over 74% of the original NH4+ions were displaced due to DMA uptake, except those conditioned at <5% RH. On the other hand, the fresh or aged bulk OA coating retarded DMA uptake by preventing the particle surface from effectively accommodating gaseous DMA molecules. Judging from the estimated DMA uptake coefficients, the retardation gradually intensified as the weight percentage of coating increased before leveling off, likely when the particle surface was completely covered by fresh or aged bulk OA. We propose that the accommodation of DMA on the particle coating is the rate-limiting step of DMA uptake. Intensive aging of the OA coating had little effect on the equilibrium particle-phase compositions but retarded DMA uptake. © 2017 American Association for Aerosol Research [ABSTRACT FROM PUBLISHER]

The ability of atmospheric particles to absorb water has extensive climate, atmospheric chemistry, and health implications, and considerable effort has gone into determining relationships between particle composition and hygroscopicity. Parallel techniques, in which co-located composition and hygroscopicity measurements are combined to infer composition-hygroscopicity relationships, may not detect the influence of external mixtures. Previous in-line measurements have been limited to single-particle composition or a limited analyte range, and are often non-quantitative and/or offline. Here, we present for the first time in-series, online, quantitative hygroscopicity-composition measurements using a Brechtel Manufacturing, Inc. Hybrid Tandem Differential Mobility Analyzer and an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer. This technique is first verified using laboratory-generated external particle mixtures, then extended to ambient measurements at a seaside sampling side at the Hong Kong University of Science and Technology. The technique successfully separated laboratory-generated particles of differing hygroscopicities and showed promise for atmospheric particles, though high mass attenuation endemic to the HTDMA dual size selection limits application to environments with at least ∼14–41 μg/m3of particulate mass, depending on composition. Copyright © 2017 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The hygroscopic tandem differential mobility analyzer (HTDMA) has been frequently used to measure the hygroscopic properties of atmospheric aerosols at a fixed high relative humidity (RH) of about 90%. To evaluate if such measurements could be used to determine the hygroscopicity of aerosols at lower RH, simultaneous hygroscopic growth factor (GF) and size-resolved composition measurements were made with an HTDMA and a high-resolution aerosol mass spectrometer (HR-AMS), respectively, at a coastal site in Hong Kong from January to June and in August 2012. A total of 58 cycles of dehydration (decreasing RH) and hydration (increasing RH) of 100 nm and 200 nm particles with organic-to-inorganic mass ratio ranging from 0.19 to 1.97 were measured at RH = 10–93%. The Kappa (κ) equation developed by Petters and Kreidenweis in the year 2007 was used to determine (i) κ at individual RHs (κRH) and (ii) best-fit κ covering the range of RHs measured (κf) for the more-hygroscopic (MH) mode, which describes more than 80% of the particles in each cycle, during dehydration. Overall, κ at 90% RH or above (κ>90) fell between 0.18 and 0.48, and was within 15% of κfin 83% of the datasets. Regression analysis between κ>90or κfand AMS mass fractions showed that κ was positively correlated with sulfate but negatively correlated with organic and nitrate. In most cases, κRHincreased as RH decreased and the average increase in κ was 45% from 90% RH to 40% RH, but these differences yielded insignificant changes in the GF-RH curves. The Zdanovskii-Stokes-Robinson (ZSR) estimated κ were mostly within 20% of κ>90and κf. GF predictions using the empirical correlation of κ with AMS mass fractions or the ZSR estimated κ were within 10% of additional measurements and hence κ>90is useful for predicting GF at lower RHs. Copyright 2015 American Association for Aerosol Research [ABSTRACT FROM PUBLISHER]

Alkylaminium sulfates are frequently detected in ambient aerosols, and are believed to be important in the nucleation of new particles in the atmosphere, despite the comparatively low gas phase concentrations of amines. In this study, water activities and osmotic coefficients have been measured, using a chilled mirror dew point technique, of aqueous mixtures of sulfuric acid and the following alkylaminium sulfates: methylaminium, ethylaminium, dimethylaminium, diethylaminium, and trimethylaminium sulf-ate. The samples were prepared by mixing solutions of the five corresponding amines and aqueous sulfuric acid and determining the exact aminium to sulfate molar ratios by ion chromatography. The results were correlated using an extended Zdanovskii–Stokes–Robinson equation to enable concentration/water activity rela-tionships to be calculated over the entire composition range from pure aqueous sulfuric acid to pure aqueous aminium sulfate. Water activities and osmotic coefficients for aminium:sulfate ratios of 1:1 (the bisulfate salts) and lower showed great similarity with ammonium bisulfate, but osmotic coefficients for the 2:1 ratio (the aminium sulfates) were significantly larger (and water activities lower) than for ammonium sulfate. These results differ from those obtained in Clegg et al.'s (2013) study. The relative values of the osmotic coefficients, in concentrated solutions, suggest that the numbers of methyl or ethyl groups in the aminium ion may have a stronger lowering effect on water activity than the alkyl chain length. Copyright 2015 American Association for Aerosol Research [ABSTRACT FROM PUBLISHER]

The adverse health effect of acidic ultrafine particles (AUFPs) has been widely recognized in scientific societies. These particles mainly deposit on the surface by diffusion and so far there is no mature method for the measurement of airborne AUFPs. The purpose of this study was to develop a diffusion sampler (DS) with iron nanofilm detectors to effectively measure the number concentration and size distribution of airborne AUFPs in indoor and outdoor environments. The developed DS was made of stainless steel with a flat and rectangular channel with 1.0 mm height, 50 mm width, and 500 mm length. The iron nanofilm detectors were deployed on rectangular recesses inside the sampler at three different locations along the length of the channel to collect the ultrafine particles. The exposed detectors were then scanned using an atomic force microscope (AFM) to numerate and distinguish the AUFPs from the nonacidic UFPs. Prior to sampling, the semi-empirical equations for the diffusive deposition efficiency of particles at the different detector locations in the sampler were obtained on the basis of theoretical diffusive mechanism and modified by the experimental data using polystyrene latex (PSL) standard particles. After calibration, the DS + AFM method and a commercially available online measurement system, i.e., scanning mobility particle sizer (SMPS) incorporated with a condensation particle counter (CPC), were simultaneously used in a 4-week field measurement. Both methods showed very good agreement in terms of total particle number concentration and size distribution. The results indicate that the diffusion sampler is effective for the quantification of ambient acidic ultrafine particles. Copyright 2014 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The Tandem Differential Mobility Analyzer (TDMA) technique coupled with aerosol humidification has been widely used for studying aerosol hygroscopicity. In this study, we evaluate the performance of a commercial Humidified TDMA (BMI HTDMA, Model 3002) with respect to DMA sizing, relative humidity (RH) control, and growth factor (GF) measurements. Unique features of this particular HTDMA include a diffusion-based particle humidifier, a DMA design allowing selection of particles up to 2 μm diameter at only 5600 volts, and the ability to study the complete deliquescence and efflorescence cycle. The sizing agreement between DMA 1 and 2 was within 2% over the 35 to 500 nm diameter range. The measured TDMA responses agreed well with theoretical calculations. The RH control and stability were tested at a suburban field site in Hong Kong. The system achieved RH equilibrium in less than 4 min when changing the RH set point. With indoor temperature changes of less than 1°C per hour, the RH control of the system was very stable at 90%, within 1% RH deviation, as confirmed by GF measurements on ammonium sulfate (AS) aerosols performed on separate days. The hygroscopic properties of various pure aerosols were examined and the results agreed well with model predictions. The application of the BMI HTDMA for field measurements was also demonstrated. Two modes were resolved from the GF distributions at 90% RH and variable hygroscopic growth with changing RH was observed. Copyright 2014 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Most aerosol chemical characterization studies to date involve bulk particle analysis. The surface chemical and physical properties of aerosol particles have rarely been analyzed, despite the particles’ potential health impacts and interactions with gas in the atmosphere. Aerosol particles ranging from 0.056 to 10 μm in size collected using a 10-stage impactor sampler from a busy walkway in a downtown area of Hong Kong were analyzed using X-ray photoelectron spectroscopy (XPS), a technique providing both elemental and chemical state information about the particle surfaces. Six key elements were detected: nitrogen (N), sulfur (S), calcium (Ca), silicon (Si), oxygen (O), and carbon (C). Carbon was the dominant species on the surfaces of all particles regardless of their sizes. A higher carbon concentration was found on the surfaces of the 0.056–0.32 μm particles. The N, Si, Ca, and O concentrations were higher on the surface of the 3.2–10 μm particles than in the smaller size fractions. Sulfur was mainly found on the surface of the 0.32–1.8 μm particles. High-resolution scans of C, N, and S were obtained to provide chemical state information about these elements. Aromatic C-H and aliphatic C-H were found to be the major carbon chemical states. Fullerenic carbon was detected on the surfaces of the finest (0.056–0.32 μm) particles. Oxygen- and nitrogen-containing organics were found on the surfaces of the 0.32–1.8 μm particles. Sulfur was present in the form of sulfates as expected. Ammonium salts, amide, and nitrate were found to form especially on the surfaces of aerosol particles in the nucleation, accumulation, and coarse modes, respectively. Silicates and carbonates were only discovered on the surfaces of coarse-mode particles (3.2–10 μm). The results suggest that both the chemical elements and their chemical states were significantly dependent on the size of the aerosol particles. Copyright 2013 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Particle size distributions are of profound interest in the study of ambient aerosols. Electrostatic classification using the Scan-ning Mobility Particle Sizer (SMPS) and more recently the Fast-Mobility Particle Sizer (FMPS) is the most commonly employed approach to establish particle size distributions for submicron par-ticles in field and laboratory applications. The FMPS enables fast size distribution measurements on a timescale of seconds but has been speculated to underestimate particle size. Aerosol mass spec-trometry has emerged as another well-accepted method for size-resolved compositional aerosol analysis with particle sizing being accomplished by flight time separation over a specified flight path under vacuum conditions. In this work, we characterized the parti-cle sizing performance of an FMPS against simultaneous measure-ments with an Aerodyne Aerosol Mass Spectrometer (AMS) and an SMPS by sampling ambient particles, as well as polydisperse and monodisperse particles from aqueous inorganic salt solutions in the size range from 50 nm to 450 nm. The particle size mea-surements by AMS and SMPS produced similar results, while the FMPS significantly underestimated particle size by 40-50%. The discrepancy was observed in all studied ambient and laboratory-generated aerosols and appeared to be largely independent of the sampled species. The observations suggest that it is crucial to eval-uate the sizing performance of the FMPS against other instruments to ensure an adequate accuracy of the particle size measurements. In this study, a simple postcorrection method for the FMPS mea-surements was applied, which was able to successfully reduce the initial underestimation. [ABSTRACT FROM AUTHOR]

The ozonolysis of maleic acid/ammonium sulfate (MA/AS, 30/70 wt%) internal mixtures was investigated at various relative humidities (RHs) (80%, 55%, 30%, and <5%) and physical states (aqueous and crystalline solid). Complementary laboratory techniques—an electrodynamic balance coupled with in situ Raman spectroscopy and a reaction flow cell coupled with offline ion chromatography (IC)—were employed to probe the mass changes, spectroscopic changes, product identities, and post-reaction hygroscopic changes in ozone-processed MA/AS mixed particles. The rate of ozonolysis was highly dependent on the physical state and the water content of the particles. Crystalline solid particles underwent negligible changes even after exposure to 10-ppm ozone for six days. Aqueous particles reacted much faster and the rate was related to the RH during reaction. It took 23 h to reduce MA to 50% of its initial mass at 80% RH, whereas it took 87 h to do the same at 55% RH or lower. We believe that the salting-out of ozone in concentrated droplets at 55% RH may have contributed to the reduced ozonolysis rate. The molar conversion of MA to glyoxylic acid was about unity in both the 55% RH and the 80% RH experiments. Exposed particles may have formed amorphous solids when dried and their hygroscopicity was different from that of the corresponding parent particles. Early deliquescence of reacted particles was observed, suggesting the strong influence of aging on aerosol hygroscopicity. Copyright 2012 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Incense burning is a common ritual in Asian communities both indoors in residential homes and outdoors in temple premises. Organic particles from burning of incense sticks, incense coils, and mosquito coils after extensive dilution (>1000×) were characterized by the Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The obtained mass spectra in general resemble those reported for biomass burning aerosols. Ion peaks with m/z values higher than 100 accounted for 15%–25% of the organic signals in the unit-mass-resolution (UMR) mass spectra. In the high-resolution (HR) mass spectra, the ion peaks at m/z 60 and 73 are found to be related to the sugar anhydrides as in particles from other biomass burning processes. In addition, the ion peaks at m/z 107, 121, 137, 151, 167, and 181, some of which (e.g., m/z 137 and 167) have been observed in particles from biomass burning but not yet assigned, were assigned to lignin-related components. Elemental analysis from the HR data reveals that a large portion of particulate organics from incense burning are oxygenated (O/C between 0.3 and 0.5) and unsaturated (and/or cyclic) in nature. Results from this study also highlight that mass spectra from HR-ToF-AMS measurements concerning primary emissions such as incense burning contain very useful information in the high m/z (>100) region about the chemical characteristics of those primary organic particles. Copyright 2012 American Association for Aerosol Research [ABSTRACT FROM PUBLISHER]

The number concentration and size-resolved properties of acidic ultrafine particles have been observed to more closely associate with adverse health effects than do indices of total particulate mass. However, no reliable measurement techniques are currently available to quantify the number concentration and the size distribution of ambient acidic ultrafine particles. In this study, a method with the use of iron nanofilm detectors for enumeration and size measurement of acid aerosols is developed and refined. Standard sulfuric acid (H2SO4) or ammonium hydrogen sulfate (NH4HSO4) droplets and sulfuric acid-coated particles were generated and deposited on the detectors causing reaction spots. The dimensions of the reaction spots were examined with Atomic Force Microscopy (AFM) to establish the correlations between the diameter of the particle and the size of the reaction spot. To validate this method, field measurements were conducted from September 06 to November 30, 2010, at Tai Mo Shan in Hong Kong. The results indicated that the particle number concentrations obtained from the AFM scanning of the exposed detectors via scanning mobility particle sizer (SMPS) and electrostatic precipitator (ESP) collection were comparable to those derived from the SMPS + CPC (condensation particle counter) measurements (p > 0.05). The average geometric mean diameter of particles at peak measured by the SMPS + CPC and the detectors scanned by the AFM was 52.3 ± 6.9 nm and 51.9 ± 3.1 nm, respectively, showing good agreement. It is suggested that the iron nanofilm detectors could be a reliable tool for the measurement and analysis of acidic particles in the atmosphere. Copyright 2012 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Black carbon emitted from vehicles or industrial plants is a very common phenomenon observed by high school students. A cost effective and user friendly measurement protocol will greatly enhance school students' capability to investigate the black carbon pollutant in their ambient environments, and hence their awareness to environmental protection. This study shows the potential of applying digital imaging as an alternative method to measure airborne elemental carbon (EC). Aerosols were collected on filters and the blackness of the filters was digitalized into RGB values using an office scanner. It was found that maximum value of transformed R, G and B values (255 - R, 255 - G and 255 - B, respectively), max {R',G',B'}, was power-law related with the EC loading of the filter (R2 = 0.85, n = 55). The power-law relationship between the max{R',G',B'} and EC loading could be applied as a cost effective and user friendly method to quantify EC by high school students. Using this method, the sampling flowrate and duration should be controlled so that the max{R',G',B'} of the filter samples lie in the optimal range between 40 and 170 to minimize the uncertainty. For samples within this range, the average percentage difference between the results of this method and the results from conventional thermal-optical method was found to be only 10.3%, which is close to common research grade instruments. [ABSTRACT FROM AUTHOR]

Grown nucleated particles > 50 nm in diameter are an important source of cloud condensation nuclei (CCN) and when the size is > 100 nm, they can also have direct influence on the climate. In this study, the nucleation and growth of new particles in the atmosphere in Hong Kong were investigated during dry season (monthly averaged RH < 75%). The maximum size of grown nucleated particles was generally less than 40 nm during new particle burst and growth events. The exception, accounting for ∼ 20% of all burst and growth events, was those induced by strong photochemical reactions, in which subsequent particle shrinkage occurred. Temporal particle and gas concentration variability and meteorological conditions support the occurrence of particle shrinkage. The shrinkage rate calculated (∼ 8 nm h-1) was close to the growth rate. The observation of particle shrinkage sheds new light on particle transformation dynamics and it would add to the understanding of particle behavior in the atmosphere. [ABSTRACT FROM AUTHOR]

Hygroscopicity and phase transitions were measured with deposited particles of ammonium sulfate (AS), ammonium nitrate (AN), malonic acid (MA), glutaric acid (GA), glyoxylic acid (GlyA), as well as two mixed particle systems AS-MA and AS-GA using micro-Raman spectroscopy. Hygroscopicity was presented in terms of water-to-solute mass ratios, which were obtained from the integrated area ratios of the Raman water band to a distinct solute peak. Deliquescence and crystallization were confirmed by abrupt changes in the Raman peak positions and the full-width-half-heights of distinct solute peaks. The results for AS, AN, MA, and GA agreed well with literature reports and model predictions. For GlyA, we detected the Raman water band at near 0% RH, indicating that the spectral technique is sensitive for hygroscopic measurements at very low RH. Additional spectral feature at ∼ 0% RH was also observed. In the case of more complicated AS-dicarboxylic acid mixed systems, the partial phase transitions of the organic components were identified using the intensity ratios of aqueous to solid C = O peaks. AS-MA particles did not completely crystallize and gradual water uptake with increasing RH from 3% was observed. Moreover, it was found that AS-GA particles showed step-wise crystallization in which the AS fraction crystallized prior to the GA fraction. The measured water content and complete DRH of both mixed systems were consistent with the published values. The results show the utility of micro-Raman spectroscopic analysis in studying hygroscopicity and phase characterizations of the chemical species in mixed particles. [ABSTRACT FROM AUTHOR]

We studied the phase transition and hygroscopicity of particles of a mixture of two atmospherically relevant species, ammonium sulfate and adipic acid. We conducted extensive investigations on ammonium sulfate-adipic acid (AS-AA) particles deposited on a hydrophobic substrate with the weight percentage (wt%) of AA ranging from 3 to 90%. Crystallization and deliquescence were observed through an optical microscope, which enabled multiple particles to be examined simultaneously. At an initial relative humidity (RH) of 94%, which was the highest RH setting, AA solids were formed from the deposited solution droplets, leading to the presence of mixed-phase particles prior to the complete crystallization recorded between 31% and 42% RH of all mixtures. The complete crystallization RH values were close to that of pure AS, indicating that the AA solids did not promote effective heterogeneous nucleation of AS. When the RH was increased, partial deliquescence in all mixtures was observed at 80% RH, which was attributed to the dissolution of the AS fractions. Full deliquescence was only observed in 3 wt% AA particles at 91% RH. We also used Micro-Raman Spectroscopy to determine the hygroscopicity of mixtures with up to 50 wt% AA. The hygroscopic behavior of the AS fraction in the mixed particles was found to resemble that of pure AS particles. However, there was observable water uptake in the solid particles at about 70% RH in mixtures with up to 70 wt% AA. This early water uptake was more pronounced in mixtures with lower weight percentages of AA (AA less than 30 wt%). [ABSTRACT FROM AUTHOR]

An electrodynamic balance was used to investigate the uptake of octanal vapor by single sulfuric acid droplets levitated under various relative humidity (RH) conditions and gas-phase octanal concentrations. In the high octanal concentration experiments (200-300 ppm), we observed that the organic mass yield depended on the acidity of the sulfuric acid droplets and significant uptake of octanal only occurred when the RH was about 10% (H2SO4 wt% ∼ 64%). Furthermore, repartitioning of only a portion of condensed organic compounds was observed after active dilution with octanal-free compressed air, indicating that low-volatility products had formed and that repartitioning is important in affecting the organic mass fraction and chemical composition of atmospheric organic aerosols. When a relatively lower octanal concentration (700-900 ppb) was used, no significant uptake of octanal vapor by the sulfuric acid droplets was observed even at 10% RH with long exposure time (>25 h). Our findings suggest that both particle acidity and gas-phase octanal concentration are the critical factors that influence the organic mass yield of levitated acidic droplets. Because of the severe conditions of low RH and high octanal conditions required to effect the reactions, the reactive uptake of aldehydes, especially those that have chemical structures and properties very similar to octanal, into acidic particles may not be an important pathway in secondary organic aerosol formation under actual atmospheric conditions. [ABSTRACT FROM AUTHOR]

Particle size (30 to 10,000 nm) distributions in number concentration were measured using an Electrical Low Pressure Impactor onboard a Mobile Real-time Air Monitoring Platform from June 2002 to August 2003 in Hong Kong to investigate the size distributions of on-road vehicular particles and their relationship with ambient temperature (T) and relative humidity (RH). In this article, we focus on the size distributions of particles >30 nm in vehicle plumes where fresh vehicular particles dominated and the NOx concentration was >400 ppb. A uni-modal size distribution with mode at either 60-108 nm or 108-170 nm was generally observed from April to October. This mode is conventionally believed to be soot particles in the literature and the different mode sizes probably depend on engine and vehicle operating conditions. From November to March, a bi-modal size distribution with a dominant mode at 30-60 nm and a minor mode at either 108-170 nm or 60-108 nm was generally observed. The existence of the 30-60 nm mode is explained in the literature by the growth of nucleated particles when vehicular exhaust cools in ambient air. Ratios of the number concentration of 30-60 nm particles to BC mass concentration correlated negatively with T, as well as with RH when RH was <60%. The ratios appeared not to be sensitive to RH when RH was >60%. The correlations suggested that T and RH exert significant influences on the formation of vehicular submicron particles, leading to varying size distributions of vehicular particles in Hong Kong. [ABSTRACT FROM AUTHOR]

Solid ammonium nitrate (NH4NO3) exists in five stable polymorphic forms (designated as phases I, II, III, IV, and V) below its melting point at around 170°C. Phase IV is stable in a temperature range of -17°C ∼ 32°C and is the only phase that has been considered by the atmospheric research community until recently. In this study, we examine the IV ↔ III phase transition of NH4NO3 and how relative humidity (RH) affects the transition path and the transition temperatures using in-situ microscopic Fourier Transform InfraRed spectroscopy. Two kinds of NH4NO3 samples, powder produced from grinding commercially produced chemicals and single particles obtained by efflorescence of droplets on PTFE filters, were studied. The powder samples exhibit the IV↔III phase transition and the transition temperature depends on the RH while the single particle samples exhibit only the IV↔II transition at about 52°C (forward) and 48°C (reverse), bypassing phase III, with transition temperatures independent of the RH. However, grinding of the particles produced through efflorescence results in the IV↔III transitions. Differences in crystal structure and moisture content may explain the distinct phase transition behaviors of the two types of samples. These results suggest that solid and pure NH4NO3 aerosol particles are stable in phase IV under ambient conditions. [ABSTRACT FROM AUTHOR]

The availability of high time-resolution (1 s full spectrum scan) particle sizers permitted atmospheric researchers to scrutinize the characteristics of ultrafine particles in rapidly varying (perhaps, random) high concentration environments such as roadside, on-road, and tunnels. These data also revealed possible artifacts associated with the slower (~30 s or more) scanning sizers when used in this kind of environment. This study discusses sampling rate artifact problems based on simulations using 1 s Engine Exhaust Particle Sizer data taken on a mobile platform. [ABSTRACT FROM AUTHOR]

An Engine Exhaust Particle Sizer was used on a mobile platform to measure ultrafine particle (5.6-560 nm) concentration profiles in streets and tunnels. Particle size distribution data along the lengths of the tunnels showed that number concentration profiles of <30 nm particles were highly nonlinear. Conventional point measurement studies inside tunnels to determine, for instance, emission factors of ultrafine particles in this range are therefore location dependent and may not be representative. The particle concentration profiles obtained in this experiment allowed the study of the transformation of 10 nm particles to 50 nm particles. Outside of the tunnels and on heavily traveled highways, particles exhibited a dominant mode at 10 nm and a minor mode at 50 nm. Inside the tunnels, 10 nm particles gradually decreased after an abrupt increase, while 50 nm particles increased from entrance to exit. The 10 nm particles decreased at 2-4 times the rate of increase of the 50 nm particles, therefore, condensational growth of the 10 nm particles alone cannot explain the decrease. The estimated hetero-coagulation rate between 10 nm particles and particles >30 nm inside the tunnel was substantially higher than that outside due to the increase in the concentration of particles >30 nm. The hetero-coagulation process is proposed to play a key role in the transformation of particle modes in the tunnel. This study provides new insight on the transformation of ultrafine particle modes in vehicular plumes. [ABSTRACT FROM AUTHOR]

The equilibrium water sorption and desorption characteristics of three pharmaceutical excipients, namely, glucose, citric acid, and sorbitol, were investigated at 25° C using an electrodynamic balance (EDB). The mass fraction of solute (mfs), defined as the ratio of the dry solute mass to the solution mass, of the additives was measured as a function of relative humidity (RH) at equilibrium. Bulk measurements and literature data suggest that these three additives all crystallize at their respective saturation condi- tions. However, crystallization or deliquescence was not observed for the suspended particles of the three additives. They behave as nondeliquescent materials and sorb and desorb water reversibly.At RH = 5%, citric acid particles still contain ∼8 wt% residual water, which corresponds to a stoichiometry of citric acid monohydrate. Glucose and sorbitol particles are essentially water-free. Universal Functional Activity Coefficient (UNIFAC) model predicts the water activities of glucose, citric acid, and sorbitol (expressed as mfs of the particles) to within ±16%, 24%, and 6% of those determined by the EDB, respectively. [ABSTRACT FROM AUTHOR]