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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 pre-treatment. 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.

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 ...

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 pa...

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...

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 fo...

Particle size distributions are of profound interest in the study of ambient aerosols. Electrostatic classification using the Scanning 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 particles 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 spectrometry 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 particle sizing performance of an FMPS against simultaneous measurements 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 measurements 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 evaluate 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 measurements was applied, which was able to successfully reduce the initial underestimation.

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

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 tha...

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 re...

The displacement of ammonium by triethylammonium (TEAH) in aerosol particles of about 15–35 μm in diameter was investigated using an electrodynamic balance (EDB) coupled with in situ Raman spectroscopy. The phase state of particles played a crucial role in the extent of triethylamine (TEA) uptake. At 50 or 75% relative humidity (RH), the heterogenous uptake of about 40-ppm TEA by aqueous ammonium salts of sulfate [(NH4)2SO4], bisulfate (NH4HSO4), nitrate (NH4NO3), chloride (NH4Cl), and oxalate [(NH4)2C2O4] led to increases in particle mass of over 90%. A complete displacement of ammonium by TEAH was confirmed by direct EDB mass measurements and the Raman spectra obtained. TEAH sulfate was formed during the exposure of aqueous droplets of (NH4)2SO4 and NH4HSO4 to TEA vapor at 50% RH; but a fraction of it decomposed to TEAH bisulfate when the TEA supply was removed. Crystalline solid particles of (NH4)2SO4 and (NH4)2C2O4 experienced small mass increases of

An electrodynamic balance was used to examine the effect of the presence of particle-phase organics on the acid-catalyzed reactive uptake of nonanal (NL) vapor. Uptake experiments were conducted by using sulfuric acid (SA) particles, oleic acid/SA (hydrophobic), and levoglucosan/SA (hydrophilic) mixed particles with 6 ppm (approximately) gas-phase NL at about 3% relative humidity. SA reacted with the mixed organics prior to NL uptake to form organic products, denoted as OleA* and Levo*, and with NL to form hydrophobic NL* (particle-phase organics). Fresh SA particles had small mass increases (5%–13%) at the start of NL exposure (0–40 min) even though they are highly acidic. However, OleA*/SA mixed particles of about 30–70 wt% of OleA* took up NL swiftly during the first 40 min. For example, the mass increase of a 33 wt% OleA* particle jumped to 120%. As the organic product, NL*, accumulated, the uptake rate of SA particle increased and the mass increase surged to 150% at 100 min. Afterwards, the mass incr...

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 an...

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.

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 c...

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 attrib...

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 m...

Solid ammonium nitrate (NH 4 NO 3 ) 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 NH 4 NO 3 and how relative humidity (RH) affects the transition path and the transition temperatures using in-situ microscopic Fourier Transform InfraRed spectroscopy. Two kinds of NH 4 NO 3 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....

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.

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, condensa...

balance (EDB). The mass fraction of solute (mfs), dee ned 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 conditions. However, crystallization or deliquescence was not observed for the suspended particles of the three additives. They behave as nondeliquescentmaterialsandsorbanddesorbwaterreversibly.At RH = 5%,citricacidparticlesstillcontain » 8wt%residualwater, which corresponds to a stoichiometry of citric acid monohydrate. Glucose and sorbitol particles are essentially water-free. Universal FunctionalActivityCoefe cient (UNIFAC)modelpredictsthewater 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.

The water activities of aqueous MgCl 2, Mg(NO3)2, and MgSO solutions and their mixtures - MgCl 2-Mg(NO3)2 (mole ratio 3:1, 1:1, 1:3), MgCl 2-MgSO4 (1:1), Mg(NO3)2-MgSO4 (1:1), and MgCl 2-Mg(NO3)2-MgSO4 (1:1:1 and 1:1:5) - were measured at room temperature from dilute solution to high supersaturation using an electrodynamic balance. The experimental results are compared with the predictions of the Zdanovskii-Stokes-Robinson (ZSR) and the Kusik and Meissner (KM) equations. Both the ZSR and the KM equations can predict the water activities of these mixtures well with an acceptable error of less than +/- 0.02. The ZSR equation is superior to the KM equation as it can give predictions for a larger range of aw where single component bulk data are available. It is also slightly more accurate than the KM equation for this system. The average standard deviations of the difference between the data measured and the predictions of the ZSR and KM equations are 0.004 and 0.01, respectively. The ZSR equation is recommen...

A mole-fraction-based thermodynamic model is used to represent available activity data, salt solubilities, and electrodynamic balance measurements for aqueous NaCl, Na2SO4, NaNO3 and their mixtures at 298.15 K. The results are presented as fitted ion-interaction parameters and equilibrium constants. These enable ion and solvent activities, and saturation with respect to five possible solid phases (NaCl, NaNO3, Na2SO4·10H2O, Na2SO4, and NaNO3·Na2SO4·H2O) to be calculated for all mixture compositions to relative humidities of ≤60%, and to high degrees of supersaturation with respect to the dissolved salts. Extension of the model to other systems is discussed.

Water activity/concentration relationships of aqueous NaCl, NaNO3, Na2SO4 and their mixtures from dilute concentration to high supersaturation have been determined at room temperature in an electrodynamic balance. Using a dynamic measurement technique based on the evaporation of the droplets, a set of water activity measurements of a solution droplet can be obtained in less than an hour. The water activities of the mixtures are compared with theoretical predictions of the Pitzer-Simonson-Clegg (PSC) model, the Zdanovskii-Stokes-Robinson (ZSR) equation, and the Kusik and Meissner (KM) model. Comparisons of the data with model predictions indicate that the PSC, ZSR, and KM approaches agree well among themselves and with the experimental data. All three models give similar maximum (±0.02) and standard (0.01) deviations in mass fraction of the solute (mfs). These deviations are close to the experimental error of ±(0.01–0.02). The agreement of the models and the need for single-component water activit...

Water activities of ammonium sulfate-sulfuric acid mixtures with ammonium to sulfate molar ratios between 0 and 2 were measured by a spherical void electrodynamic levitator at relative humidities (RH) of 0.18–0.90. Since the composition of the solid particles is subject to uncertainty, solution properties were determined relative to the known properties at RH about 0.75–0.90. The data were compared with other measurements and the estimates from the Zdanovskii-Stokes-Robinson (ZSR) method and SCAPE, a newly developed gas-particle equilibrium model, and generally were found to be in good agreement.