Aerodynamic Size Distribution Research Articles

SEASONAL VARIATION OF THE AEROSOL CHEMICAL COMPOSITION IN A ROMANIAN PERI-URBAN AREA

Chemical composition variations of aerosols during summer and winter seasons have been investigated using an Aerosol Mass Spectrometer (AMS) at Magurele, a city near Bucharest, Romania. Proportions of chemical species, concentration time series, mass range distribution and aerodynamic size distribution of aerosols have been analyzed. AMS measurements proved that the organics, nitrate, sulphate and ammonium, with small amounts of chloride were the main species at ground level during the investigated period. Important differences have been noticed between hot and cold seasons related to the total submicronic aerosol concentration and composition. The submicronic aerosols loadings are almost two times higher in summer in comparison with wintertime. Proportions of the species are evenly distributed during wintertime. One accumulation peak can be depicted with diameter between 450 to 600 nm. The fine particle mode consisted typically of organics, sulphate and ammonium during summertime, with size distribution centered at about 400-500 nm.

Evaluating the influence of laser wavelength and detection stage geometry on optical detection efficiency in a single-particle mass spectrometer

Abstract. Single-particle mass spectrometry (SPMS) is a useful tool for the online study of aerosols with the ability to measure size-resolved chemical composition with a temporal resolution relevant to atmospheric processes. In SPMS, optical particle detection is used for the effective temporal alignment of an ablation laser pulse with the presence of a particle in the ion source, and it gives the option of aerodynamic sizing by measuring the offset of particle arrival times between two detection stages. The efficiency of the optical detection stage has a strong influence on the overall instrument performance. A custom detection laser system consisting of a high-powered fibre-coupled Nd:YAG solid-state laser with a collimated beam was implemented in the detection stage of a laser ablation aerosol particle time-of-flight (LAAP-TOF) single-particle mass spectrometer without major modifications to instrument geometry. The use of a collimated laser beam permitted the construction of a numerical model that predicts the effects of detection laser wavelength, output power, beam focussing characteristics, light collection angle, particle size, and refractive index on the effective detection radius (R) of the detection laser beam. We compare the model predictions with an ambient data set acquired during the Ice in Clouds Experiment – Dust (ICE-D) project. The new laser system resulted in an order-of-magnitude improvement in instrument sensitivity to spherical particles in the size range 500–800 nm compared to a focussed 405 nm laser diode system. The model demonstrates that the limit of detection in terms of particle size is determined by the scattering cross section (Csca) as predicted by Mie theory. In addition, if light is collected over a narrow collection angle, oscillations in the magnitude of Csca with respect to particle diameter result in a variation in R, resulting in large particle-size-dependent variation in detection efficiency across the particle transmission range. This detection bias is imposed on the aerodynamic size distributions measured by the instrument and accounts for some of the detection bias towards sea salt particles in the ambient data set.

In Vitro Pulmonary Cell Culture in Pharmaceutical Inhalation Aerosol Delivery: 2-D, 3-D, and In Situ Bioimpactor Models

The use of non-invasive inhaled aerosols for pulmonary drug delivery continues to grow. This is due to the many unique advantages this delivery route offers for the treatment of both local and systemic diseases. The physicochemical properties of the formulated drugs as well as the physiology of the lungs play a key role in both the deposition and absorption of the particles. The airway and the alveolar epithelium are targets for the treatment of respiratory diseases. However, particles have to overcome biological barriers before they reach their target and produce an effect. In vitro aerosol dispersion performance (i.e. aerodynamic size and aerodynamic size distribution) of inhalable particles is quantified by inertial impaction, as required by regulatory agencies for an investigational pharmaceutical inhalation aerosol formulation to be approved for use in patients as a marketed pharmaceutical product. Using inertial impaction in conjunction with cell cultures of various pulmonary cells in situ as bioimpactors has unique aspects in correlating aerodynamic properties with pulmonary cellular behavior including viability and uptake. These can be as co-culture or in single culture, as 3-D multicellular spheroids or 2-D cellular monolayer using different conditions to grow them, such as air-liquid interface culture (ALI) or in liquid covered culture (LCC). evaluation of the currently available in vitro models and the challenges in developing reliable cellular tools to predict the deposition of inhalable particles in the lungs as a function of aerodynamic particle properties is presented in the manuscript. The mechanistic aerodynamic and biophysical properties of inhaled aerosol particles on the entire respiratory tract at the cellular level based on aerodynamic size and aerodynamic size distribution will be better understood with the development of in vitro methods which are described in this work.

Effect of Flow Rate on In Vitro Aerodynamic Performance of NEXThaler® in Comparison with Diskus® and Turbohaler® Dry Powder Inhalers

Background: European and United States Pharmacopoeia compendial procedures for assessing the in vitro emitted dose and aerodynamic size distribution of a dry powder inhaler require that 4.0 L of air at a pressure drop of 4 kPa be drawn through the inhaler. However, the product performance should be investigated using conditions more representative of what is achievable by the patient population. This work compares the delivered dose and the drug deposition profile at different flow rates (30, 40, 60, and 90 L/min) of Foster NEXThaler® (beclomethasone dipropionate/formoterol fumarate), Seretide® Diskus® (fluticasone propionate/salmeterol xinafoate), and Symbicort® Turbohaler® (budesonide/formoterol fumarate).Methods: The delivered dose uniformity was tested using a dose unit sampling apparatus (DUSA) at inhalation volumes either 2.0 or 4.0 L and flow rates 30, 40, 60, or 90 L/min. The aerodynamic assessment was carried out using a Next Generation Impactor by discharging each inhaler at 30, 40, 60, or 90 L/min for a time sufficient to obtain an air volume of 4 L.Results: Foster® NEXThaler® and Seretide® Diskus® showed a consistent dose delivery for both the drugs included in the formulation, independently of the applied flow rate. Contrary, Symbicort® Turbohaler® showed a high decrease of the emitted dose for both budesonide and formoterol fumarate when the device was operated at airflow rate lower that 60 L/min. The aerosolizing performance of NEXThaler® and Diskus® was unaffected by the flow rate applied. Turbohaler® proved to be the inhaler most sensitive to changes in flow rate in terms of fine particle fraction (FPF) for both components. Among the combinations tested, Foster NEXThaler® was the only one capable to deliver around 50% of extra-fine particles relative to delivered dose.Conclusions: NEXThaler® and Diskus® were substantially unaffected by flow rate through the inhaler in terms of both delivered dose and fine particle mass.

A New Method to Measure Aerosol Particle Bounce Using a Cascade Electrical Low Pressure Impactor

The phase state of secondary organic aerosol (SOA) in the atmosphere is of scientific interest as it can impact SOA growth and reactivity. For this purpose, a simplified method is described herein to estimate SOA bounce factor to gain an improved understanding of the phase state of atmospheric aerosols. This new method involves the use of a multistage electrical low pressure cascade impactor operating with either smooth or sintered impaction plates. Measurement of the raw current on smooth and sintered plates allows one to calculate the bounce factor, eliminating the need for a scanning mobility particle sizer to independently measure the SOA aerodynamic size distribution. The proposed method provides the temporal resolution necessary to measure phase changes in a continuously evolving SOA parcel.We validate our method by measuring the bounce factor of solid and liquid aerosols, namely ammonium sulfate (AS), dioctyl sebacate (DOS), oleic acid (OA) and ozonized OA, and also present bounce factor evolution ...

Novel Inhaled Combination Powder Containing Amorphous Colistin and Crystalline Rifapentine with Enhanced Antimicrobial Activities against Planktonic Cells and Biofilm of Pseudomonas aeruginosa for Respiratory Infections.

Colistin has been increasingly used for the treatment of respiratory infections caused by Gram-negative bacteria. Unfortunately parenteral administration of colistin can cause severe adverse effects. This study aimed to develop an inhaled combination dry powder formulation of colistin and rifapentine for the treatment of respiratory infections. The combination formulation was produced by spray-drying rifapentine particles suspended in an aqueous colistin solution. The combination dry powder had enhanced antimicrobial activities against planktonic cells and biofilm cultures of Pseudomonas aeruginosa, with both minimum inhibitory concentration (MIC) and minimum biofilm inhibitory concentration (MBIC) values (2 and 4 mg/L, respectively) being half that of pure colistin (MIC 4 mg/L and MBIC 8 mg/L) and 1/16th that of pure rifapentine (MIC 32 mg/L and MBIC 64 mg/L). High aerosol performance, as measured via an Aerolizer device, was observed with emitted doses>89% and fine particle fraction (FPF) total>76%. The proportion of submicron particles of rifapentine particles was minimized by the attachment of colistin, which increased the overall particle mass and aerodynamic size distribution. Using the spray-drying method described here, stable particles of amorphous colistin and crystalline rifapentine were distributed homogeneously in each stage of the impinger. Unlike the colistin alone formulation, no deterioration in aerosol performance was found for the combination powder when exposed to a high relative humidity of 75%. In our previous study, surface coating by rifampicin contributed to the moisture protection of colistin. Here, a novel approach with a new mechanism was proposed whereby moisture protection was attributed to the carrier effect of elongated crystalline rifapentine particles, which minimized contact between hygroscopic colistin particles. This inhaled combination antibiotic formulation with enhanced aerosol dispersion efficiency and in vitro efficacy could become a superior treatment for respiratory infections.

A spectrometer for measuring particle size distributions in the range of 3 nm to 10 μm

A spectrometer combining electrical mobility sizing and aerodynamic sizing was developed to measure aerosol size distributions in the range of 3 nm to 10 μm. It includes three instruments which cover different size ranges (a nano scanning mobility particle sizer (NSMPS, 3–60 nm), a regular scanning mobility particle sizer (RSMPS, 40–700 nm), and an aerodynamic particle sizer (APS, 550 nm–10 μm)). High voltage and sheath flow of the NSMPS and RSMPS were supplied using two home-built control boxes. A LabVIEW program was developed for spectrometer automatic operation. A linear inversion method was applied to correct particle multiple charging effects and to integrate data from the three instruments into a wide-range size distribution. Experiments were conducted to compare distributions in the overlap size ranges measured by three instruments. Good agreement between the NSMPS and RSMPS was achieved after correcting for the difference in counting efficiencies of the two particle counters. Aerodynamic size distributions reported by the APS were converted to mobility size distributions by applying an effective density method. Distributions measured by the RSMPS and APS were consistent in the overlap size range of 550–700 nm. A full spectrum in the size range of 3 nm to 10 μm was demonstrated by measuring aerosol generated using a mixture of different sized polystyrene latex spheres.

Comparison of models and measurements of angle-resolved scatter from irregular aerosols

We have developed and validated a method for modeling the elastic scattering properties of biological and inert aerosols of irregular shape at near- and mid-wave infrared wavelengths. The method, based on Gaussian random particles, calculates the ensemble-average optical cross section and Mueller scattering matrix, using the measured aerodynamic size distribution and previously-reported refractive index as inputs. The utility of the Gaussian particle model is that it is controlled by only two parameters (σ and Γ) which we have optimized such that the model best reproduces the full angle-resolved Mueller scattering matrices measured at λ=1.55µm in the Standoff Aerosol Active Signature Testbed (SAAST). The method has been applied to wet-generated singlet biological spore samples, dry-generated biological spore clusters, and kaolin. The scattering computation is performed using the Discrete Dipole Approximation (DDA), which requires significant computational resources, and is thus implemented on LLGrid, a large parallel grid computer. For the cases presented, the best fit Gaussian particle model is in good qualitative correspondence with microscopy images of the corresponding class of particles. The measured and computed cross sections agree well within a factor of two overall, with certain cases bearing closer correspondence. In particular, the DDA reproduces the shape of the measured scatter function more accurately than Mie predictions. The DDA-computed depolarization factors are also in good agreement with measurement.

Particulate PAH emissions from residential biomass combustion: time-resolved analysis with aerosol mass spectrometry.

Time-resolved emissions of particulate polycyclic aromatic hydrocarbons (PAHs) and total organic particulate matter (OA) from a wood log stove and an adjusted pellet stove were investigated with high-resolution time-of-flight aerosol mass spectrometry (AMS). The highest OA emissions were found during the addition of log wood on glowing embers, that is, slow burning pyrolysis conditions. These emissions contained about 1% PAHs (of OA). The highest PAH emissions were found during fast burning under hot air starved combustion conditions, in both stoves. In the latter case, PAHs contributed up to 40% of OA, likely due to thermal degradation of other condensable species. The distribution of PAHs was also shifted toward larger molecules in these emissions. AMS signals attributed to PAHs were found at molecular weights up to 600 Da. The vacuum aerodynamic size distribution was found to be bimodal with a smaller mode (Dva ∼ 200 nm) dominating under hot air starved combustion and a larger sized mode dominating under slow burning pyrolysis (Dva ∼ 600 nm). Simultaneous reduction of PAHs, OA and total particulate matter from residential biomass combustion may prove to be a challenge for environmental legislation efforts as these classes of emissions are elevated at different combustion conditions.

Characterization of a Vortex Shaking Method for Aerosolizing Fibers.

Generation of well-dispersed, well-characterized fibers is important in toxicology studies. A vortex-tube shaking method is investigated using glass fibers to characterize the generated aerosol. Controlling parameters that were studied included initial batch amounts of glass fibers, preparation of the powder (e.g., preshaking), humidity, and airflow rate. Total fiber number concentrations and aerodynamic size distributions were typically measured. The aerosol concentration is only stable for short times (t < 10 min) and then falls precipitously, with concomitant changes in the aerosol aerodynamic size distribution; the plateau concentration and its duration both increase with batch size. Preshaking enhances the initial aerosol concentration and enables the aerosolization of longer fibers. Higher humidity strongly affects the particle size distribution and the number concentration, resulting in a smaller modal diameter and a higher number concentration. Running the vortex shaker at higher flow rates (Q > 0.3 lpm), yields an aerosol with a particle size distribution representative of the batch powder; running the vortex shaker at a lower aerosol flow rate (Q ~ 0.1 lpm) only aerosolizes the shorter fibers. These results have implications for the use of the vortex shaker as a standard aerosol generator.

Use of beryllium-7 as a surrogate to determine the deposition of metal and polycyclic aromatic hydrocarbon through urban aerosols in Nantes, France

The aerodynamic size distribution of the natural radionuclide beryllium-7 (7Be) and associated trace metals, pesticides and polycyclic aromatic hydrocarbons (PAHs) in Nantes (France) was determined during 6 campaigns (conducted from June 2010 to March 2012) using a low-pressure impactor. The activity distribution of 7Be, as measured by a gamma ray spectrometer, was associated with the accumulation mode (with around 85% of the mass < 1 μm) and explained by post condensation processes on the airborne particle. The mass size distribution of heavy metals and PAHs was mainly focused on the accumulation mode as well. Trace metals generally show a unimodal distribution, although bimodal distributions for Zn, Cu, As and Cr could also be observed; these were attributed to re-suspension processes. For PAHs, the size distribution remained unimodal with a mass median diameter (MMD) ranging from 0.31 to 0.68 μm, thus confirming the urban origin. Pesticide concentrations in the atmosphere were low and did not allow determining the size distribution. Given their identical aerodynamic distribution as 7Be, trace metals and PAHs are expected to display a similar dry deposition velocity, which will then allow us to calculate the dry deposition fluxes over this urban area.

Influence of Batch Cooling Crystallization on Mannitol Physical Properties and Drug Dispersion from Dry Powder Inhalers

This study provides, for the first time, an evaluation of the physicochemical properties of batch cooling crystallized mannitol particles combined with how these properties correlated with the inhalation performance from a dry powder inhaler (Aerolizer). The results showed that the type of polymorph changed from β-form (commercial mannitol) to mixtures of β- + δ-mannitol (cooling crystallized mannitol crystals). In comparison to mannitol particles, crystallized at a higher supersaturation degree, a lower degree of supersaturation favored the formation of mannitol crystals with a more regular and elongated habit, smoother surface, higher specific surface area, higher fine particle content, higher bulk density, and higher tap density. Cooling crystallized mannitol particles demonstrated considerably lower salbutamol sulfate–mannitol adhesion in comparison to commercial mannitol, with a linear reduction as surface roughness decreased and fines content increased. Also, mannitol crystals with smoother surfaces demonstrated a reduction in salbutamol sulfate content uniformity (expressed as %CV) within salbutamol sulfate–mannitol formulations. Despite the different physical properties, all mannitol products showed similar flow properties and similar emission of salbutamol sulfate upon inhalation. However, mannitol crystals grown from lower supersaturation (reduced roughness and increased fines) generated a finer aerodynamic size distribution and consequently deposited higher amounts of salbutamol sulfate on lower stages of the impactor. Regression analysis indicated linear relationships showing higher fine particle fraction of salbutamol sulfate in the case of mannitol particles having a more elongated shape, higher fines content, higher specific surface area, higher bulk density, and higher tap density. In conclusion, a cooling crystallization technique could be controlled to produce mannitol particles with controlled physical properties that could be used to influence aerosolization performance of a dry powder inhaler product.

Emissions from soy biodiesel blends: A single particle perspective

Biodiesel has recently reemerged as a common fuel. However, emissions from biodiesel combustion have been studied in much less detail than those from traditional petroleum-based diesel. In this experiment, emissions from the combustion of different fuel blends (B00, B02, B20, B99, where the number after B indicates the percentage, by volume, of biodiesel in the fuel) in a VW TDi engine were analyzed by aerosol time-of-flight mass spectrometers (ATOFMS) for single-particle composition and vacuum aerodynamic size. The ATOFMS results show that the PAH molecular weight distribution is not significantly affected by the fuel composition, and that sulfates are reduced by increased biodiesel content. Octanedioic acid (a carbonyl species) is increased with increased biodiesel concentration. Clustering results from the single-particle spectra show that the particles fall in five main types by chemical composition. The aerodynamic size distribution of these individual clusters was also determined. These results also show that methods used to identify diesel particle emissions for source apportionment are not applicable when significant concentrations of biodiesel are used in fuels.

Aerodynamic and Mobility Size Distribution Measurements to Reveal Biodiesel Effects on Diesel Exhaust Aerosol

This article examines the effect of biodiesel blends on the exhaust aerosol from a Euro 3 passenger car. Five different feedstock oils (soybean, palm, sunflower, rapeseed, and used frying oil) were used to produce fuels with 10% vol. content in biodiesel (B10). Use of the B10 blends led to a systematic reduction of PM mass emissions in the range of ∼9% (rapeseed) to 23% (used frying oil) on average. The combination of particle size distributions based on the aerodynamic and the mobility diameters led to the estimation of the fractal dimension (DF) for non-volatile particles. This was found to range from 2.52 for the baseline (fossil) fuel to 2.62 for the palm oil blend, suggesting that biodiesel can affect the particle morphology, even at this low blending ratio. The differences were statistically significant. The increase of the DF is translated to more compact particle structure, which in turn denotes lower specific surface area. The volatile fraction of PM lies within a range of 1–9% when fossil diesel...

Extending the Capabilities of Single Particle Mass Spectrometry: II. Measurements of Aerosol Particle Density without DMA

Particle density is an important and useful property that is difficult to measure because it usually requires two separate instruments to measure two particle attributes. As density measurements are often performed on size-classified particles, they are hampered by low particle numbers, and hence poor temporal resolution. We present here a new method for measuring particle densities using our single particle mass spectrometer, SPLAT. This method takes advantage of the fact that the detection efficiency in our single particle mass spectrometer drops off very rapidly as the particle size decreases below 100 nm creating a distinct sharp feature on the small particle side of the vacuum aerodynamic size distribution. Thus, the two quantities needed to determine particle density, the particle diameter and vacuum aerodynamic diameter, are known. We first test this method on particles of known compositions and densities to find that the densities it yields are accurate. We then apply the method to obtain the dens...

The production of ‘aerodynamically equivalent’ drug and excipient inhalable powders using a novel fractionation technique

Inhalation particles can be produced by various techniques such as milling, controlled crystallisation and spray-drying, but current methods cannot, to-date, precisely control the aerodynamic size distribution of produced powders. The aim of this study was to develop and validate a novel preparative technique whereby the efficient and reproducible aerodynamic fractionation of drug and excipient powders could be achieved. Salmeterol xinafoate (SX), fluticasone propionate (FP) and fine α-lactose monohydrate (FL) were chosen as model compounds. Powders were aerosolised using a dry powder feeder into a Next Generation Impactor operated at 60Lmin−1. Powders deposited on NGI stages were then collected and analysed. The fractionation process was successful for all powders producing significant linear correlations between the pre-set aerodynamic cut-off limits and geometric size measurements. For each of SX, FP and FL, sufficient powder quantities were recovered from NGI stages 1–6 producing six fractions with sequential aerodynamic and geometric particle size distributions. The fractionation technique was efficient and reproducible for all powders studied. The method can be equally applied to various drugs and excipients regardless of their previous production/processing history. Therefore, the aerodynamic fractionation technique may be used to compare and contrast samples produced by different processes.

The use of modeling in spray drying of emulsions and suspensions accelerates formulation and process development

In respiratory drug delivery, porous, lipid-based microparticles have shown performance advantages over microcrystalline particles ( Hirst et al., 2002). These engineered particles are manufactured by spray drying of emulsions or suspensions. The purpose of this work is to combine an understanding of underlying mechanisms with particle characterization data to develop models in support of a quality-by-design (QbD) approach to drug development. Powders were manufactured using a laboratory scale spray dryer. A thermodynamic model was developed for the spray drying process, allowing prediction of outlet temperature and powder residual water content. A stochastic model based on a Monte-Carlo simulator was developed to analyze the effects of relevant parameters on the aerodynamic size of the particles; modeled results were verified by comparison to measured aerodynamic size distributions of spray-dried powders. These modeling techniques allowed rank-ordering of the relative importance of formulation and process variables, and aided in developing an understanding of the drying unit operation.

Effect of Surface Treatment of Carrier on Toner Charging Characteristics

Nowadays the two-component developers are widely used in electrophotography. In two-component developing system toner is tribo-electrically charged by contacting with carrier. In this study, two types of crushed toners categorized according to the constitution of external additives and four types of carriers which are also categorized according to the surface treatment were used for the experiment. The toner charge distribution is changed by the type of carrier coating and concentration of external additives as well. The Aerodynamic size and electrostatic charge distribution of the toner particles are measured individually by E-SPART (Electrical Single Particle Aerodynamic Relaxation Time) analyzer.

Beryllium-7 Aerosols in Ambient Air

The aerodynamic size distribution of 7Be aerosol particles in ambient air was determined by using 1-ACFM and high-volume (HVI) cascade impactors, 20 m above the ground on the roof of the Faculty of Science building, Aristotle University of Thessaloniki at Thessaloniki Greece (40o38’N, 22o58’E) from November 2006 to June 2008. The activity concentration of 7Be aerosols was determined by gamma spectrometry (Eγ = 0.477 MeV). The activity size distribution of 7Be-aerosols was largely associated with submicron aerosol particles in the accumulation mode (0.4-2.0 μm). The activity median aerodynamic diameter, AMAD varied from 0.76-1.18 μm (average 0.90 μm), indicating post-condensation growth either in the upper atmosphere or after mixing into the boundary layer. The geometric standard deviation, σg varied from 1.86-2.77 (average 2.24). The activity size distribution of 7Be-aerosols peaked in the 0.7-1.1 μm size range in the 65% of the measurements carried out by the 1-ACFM cascade impactor. In estimating lifetimes of 7Be-aerosols in ambient air, a mean residence time of about 8 days averaged for atmospheric aerosols of 0.90 μm AMAD size.

Radon Decay Product Aerosols in Ambient Air

The aerodynamic size distributions of radon decay product aerosols, i.e. 214Pb, 212Pb, and 210Pb were measured using low-pressure (LPI) as well as conventional low-volume 1-ACFM and high-volume (HVI) cascade impactors. The activity size distribution of 214Pb and 212Pb was largely associated with submicron aerosols in the accumulation mode (0.08 to 2.0 μm). The activity median aerodynamic diameter “AMAD” varied from 0.10 to 0.37 μm (average 0.16 μm) for 214Pb-aerosols and from 0.07 to 0.25 μm (average 0.12 μm) for 212Pb-aerosols. The geometric standard deviation, σg averaged 2.86 and 2.97, respectively. The AMAD of 210Pb-aerosols varied from 0.28 to 0.49 μm (average 0.37 μm) and the geometric standard deviation, σg varied from 1.6 to 2.1 (average 1.9). The activity size distribution of 214Pb-aerosols showed a small shift to larger particle sizes relative to 212Pb-aerosols. The larger median size of 214Pb-aerosols was attributed to α-recoil depletion of smaller aerosol particles following the decay of the aerosol-associated 218Po. Subsequent 214Pb condensation on all aerosol particles effectively enriches larger-sized aerosols. Pb-212 does not undergo this recoil-driven redistribution. Even considering recoil following 214Po α-decay, the average 210Pb-labeled aerosol grows by a factor of two during its atmospheric lifetime. Early morning and afternoon measurements indicated that similar size associations of 214Pb occur, despite humidity differences and the potential for fresh particle production in the afternoon. In estimating lifetimes of radon decay product aerosols in ambient air, a mean residence time of about 8 days could be applied to aerosol particles in the lower atmosphere below precipitation cloud levels.