Impactor Measurements Research Articles

A field-based approach for deterimining ATOFMS instrument sensitities to ammonium and nitrate.

Aerosol time-of-flight mass spectrometry (ATOFMS) instruments measure the size and chemical composition of individual particles in real-time. ATOFMS chemical composition measurements are difficult to quantify, largely because the instrument sensitivities to different chemical species in mixed ambient aerosols are unknown. In this paper, we develop a field-based approach for determining ATOFMS instrument sensitivities to ammonium and nitrate in size-segregated atmospheric aerosols, using tandem ATOFMS-impactor sampling. ATOFMS measurements are compared with collocated impactor measurements taken at Riverside, CA, in September 1996, August 1997, and October 1997. This is the first comparison of ion signal intensities from a single-particle instrument with quantitative measurements of atmospheric aerosol chemical composition. The comparison reveals that ATOFMS instrument sensitvities to both NH4+ and NO3- decline with increasing particle aerodynamic diameter over a 0.32-1.8 microm calibration range. The stability of this particle size dependence is tested overthe broad range of fine particle concentrations (PM1.8) = 17.6 +/- 2.0-127.8 +/- 1.8 microg m(-3)), ambient temperatures (23-35 degrees C), and relative humidity conditions (21-69%), encountered during the field experiments. This paper describes a potentially generalizable methodology for increasing the temporal and size resolution of atmospheric aerosol chemical composition measurements, using tandem ATOFMS-impactor sampling.

An Evaluation of Mass-Weighted Size Distribution Measurements with the Model 3320 Aerodynamic Particle Sizer

The ability of the Model 3320 aerodynamic particle sizer (APS) to make accurate mass-weighted size distribution measurements was investigated. Significant errors were observed in APS size distribution measurements with measured mass median aerodynamic diameters (MMADs) as much as 17 times higher than from cascade impactor measurements. Analysis of APS correlated time-of-flight and light scattering data indicated that the MMAD distortions were due to a few anomalous large particle measurements (∼0.1% of the total measurements) with surprisingly low scattered light. Computational fluid dynamics modeling indicated that these anomalous measurements were due to particles that deviated from the intended aerosol pathway and recirculated through the APS measurement volume at low velocities leading to erroneous large particle measurements. A technique for removing erroneous measurements based on correlated aerodynamic diameter and light scattering values is presented. When this technique was used, APS and cascade impactor size distribution measurements agreed well.

Chemical classes of atmospheric aerosol particles at a rural site in Central Europe during winter

Ambient atmospheric particles were studied at an ecosystem research site in the Fichtelgebirge mountains in Central Europe by single-particle analysis and bulk impactor measurements. Fuzzy clustering analysis of mass spectra of individual aerosol particles allowed chemical classification of the atmospheric aerosol. During the campaign, inorganic salts, mineral particles, and carbonaceous particles, with varying degrees of secondary components, were identified. These chemical classes exhibited preferential size ranges leading to a characteristic pattern of relative abundances with respect to particle size. A more detailed analysis revealed that 65–80% of all particles were assigned almost exclusively to one chemical class. These particle populations are assumed to be externally mixed with respect to the identified chemical classes. The temporal variations of the ratio of nitrate to ammonium (ranging between 0.37 and 0.81) determined by both impactor measurements and single-particle analyses were in good agreement. Through Monte-Carlo-type simulations, confidence intervals of the mean NO 3 −/NH 4 + ratio were calculated for sub-samples of the total particle population.

Chamber measurements of CI depletion in cloud‐processed sea‐salt aerosol

Cl− depletion in sea‐salt CCN was measured in chamber experiments after cloud formation in the presence of SO2 and O3. In each experiment, two successive clouds were formed on a sea‐salt aerosol generated from the nebulization of filtered seawater, while chamber SO2 (g) and O3 (g) concentrations ranged from 4.8–8.3 ppb and 69–75 ppb. Particle growth from cloud processing reactions was measured with a differential mobility analyzer (DMA) immediately after cloud dissipation, and particles activated from dry radii of 0.02–0.03 μm showed growth to 0.08 and 0.12 μm, with total aerosol mass increases of 7.0 and 2.4 μg m−3 (assuming a particle density of 2.4 g cm−3). Particle growth was greater than that predicted by standard SO2‐O3 oxidation kinetics and cannot be accounted for by the buffering capacity of the sea‐salt CCN. Micro‐orifice impactor (MOI) measurements of inorganic ions in the postcloud aerosol size distributions show, within analytical error, a 1:1 displacement of Cl− with nss ‐ SO42−, until the SO42− is in excess and the Cl− is completely depleted at the smaller CCN sizes. Calculations of Cl− depletion, based upon the input size distribution and assuming particle growth via sulfate addition, show greater Cl− depletion on smaller size particles than the measured values. However, there is agreement within the uncertainty of the measured values.

A new method for measuring the dependence of particle size distributions on relative humidity, with application to the Southeastern Aerosol and Visibility Study

A relative humidity‐moderated differential mobility optical particle size spectrometer (RH‐DMOPSS) was developed to measure particle size distributions at different humidity conditions in the 0.1‐ to 1.1‐μm size range. The system has a high‐flow differential mobility analyzer followed by an optical counter and condensation particle counter in parallel. An aerosol dryer or humidity conditioner can be placed in line ahead of the mobility analyzer. A second dryer ahead of the optical counter permits measurements of the decrease in particle size when particles classified at high relative humidity were dried. The RH‐DMOPSS data set includes (1) optical counter response to monodisperse ambient aerosols, (2) accumulation mode particle size distributions for dry and humid conditions, (3) selected hygroscopic growth factors for humid diameters of 0.2 to 1.1 μm, (4) inferred volatile particle water volume, and (5) inferred refractive index for dried particles. The RH‐DMOPSS was operated in Smoky Mountains National Park as part of the Southeastern Aerosol and Visibility Study and compared with other collocated size and aerosol growth measurements. For dried ambient particles, size distribution parameters were correlated with those from impactor measurements, but volume median diameters were 12% smaller, on average. Growth factors for particles below 0.5‐μm diameter were in the same range as those measured by a tandem differential mobility analyzer and, for all sizes, were consistent with the measured differences between dried and humidified particle size distributions. The mean ambient particle real refractive index inferred from the RH‐DMOPSS dry‐particle calibration data was 1.48, equal to that measured by a multiangle light‐scattering instrument.

Respiratory Deposition Patterns of Salbutamol pMDI with CFC and HFA-134a Formulations in a Human Airway Replica

This paper describes a technique that uses a well-defined human airway replica and gamma counting as a standard method for evaluating and comparing the performance of medical inhalers and spacers. High-fidelity replicas reproduced as needed from master casts made from human cadavers include the oropharyngeal cavity, larynx, trachea, and five to nine generations of bronchi. Deposition in the small airways and alveoli region of the cast is simulated by material that passes through the upstream airways and is collected on foam filters. Deposition patterns in the respiratory tract replica were obtained by using radiolabel in the medical inhaler and by gamma scintigraphy. This technique was used to determine respiratory deposition patterns of salbutamol in a pressurized metered dose inhaler (pMDI) with chlorofluorocarbon (CFC, in-house formulation) and HFA-134 formulations (Proventil hydrofluoroalkane [HFA]). At an inspiration flow of 30 L/min, patterns in the salbutamol/CFC formula showed a high deposition in the oropharyngeal airway (78%) and a 16% deposition in the lung, similar to in vivo measurements reported in the literature. However, the salbutamol/HFA formula showed lower oral deposition (56%) but higher lung deposition (24%). The difference in the oral deposition patterns may be attributed to lower initial spray velocity, initial droplet evaporation rate, and possibly initial droplet sizes of Proventil HFA. The small orifice diameter (0.25 mm) of the Proventil HFA actuator produced a softer plume with a smaller impact force, resulting in lower oropharyngeal deposition. Cascade impactor measurements showed similar aerodynamic particle size distribution of the CFC and HFA formulations. We also showed that using spacers in the Proventil HFA resulted in a lower oropharyngeal deposition and higher lung deposition, indicating beneficial effects. Comparison of oropharyngeal deposition and those predicted by artificial throats used in the impactor measurements showed that, in general, the artificial throat predicted a lower deposition.

PM-10, PM-2.5 and impactor measurements in Mexico City

PM-10, PM-2.5 and impactor measurements in Mexico City

A new analytical approach for size‐resolved speciation of organic compounds in atmospheric aerosol particles: Methods and first results

A new analytical approach for the characterization of organic compounds in size‐segregated atmospheric aerosol particles is described. Its potential is demonstrated for samples that were collected during the Second Aerosol Characterization Experiment in Sagres (Portugal) and at the Institut für Troposphärenforschung research station in Melpitz (Germany). Curie point pyrolysis gas chromatography/mass spectrometry is used to determine semivolatile compounds, capillary electrophoresis for organic acids. The main advantages are low sample amount and almost no sample preparation. Size distributions of alkanes, fatty acids, polycyclic aromatic hydrocarbons (PAHs), oxygenated polycyclic aromatic hydrocarbons (oxy‐PAHs), dicarboxylic acids, hydroxylated dicarboxylic acids, organic and elemental carbon, main water soluble ions, and gravimetric mass were determined by impactor measurements and are discussed. Compared to central Europe (hydroxylated) dicarboxylic acids show a higher concentration in marine air influenced by long‐range transport through European polluted air masses. Dicarboxylic acids show a maximum concentration in the coarse mode, whereas hydroxylated dicarboxylic acids are mostly found in submicrometer particles. Alkanes are found in higher concentrations and oxy‐PAHs are only found in central Europe. For the selected samples, 12–15% of the organic carbon could be assigned to individual species in the case of central Europe, and 20–40% could be assigned in the marine aerosol of Portugal. In all cases, semivolatile organic species such as alkanes and PAHs account for the major fraction of the speciated organic carbon.

Seasonal variation of primary biological aerosol particles in the remote continental region of Lake Baikal/Siberia

During a complete year (April 1996–April 1997) impactor measurements were performed in the remote continental region of Lake Baikal/Siberia and were analysed by single-particles analysis to determine the content of primary biological aerosol particles. The results show an amount of 20% of the number concentration and almost 30% of the volume concentration that were biological, with no seasonal variation. Distinct differences were seen looking at the giant particles ( r>2 μm), where strong phenological variations were seen within the size distributions.

Contribution of carbonaceous material to cloud condensation nuclei concentrations in European background (Mt. Sonnblick) and urban (Vienna) aerosols

During four intensive measurement campaigns (two on Mt. Sonnblick, European background aerosol, and two in Vienna, urban aerosol), cloud condensation nuclei (CCN) were measured at supersaturations of 0.5%. Impactor measurements of the mass size distribution in the size range 0.1–10 μm were performed and later analyzed for Cl -, NO - 3, SO 2- 4, Na +, NH + 4, K +, Ca 2+ and Mg 2+ by ion chromatography, for total carbon (TC) using a combustion method, and for black carbon (BC) by an optical method (integrating sphere). Organic carbon (OC) was defined as the difference between TC (minus carbonate carbon) and BC. At all sites, the mass fraction of BC in the submicron aerosol was comparable (4–5%). CCN concentrations on Mt. Sonnblick were found to be 10–30% of those measured in Vienna, although high Mt. Sonnblick concentrations were comparable to low Vienna concentrations (around 800 cm -3). The contribution of organic material was estimated from the mass concentrations of the chemical species sampled on the impactor stage with the lowest cut point (0.1–0.215 μm aerodynamic equivalent diameter). On Mt. Sonnblick, TC material contributed 11% to the total mass in fall 1995, and 67% in summer 1996, while the OC fraction was 6 and 61%. The combined electrolytes and mineral material contributed 18 and 16% in fall and summer. During the Vienna spring campaign, the contributions of OC and electrolytes to the total mass concentration in this size range were 48 and 36%, respectively.

Undersizing of droplets from a vented nebulizer caused by aerosol heating during transit through an anderson impactor

The use of Anderson cascade impaction to measure droplet sizes from a vented nebulizer connected directly to the impactor was examined by performing two sets of impactor measurements. In the first set, the impactor was operated in room temperature air. In the second set, the impactor was immersed in a cooled water bath at the same temperature as the aerosol exiting the nebulizer (10°C). Normal saline was nebulized in five Pari LC Jet+nebulizers driven by a single Pulmo-Aide compressor, operating under ambient conditions of 36% RH and 22°C. Impaction was done once steady temperatures were reached. When the impactor was operated in room temperature air, the air travelling through the impactor warmed from 10°C at the entrance to the impactor to room temperature at the exit of the impactor. This heating resulted in significant droplet shrinkage due to humidification by evaporation from the droplets, since the impactor immersed in the cooled water bath gave an MMAD that was on average 70% larger than was obtained when the impactor was operated in room air (3.4 μm vs 2.1 μm). These results emphasize the need for caution when using impactors to measure nebulized hygroscopic aerosols, since even if these aerosols enter the impactor in vapor pressure equilibrium with their surrounding air, significant size changes can occur during transit through the impactor if the temperature of the aerosol differs significantly from that surrounding the impactor.

Measurement of resuspended aerosol in the Chernobyl area. Part III. Size distribution and dry deposition velocity of radioactive particles during anthropogenic enhanced resuspension.

During anthropogenic activities, such as agricultural soil management and traffic on unpaved roads, size distribution measurements were performed of atmospheric particulate radionuclides at a site in the Chernobyl 30-km exclusion zone. Analysis of cascade impactor measurements showed an increase of the total atmospheric radioactivity. In the cases of harrowing by a tractor and traffic on unpaved roads, a common shape of the size distribution was found with two maxima, the first in the 2-4 microm range, the second in the 12-20 microm range. The size distributions were compared to measurements during wind-driven resuspension. Particle number concentration measurements with an Aerodynamic Particle Sizer showed a dynamic dependence of the particle concentration in different size ranges on anthropogenic action. The increase of the mean concentration was for the large particles more than one order of magnitude higher than for fine particles during anthropogenic enhanced resuspension. From the measurement of the mass concentration, the radioactive loading could be estimated. An enrichment of radionuclides on resuspended particles (compared to soil particles) was found, with the highest enrichment for large particles. Micrometeorological considerations showed that large particles may frequently be subject to medium range transport. The dry deposition velocity was measured; the mean value of 0.026 m s(-1) +/- 0.016 m s(-1) is typical for 6-9 microm diameter particles.

Inversion of impactor and diffusion battery measurements

Inversion of impactor and diffusion battery measurements

Substitution of chloride in sea-salt particles by inorganic and organic anions

Depletion of chloride in sea-salt particles was studied at a site near the Arctic Ocean. The investigation was based on size-segregated particle sampling using a Berner low-pressure impactor. According to the impactor measurements, average chloride losses were close to 100% for submicron particles. However, this is successively less for increasing particle size in the supermicron size range. The main constituents replacing chloride from supermicron sea-salt particles were sulfate and nitrate followed by MSA - and oxalate, and with malonate and succinate giving a minor contribution. Anions of organic dicarboxylic acids became more important for air spending a longer time over the continent. Our analysis suggests that principal mechanisms accumulating sulfate into sea-salt particles are cloud processing and, to a lesser degree, heterogeneous reactions taking place in deliquescent sea-salt particles. Mechanisms for the chloride replacement by nitrate are less clear. The distributions of MSA - and oxalate over the sea-salt particle size range were similar to each other, whereas other organic anions analyzed here had a peak concentration at a somewhat larger particle size. Better understanding on the chemistry associated with sea-salt particles requires investigating not only reactions in deliquescent sea-salt particles, but also the interactions between these particles and clouds.

Filter and impactor measurements of anions and cations during the First Aerosol Characterization Experiment (ACE 1)

During the First Aerosol Characterization Experiment (ACE 1), we measured the concentrations of aerosol non‐sea‐salt sulfate (NSS), methane sulfonate (MS), ammonium, sodium, nitrate, and a variety of other anions and cations from the National Center for Atmospheric Research C‐130Q aircraft and the Cape Grim Baseline Air Monitoring Station. We used the data to test a variety of hypotheses concerning the sources and properties of aerosol in remote environments. We noted a large gradient in free tropospheric (FT) MS, with much more in the spring (southern) hemisphere, but FT NSS showed no latitudinal gradient. Most species had strong vertical concentration gradients, generally with more material near the biogenic sources of the marine boundary layer (MBL) than in the FT. During the Lagrangian experiments and in time series measurements at Cape Grim, we saw daytime increases of photochemically derived MS, with more constant or decreasing concentrations at night. NSS also increased in the daytime during the Lagrangian experiments. At Cape Grim, we were unable to account for the gravimetric mass of particles on impactor substrates using the measured ions, which implies that a substantial fraction of the aerosol at that location must be either organic or mineral matter. Our FT ratios of MS/NSS exhibited a strong latitudinal gradient, with the southernmost values similar to those found in Antarctic plateau snowfall.

Exposure to dust and its particle size distribution in California agriculture.

The aim of this study was to measure personal dust exposure levels and the dust particle size distribution during various agricultural operations in California. Personal dust exposure levels were measured with four-stage cascade impactors and respirable dust cyclones during field crop, fruit and nut farming, and dairy operations at three farms. Altogether, 103 cascade impactor measurements and 108 cyclone measurements were taken. High personal dust exposure levels were measured during various operations in particular during ground preparation operations such as land planing (geometric mean [GM] = 57.3 mg/m3, geometric standard deviation [GSD] = 2.4) and discing (GM = 98.6 mg/m3, GSD = 2.9). Dust particles were relatively large and the great majority belonged to the extrathoracic fraction. Dust levels were considerably lower when an enclosed cabin was present on the tractor; for example, during discing, dust exposure levels were reduced more than sixtyfold for the larger dust particle fraction and more than fourfold for the respirable dust fraction.

Determinants of personal dust exposure during field crop operations in California agriculture.

The aim of this study was to identify determinants, if any, of personal dust exposure levels during agricultural field crop operations in California. Personal dust concentrations were measured with four-stage cascade impactors and respirable dust cyclones. Altogether, 57 cascade impactor measurements and 63 cyclone measurements were used for analyses. High personal dust concentrations were measured during most operations, in particular during ground preparation operations such as land planing and discing. For the larger dust particles, which is dust collected on all stages of the cascade impactor and dust with a 50% cutoff < 9.8 microns, the most important determinants of personal dust exposure were the presence of an enclosed cabin, relative humidity, type of operation, and tractor speed. The presence of an enclosed cabin on the tractor, higher relative humidity, and lower tractor speed were all associated with a decrease in personal dust levels. For smaller dust particles, which is dust with a 50% cutoff < 3.5-4 microns, the presence of an enclosed cabin, relative humidity, and soil temperature appear to be related to dust levels. The results of this study could be used to reduce the high levels of personal dust exposure currently experienced by those who work in field crop farming in California.

The influence of the dry deposition on the lead concentration in aerosols dependent on the particle size over the Baltic Sea

In recent studies concerning modelling aerosol transport and deposition over Europe widely emission estimates of heavy metals, associated with particles, integrated over the size distribution have been used. This is a crucial simplification of the real conditions and reflects an important weakness of the emission inventories. Also the dry deposition parameterization often performed by a single value of the dry deposition velocity representative for mean particle-size distributions can be expected to be rather uncertain. The results of many investigations which provide estimates of atmospheric depositions are based on such assumptions. However compared to the emissions about the dry deposition process there is a much better knowledge which can be used for modelling. The idea in this study was to combine the best available information about particle size dependent emissions and a dry deposition scheme applicable for different particle sizes. The motivation was the evaluation of impactor measurements of particle size segregated lead concentrations in aerosols at the coast of the Baltic Sea. In order to determine the dry deposition over the Baltic Sea the deposition module of the ADOM model was adapted for different sizes of lead containing particles. The model referred to as ADOM (Acid Deposition and Oxidant Model) originally has been developed under the sponsorship of Environment Canada, Atmospheric Environment Service(AES), the Ontario Ministry of Environment and Energy (OMEE), the German Umweltbundesamt and the Electric Power Research Institute (EPRI) for simulating long-range transport and deposition of acidic pollutants in Europe and North America (Venkatram et al., 1988). Presently it is modifyed for heavy metals (Krüger and Petersen, 1996). The dry deposition module of ADOM is formulated by means of an electrical resistance analog. The basis for the electrical analog is the Ohm's law which relates voltage to resistance and current. In the analogous relation for the dry deposition the concentration is analogous to voltage the flux is analogous to current and the inverse of the deposition velocity is analogous to total resistance. For the estimation of the flux the pollutant transfer is thought to occur in the constant flux layer and the deposition layer. In the dry deposition scheme small particles (diameter < 0.05μm) within the deposition layer are transported much in the same way like gases. The Brownian diffusion is considered as the primary transport mechanism. The deposition of larger particles is parameterized by the settling velocity and the stopping time, which are dependent on the Stokes number and collection efficiencies. The roughness lengths of 12 land use categories identical with those of the PHOXA land use data base for Europe are taken into account.The ADOM dry deposition module was applied to air mass movements over the Baltic Sea. Typical trajectories for the receptor point Kap Arkona located in the North East of Germany were investigated. As initial concentrations emission data for lead which are documented in van den Hout, K.D. (1994) were used for the calculations. This inventory contains a general separation of the aerosol size distribution into the 5 diameter intervals (<0.95μm, 0.95–4μm, 4–10μm, 10–20μm, >20μm). The meteorological data along the trajectories were taken from the HIRLAM weather forecast model (Källen, 1996). The dry deposition was calculated along air mass back trajectories for the receptor point Kap Arkona. The results indicate that there is a pronounced decrease of the initial concentration during the first hours of transport. Consequently after some hour simulation time the lead is only concentrated in aerosols of diameters lower than 4μm (figure 1). These findings are quite different compared to the lead concentrations found in size distributions measured at Kap Arkona. The present interpretation of the investigations is that interactions of aerosol particles of different sizes seem to have a strong influence. Possibly particles from sea spray play an important role during the transport over the sea.▪

Formation of particulate MSA: deductions from size distribution measurements in the Finnish Arctic

Formation of particulate MSA was studied by conducting a field campaign in the Finnish Arctic. Based on continuous mass size distributions extracted from impactor measurements, 4 MSA modes were identified: an accumulation mode centred between 0.3-0.5 μm of particle aerodynamic diameter, an Aitken mode below 0.1 μm, and 2 supermicron modes that peaked at 2-3 and 7-10 μm, respectively. The lower supermicron mode resulted primarily from the reaction of gaseous MSA with sea salt particles, and the upper mode probably from its reaction with soil-derived particles. From 70 to 90% of the MSA was found in the accumulation mode, where it was distributed very similarly to ammonium. The overall MSA to nss-SO42- ratio, R, ranged from 2 to 34%, with most of this variation resulting from different degrees of anthropogenic perturbation in the measured air masses. When comparing different-size particles, R was clearly the highest in the Aitken mode, suggesting that MSA and sulfate contribute with comparable magnitudes to nuclei condensational growth at high latitudes during the summer. The evident variation of R with particle size, together with potential enhanced MSA production in air influenced by pollution, demonstrate further that one needs to be extremely careful when using observed MSA to nss- SO42- ratios for estimating the contribution of biogenic sources to total particulate sulfate in different environments. Strong indications on the saturation of MSA over accumulation-mode particles were found. The saturation effect was hypothesized to explain, in part, the observed large partitioning of MSA in the supermicron mode in warm and very acidic aerosol systems. The distribution of submicron MSA, including potential saturation, was shown to be affected significantly by cloud processing.

Evaluation of Two Inversion Techniques for Retrieving Health-Related Aerosol Fractions from Personal Cascade Impactor Measurements

Personal cascade impactors are widely used in occupational aerosol exposure assessment. Appropriate algorithms must be used to determine the total particle size distribution from masses collected on the stages of the cascade impactor. Such algorithms should be regarded as integral components of the measurement system. When evaluating algorithms for reconstruction of size distributions from cascade impactor data, the eventual use of the size distribution must be considered. So, from an industrial hygiene perspective, an appropriate basis for comparison of given measurement systems is the accurate retrieval of the inhalable, thoracic, and respirable aerosol fractions as described by the new, internationally accepted, particle size-selective sampling conventions (which are expected to form the basis of future aerosol standards). This article compares two inversion routines in terms of their abilities to retrieve these aerosol mass fractions relative to the masses that would have been obtained using an ideal sampler that perfectly followed the sampling convention. The routines were used to invert measurements made with the Institute of Occupational Medicine personal inhalable dust spectrometer, a miniature cascade impactor that aspirates the inhalable aerosol fraction, and the results are presented graphically as contours of equal mass bias. The simplest algorithm, based on the a priori assumption of lognormality, appears to provide the best results.