Characterizing the transport pathways of Asian dust Tsai, Fujung; Chen, George Tai-Jen; Liu, Tsun-Hsien ...
Journal of Geophysical Research - Atmospheres,
16 September 2008, Volume:
113, Issue:
D17
Journal Article
Peer reviewed
Open access
Backward trajectories, synoptic analyses, and regional dust models are applied to analyze the aerosol particles obtained from aircraft measurements during major dust events in the Asian Pacific ...Regional Aerosol Characterization Experiment (ACE‐Asia) to understand the controlling mechanism of Asian dust transport. The selected high concentrations of aerosol measurements are traced back to their source areas, and the summarized eleven representative trajectories of dust particles are categorized into three types, including the upper‐level (U type), lower‐level (L type), and descending (D type) transport. The characteristics of the dust transport during different stages for each of the three types are inspected. It is found that the transport distance of a dust event strongly depended on the source areas relative to the synoptic conditions. For source areas ahead of a trough, the dust particles can be uplifted into the free troposphere. If the uplifted dust particles move to the ascending areas of a trough or ascend due to other processes in the later stage, then the dust particles likely travel longer distances. When dust particles ascend in an area too close to the trough, their subsidence may be expected as sinking motion could occur after trough passage, and thus the transport distance is limited. Uplifted dust particles are typically found at a height of 400–700 hPa (2–4.5 km above ground level) over source regions. For dust particles that are generated behind a trough, the vertical motion is limited to a layer around 700 hPa, and the transport distance is also shorter.
Cascade impactors are commonly used to assess the size characteristics of aerosols in toxicology and pharmaceutical applications. These aerosol instruments have been developed and refined over ...decades. However, a number of questions remain related to impactor performance, including the influence of electrostatic charge on measured size distributions. The objective of this study was to develop a validated CFD model of the Mark II Andersen cascade impactor (ACI) and apply this model to evaluate the effects of particle charge on deposition. The flow field was simulated using a commercial CFD code for incompressible laminar and transitional flows. Particle trajectories and deposition were evaluated using a well tested Lagrangian tracking approach that accounts for impaction, sedimentation, diffusion, and electrostatic attraction. Particle charge levels typical of dry powder inhaler (DPI) and metered dose inhaler (MDI) aerosols were considered for a particle size range of 0.3–12
μm. As a model validation, computational predictions of cutoff
d
50 diameters for each of the eight ACI stages were found to be within 10% difference of existing experimental and manufacturer data. Results indicated that charges consistent with DPI and MDI aerosols increased deposition fraction in Stages 0–3 by up to 30% and increased deposition fraction in Stages 4–7 by up to an order of magnitude. For Stages 0–3, both DPI and MDI charges reduced the
d
50 value by approximately 10% or less. In contrast, charged aerosols reduced the
d
50 values in Stages 4 and 5 by 200% and 60%, respectively. All charged submicrometer aerosols considered deposited in Stages 6 and 7. Increasing the particle charge by an order of magnitude from DPI to MDI values had a relatively small effect on further decreasing the cutoff size of each stage. In conclusion, these results can be used to approximate the actual aerodynamic diameter of a charged pharmaceutical aerosol based on measurements in a standard ACI. Future applications of the developed ACI model include evaluating the influence of space charge on deposition and quantifying the effects of aerosol condensation and evaporation on size assessment.
An on-line method is described and validated to measure the thickness of coatings on gas-borne nanoparticles. The method is essentially a tandem technique which measures the aerodynamic diameter of a ...particle twice—before and after coating—by a single-stage low-pressure impactor (SS-LPI) for the
same
mobility equivalent diameter preselected via differential mobility analyzer (DMA). A shell thickness is then derived from the change in effective particle density determined by the SS-LPI. The method requires a difference in mass density between carrier particle and coating material. Its theoretical sensitivity is shown to range between about 0.1 and 1 nm, depending on the density ratio. One advantage of this approach is that both DMA and SS-LPI are situated in series but downstream of the coating step, so as not to interfere with the coating process. The method was validated against transmission electron microscopy (TEM) measurements, using spherical silica–titania particles coated with conformal shells of molybdenum and bismuth oxide by chemical vapor deposition (CVD). For such spherical particles, the agreement with TEM was excellent. The technique was able to provide layer thicknesses for sub-nanometer layers barely or not resolved by TEM. The paper also discusses the impact of ‘non-ideal’ phenomena such as the formation of doublet particles by coagulation, the effect of multiply charged particles, or the onset of homogeneous decomposition of the coating precursor. With supporting experimental data, it is shown that such phenomena can be interpreted reliably from certain features of the impactor penetration curve. The on-line method can thus be used for fast screening of process parameters and reliable process monitoring for gas-phase synthesis of composite nanopowders.
Multi‐wavelength Raman light detection and ranging (lidar) observations were analyzed, which were performed in Évora, Portugal, during more than 2 years on a regular basis in the framework of the ...European Aerosol Research Lidar Network (EARLINET). An aerosol characterization in terms of the lidar ratios at 355 and 532 nm and the extinction and backscatter related Ångström exponents is presented. Aerosol layers in the free troposphere were classified according to their origin. Clear differences in the intensive optical properties were found for layers of mineral dust from the Sahara and from Asia, of anthropogenic aerosol from Europe and from North America, as well as of biomass burning smoke from the Iberian Peninsula and from North America, respectively. In general, the mean Ångström exponents of aerosol layers of the same type, but from closer source regions, were smaller than those from aerosol layers transported over a longer distance. This hints at the deposition of large particles along the transportation path, especially for anthropogenic aerosol and mineral dust. Besides, the seasonal behavior of aerosol in the free troposphere over Évora was studied. Seventy‐three percent of the detected layers were observed during spring and summer. On average, the layers were highest in summer with an overall mean layer height of (3.8 ±1.9) km above sea level (asl), and lowest in winter with (2.3 ±0.9) km asl.
Key Points
Multi‐wavelength Raman lidar measurements from more than 2 years were analyzed.An aerosol characterization was done, regarding optical particle properties.Clear differences were found between the investigated aerosol classes.
The occurrence of Asian dust storms, and the subsequent transport of yellow dust (YD) greatly influences the air quality of lee-side countries such as Korea and Japan. The dust is also frequently ...transported in a southward direction by a strong cold high-pressure system that affects the air quality in Taiwan. This study reports the aerosol properties that were monitored continuously at the Taiwan aerosol supersite during YD events in 2002. Based on the observations of meteorology and aerosols, we divided the time interval of a YD event into a
before period,
during period, and
after period. Among the seven observed YD events, the second event was marked with the maximum hourly PM
10 level at 502
μg
m
−3, and with the longest
during period for a total of 147
h. The averages of the hourly PM
10 and PM
2.5−10 were much higher in the
during period as compared to those in the
before period. It is interesting to note that the time lapse in the
during period was well correlated with the maximum level of both PM
10 and PM
2.5−10. It must be noted that the PM
2.5 levels were dramatically increased in the
after period, which was due to the accumulation of particles influenced by the anticyclonic outflow. The aerosol size distribution in the third YD event verified that supermicron particles dominated in the
during period, and that submicron particles were predominant in the
before and
after periods. For the chemical properties of the aerosols, time series results indicated that sulfates were mostly contributed by the dust transport, and the others were more related to vehicle exhausts. However, they all accumulated in the period of atmospheric stagnancy.
Aerosol mass measurements are a key air pollution parameter that is regulated in most countries. Beyond mass measurements, the precise composition of the aerosol is essential in identifying sources ...and impacts on health and climate. The conventional method for simultaneously quantifying mass and composition is to collect aerosol onto filter or impactor samples followed by laboratory analysis. This approach requires long collection times—providing poor time resolution for mass measurements—and long sample preparation prior to analysis. The first limitation can be circumvented with microresonators, which are novel particulate mass sensors with high mass sensitivities and time resolutions. In addition, direct surface analysis techniques, like liquid extraction surface analysis mass spectrometry (LESA–MS), shorten sample preparation times. This work combines, for the first time, the high time resolution mass measurements of a microresonator with the integrated compositional analysis of LESA–MS. Laboratory-produced secondary organic aerosol were collected onto a microresonator via impaction with LESA–MS being used to analyze the chemical composition afterwards. The results were compared with classic filter extraction methods and literature with the final spectra matching the expected reaction products. The combined technique demonstrates an extension to current microresonator applications and illustrates their potential for ambient aerosol studies.
Long-term characterization of tropospheric aerosol has been carried out at Chiba, Japan, using a compact, stand-alone spectroradiometer under clear-sky conditions between August 2007 and March 2009. ...The spectra of direct solar radiation, aureole, and scattered solar radiation in various directions are observed in a wavelength range between 350 and 1050
nm with an optical resolution of 10
nm. Radiative transfer calculation using the MODTRAN4 code is employed to retrieve aerosol optical parameters such as aerosol optical depth (AOD), extinction coefficient, single-scattering albedo, scattering phase function, and asymmetry parameter, as well as water vapor column amount. The retrieved value of AOD varies in the range 0.1–0.5, while the water vapor column amount changes from 0.2 to 4
g/cm
2, showing reasonable agreements with the concurrent measurements with a sunphotometer and a microwave radiometer, respectively. The seasonal variation of the retrieved parameters indicates the major impacts of dust particles in spring, sea salt particles in summer, and anthropogenic fine particles in winter.
Aerosol samples were collected at Catania (Italy), from 16 March to 13 June 2005. The sampling was performed using a low pressure five-stage Berner cascade impactor. The samples were analysed for ...total aerosol mass, Water Soluble Organic Carbon (WSOC), Total Carbon (TC) and main inorganic ionic species. The Water-Insoluble Carbon (WINC) was derived by the difference: TC-WSOC. The samples share some common features: ammonium sulphate and carbon-containing species (both soluble and insoluble) are the largest contributors of fine particle mass, while coarse particles essentially consist of sea-salt, sodium nitrate and unaccounted PM (probably crustal material). The WINC/WSOC ratio decreases from the smallest size range to the large accumulation mode range (0.42-1.2 μm), while the graphic removed and graphic removed contribution rises. The water-insoluble carbonaceous matter is the dominant component in the smallest particles (0.05-0.14 μm). We identified four different aerosol types, corresponding to different sources, contributing to the total particles load of the investigated urban environment: vehicular traffic, producing primary carbonaceous insoluble particles, secondary aerosols, dominating the composition of accumulation mode particles, and marine particles and mineral dust (both important components of the coarse aerosol fraction).
Characterizations of urban and regional sources of particulate matter (PM) were performed in the Milan area (North of Italy) during Föhn and stagnant (non-Föhn) conditions. The measurements were ...performed at two different places: in an urban area North of Milan (Bresso) and in a regional area at the EMEP-GAW station in Ispra (about 65 km NW from Milan) during the winter periods of the years 2002–2007. Particle size distributions and chemical bulk analysis of aerosols are combined with single particle mass spectrometry to obtain information about the chemical content of the particles and their mixing state. Föhn conditions are characterized by extremely clean background air from which background aerosol is scavenged, and consequently local sources (here defined as sources between the sampling sites and the mountain range top about 100–150 km away depending on the wind direction) determine the aerosol properties.
It was observed that during Föhn events the accumulation mode in the size range 50 nm
<
d
<
300 nm practically disappears and that the size fraction below 50 nm dominates the total number distribution. The significant change in the number size distribution and the large decrease in PM10 mass during Föhn events are accompanied by a significant change in the chemical composition of the particles. Results from bulk chemical analysis showed high amounts of carbonaceous compounds and very low concentrations of ammonium nitrate (as indicator for secondary chemistry) during Föhn episodes, in contrast to stagnant conditions, when secondary components are dominating the aerosol composition. Single particle measurements confirm the high contribution of carbonaceous compounds in locally emitted particles.
It was concluded that particles that originated in the urban area come mainly from combustion processes, especially direct traffic emissions, domestic heating and industrial activities, whereas the regionally emitted particles are different with much less traffic contribution.
We estimate that under prevailing (non-Föhn) winter conditions, about 50–65% of the aerosol mass load in the city of Milan are caused by local emissions, and about 35–50% come from regional background. This finding suggests that in order to improve air quality in a big city like Milan, it is important to combine local traffic restriction interventions with other long-term regional scale air-quality-measures.
The synthesis of silicon quantum dots is performed in the 3–5 nm range using CO2 laser pyrolysis of SiH4. This size range is particularly relevant for potential applications in photovoltaic devices ...and biomedical imaging. The laser pyrolysis technique offers convenient control of the synthesis parameters in the case of nanoparticle production. However, controlling the size of small silicon objects remains difficult. The original approach consists here in a time-control of the energy injected into the reaction by gating the laser. The laser gate-on duration is adjusted in the range of 10 to 80 μs while keeping the average power constant. In parallel, supersonic expansion and on-line time-of-flight mass spectrometry are performed for on-line size characterization. A monotonic increase of the size as a function of the gate-on duration is observed for several SiH4 volume concentrations. The results are discussed qualitatively.
PDF
