Shape Of Aerosol Particles Research Articles

FLEXPART version 11: improved accuracy, efficiency, and flexibility

Abstract. Numerical methods and simulation codes are essential for the advancement of our understanding of complex atmospheric processes. As technology and computer hardware continue to evolve, the development of sophisticated code is vital for accurate and efficient simulations. In this paper, we present the recent advancements made in the FLEXible PARTicle dispersion model (FLEXPART), a Lagrangian particle dispersion model, which has been used in a wide range of atmospheric transport studies over the past 3 decades, extending from tracing radionuclides from the Fukushima nuclear disaster, to inverse modelling of greenhouse gases, and to the study of atmospheric moisture cycles. This version of FLEXPART includes notable improvements in accuracy and computational efficiency. (1) By leveraging the native vertical coordinates of European Centre for Medium Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) instead of interpolating to terrain-following coordinates, we achieved an improvement in trajectory accuracy, leading to a ∼8 %–10 % reduction in conservation errors for quasi-conservative quantities like potential vorticity. (2) The shape of aerosol particles is now accounted for in the gravitational settling and dry-deposition calculation, increasing the simulation accuracy for non-spherical aerosol particles such as microplastic fibres. (3) Wet deposition has been improved by the introduction of a new below-cloud scheme, by a new cloud identification scheme, and by improving the interpolation of precipitation. (4) Functionality from a separate version of FLEXPART, the FLEXPART CTM (chemical transport model), is implemented, which includes linear chemical reactions. Additionally, the incorporation of Open Multi-Processing parallelisation makes the model better suited for handling large input data. Furthermore, we introduced novel methods for the input and output of particle properties and distributions. Users now have the option to run FLEXPART with more flexible particle input data, providing greater adaptability for specific research scenarios (e.g. effective backward simulations corresponding to satellite retrievals). Finally, a new user manual (https://flexpart.img.univie.ac.at/docs/, last access: 11 September 2024) and restructuring of the source code into modules will serve as a basis for further development.

Study on inversion of atmospheric aerosol nonsphericity based on satellite and ground observations

The lack of aerosol shape data is one of the causes for the uncertainty of aerosol radiative forcing. In this paper, the parameterization of aerosol particle shape is successfully implemented based on the written computer program using satellite and ground observation data. The method can use aerosol optical (microphysical) properties and linear depolarization ratio from multiple data sources for obtaining aspect ratio based on the ellipsoidal model, or aspect ratio and roundness based on the super-ellipsoidal model. Subsequently, the distribution characteristics of aerosol depolarization ratio have been analyzed, the aerosol nonsphericity in global regions have been inverted based on the method, and the solar radiance of different study regions has been calculated according to the aerosol nonsphericity and verified by the sun photometer data. The results show that the aerosol depolarization ratio data is mainly distributed below 6 km. In urban regions, the depolarization ratio is generally below 20%, while in desert regions, it is mostly distributed between 20% and 40%. The aerosol aspect ratio based on the ellipsoidal model obtained by inversion ranges from 1.00 to 1.55 and vary considerably with region, generally less than 1.20 in urban regions and generally greater than 1.20 in desert regions. Meanwhile, the aerosol aspect ratio and roundness based on the super-ellipsoidal model obtained by inversion range from 1.00 to 1.40 and 1.00 to 1.30, respectively. The aspect ratio and roundness in urban regions are distributed between 1.00 and 1.15 (1.10), while in desert regions are generally greater than 1.10. Besides, the calculation results of solar radiance show that the parameterized data of aerosol nonsphericity can provide an effective shape input for aerosol optical modeling.

Polarized light illuminated scattering characteristics of single airborne particle

We demonstrate a method of classifying aerosol particles with different shapes based on the two separated vertical and horizontal polarized beams spaced by 90-100 µm. Model 3321 Aerodynamic Particle Sizer Spectrometer (APS3321) was used to set up an experimental apparatus to test aerosol samples of different shapes, such as spherical oleic acid droplets and elongated crystallized riboflavin. The receiver operating characteristic (ROC) curve was applied to analyze the discrimination ability of the difference in light-scattering intensity to samples with different shapes in the different particle size ranges. The results show that under our experimental conditions, for spherical particles and elongated particles, the distribution trend of the Pf parameter defined by ourselves are different in each particle size range, and the difference between the two particles in the smaller particle size range (about less than 1 µm) seems to be greater than that in the larger one. The relationship between the difference in light-scattering intensity and the elements of the Muller matrix was deduced theoretically. On this basis, the difference in light-scattering intensity of particles with different shapes was calculated by T-matrix. The calculation results show that the method can distinguish spherical and elongated particles well in a certain range of particle sizes (it's approximately 0.3-1.0 µm in diameter). The good agreement between the experimental results and the theoretical calculations indicate the feasibility of the polarized light illuminated scattering characteristics of single airborne particle combined with aerodynamic size information as a new method for aerosol particle shape rapid discrimination.

Imaging atmospheric aerosol particles from a UAV with digital holography

The lack of quantitative characterization of aerosol particles and their loading in the atmosphere is one of the greatest uncertainties in climate-change science. Improved instrumentation capable of determining the size and shape of aerosol particles is needed in efforts to reduce this uncertainty. We describe a new instrument carried by an unmanned aerial vehicle (UAV) that images free-floating aerosol particles in the atmosphere. Using digital holography, the instrument obtains the images in a non-contact manner, resolving particles larger than ten micrometers in size in a sensing volume of approximately three cubic centimeters. The instrument, called the holographic aerosol particle imager (HAPI), has the unique ability to image multiple particles freely entering its sensing volume from any direction via a single measurement. The construction of HAPI consists of 3D printed polymer structures that enable a sufficiently low size and weight that it may be flown on a commercial-grade UAV. Examples from field trials of HAPI show images of freshly emitted tree pollen and mineral dust.

Variations of flow in human airways as a consequence of lung diseases

The efficiency of drug delivery administered by inhalation depends, among other factors, such as size and shape of aerosol particles, significantly also on the flow in the airways. As many lung diseases change both the breathing pattern and the shape of airways, we focus in this study on the influence of several selected diseases on the distribution of flow between the lung lobes and on changes the diseases induce on the course of flowrate. First, we present results of a literature survey focused on the published records of pathological breathing patterns. In the second part, we describe the newly designed breathing simulator and the implementation of the patterns into it. The last part is focused on the experimental verification of fidelity of the simulated breathing patterns.

Comprehensive tool for calculation of radiative fluxes: illustration of shortwave aerosol radiative effect sensitivities to the details in aerosol and underlying surface characteristics

Abstract. The evaluation of aerosol radiative effect on broadband hemispherical solar flux is often performed using simplified spectral and directional scattering characteristics of atmospheric aerosol and underlying surface reflectance. In this study we present a rigorous yet fast computational tool that accurately accounts for detailed variability of both spectral and angular scattering properties of aerosol and surface reflectance in calculation of direct aerosol radiative effect. The tool is developed as part of the GRASP (Generalized Retrieval of Aerosol and Surface Properties) project. We use the tool to evaluate instantaneous and daily average radiative efficiencies (radiative effect per unit aerosol optical thickness) of several key atmospheric aerosol models over different surface types. We then examine the differences due to neglect of surface reflectance anisotropy, nonsphericity of aerosol particle shape and accounting only for aerosol angular scattering asymmetry instead of using full phase function. For example, it is shown that neglecting aerosol particle nonsphericity causes mainly overestimation of the aerosol cooling effect and that magnitude of this overestimate changes significantly as a function of solar zenith angle (SZA) if the asymmetry parameter is used instead of detailed phase function. It was also found that the nonspherical–spherical differences in the calculated aerosol radiative effect are not modified significantly if detailed BRDF (bidirectional reflectance distribution function) is used instead of Lambertian approximation of surface reflectance. Additionally, calculations show that usage of only angular scattering asymmetry, even for the case of spherical aerosols, modifies the dependence of instantaneous aerosol radiative effect on SZA. This effect can be canceled for daily average values, but only if sun reaches the zenith; otherwise a systematic bias remains. Since the daily average radiative effect is obtained by integration over a range of SZAs, the errors vary with latitude and season. In summary, the present analysis showed that use of simplified assumptions causes systematic biases, rather than random uncertainties, in calculation of both instantaneous and daily average aerosol radiative effect. Finally, we illustrate application of the rigorous aerosol radiative effect calculations performed as part of GRASP aerosol retrieval from real POLDER/PARASOL satellite observations.

Retrieval of dust aerosol optical and microphysical properties from ground-based Sun-sky radiometer measurements in approximation of randomly oriented spheroids

Deserts currently covering more than one tenth of the land surface gradually spread out over more and more areas due to desertification. Mineral dust as one of the most widespread aerosol types plays increasingly important role in atmospheric processes. An accurate accounting for physical and chemical characteristics of desert dust, particles shape in particular, in methods for aerosol retrievals from remote sensing data, is needed. The neglect of dust particle non-sphericity may lead to errors in quantifying the aerosol radiative and regional climatic effects.Earlier we developed a software package Sun-Sky Measurements for Aerosol ReTrieval (SSMART) to retrieve aerosol optical and microphysical characteristics from spectral ground-based Sun and sky radiance measurements. The single scattering albedo and scattering phase function were retrieved in two ways: (i) directly from the almucantar measurement data without any assumptions about the shape of aerosol particles and (ii) through the calculation on the basis of the Mie theory using the retrieved particle size distribution function and complex refractive index. However, there is experimental evidence that in modeling the optical properties of desert dust, the effects of particle non-sphericity should be taken into consideration.In this paper, we propose an improved version of the SSMART algorithm, in which the inverse light scattering problem is solved using the model of a mixture of randomly oriented polydisperse spheroids. On the basis of closed numerical experiments, the accuracy of aerosol retrievals is investigated in error-free conditions and in the presence of measurement errors. The results of the algorithm applying to almucantar scans obtained at Dakar AERONET site during desert dust events in 2012 are discussed. The retrieved aerosol optical and microphysical characteristics are compared with those obtained by the AERONET inversion algorithm.

T‐matrix studies of aerosol particle shape effects on IR resonance spectral line profiles and comparison with an experiment

Experimental infrared resonance absorption line profiles are compared with results from T‐matrix theory calculations for several mineral components of atmospheric dust (illite, kaolinite, montmorillonite, quartz, and calcite). The model results are used to infer general characteristics of the aerosol particle shape distribution. For the silicate clays the spectral line profiles are best fit by a shape distribution of highly eccentric oblate spheroids, consistent with the expected sheet‐like nature of the clay minerals. For quartz dust the spectral line profiles are best fit by a very broad distribution including both extreme oblate and prolate spheroids. For calcite a spheroid model with moderate shape parameters gives the best fit. Our results suggest that high‐resolution IR extinction measurements may offer useful insight into the shape distributions of atmospheric mineral dust.

Radiative properties of aerosol mixture observed during the dry season 2006 over M'Bour, Senegal (African Monsoon Multidisciplinary Analysis campaign)

During the dry season in the Western Africa region the outbreaks of biomass burning aerosols occur on a background of frequent dust storms. In the frame of the African Monsoon Multidisciplinary Analysis (AMMA) campaign we studied optical properties and radiative effects of aerosol mixture and dust observed from January to March 2006 over M'Bour, Senegal. The aerosol burden during the observing period was notably below the average of previous years. However, variability of aerosol sizes and spectral absorption were found as representative. The lidar obtained extinction profiles suggested aerosol transport in two layers (or one elevated layer) for events with two different aerosol types, called aerosol mixture in the paper. Low‐altitude transport was observed for dust. The broadband solar radiative fluxes and aerosol radiative effects were analyzed by combining measurements and simulations. The simulations relied on retrieved column aerosol properties and are products of Aerosol Robotic Network data processing. The 24‐h average aerosol radiative effect yielded cooling at the bottom and at the top of the atmosphere for all the analyzed events of aerosol mixture and dust. The radiative efficiency (radiative effect scaled by optical thickness) at the bottom of the atmosphere and within the aerosol layer appeared to be stronger in case of mixture than in case of pure dust due to enhanced absorption ability of former. We also evaluated the importance of accounting for dust particles nonsphericity for simulating radiative effects. Our test showed that neglecting of aerosol particle shape can result in pronounced bias, up to 10% overestimation of daily average aerosol radiative effect.

Simultaneous retrieval of aerosol and surface properties from a combination of AERONET and satellite data

We present a method for simultaneously retrieving aerosol and surface parameters from ground-based and satellite observations collocated in space and time. We show that a combination of down and up-looking observations provides sufficient measurement constraints for characterizing both aerosol and surface properties with minimal assumptions. In order to employ this concept in AERONET processing, the standard inverse algorithm [Dubovik, O. & King, M. D. (2000), A flexible inversion algorithm for retrieval of aerosol optical properties from sun and sky radiance measurements. Journal of Geophysical Research, 105, 20673–20696] has been modified to retrieve surface reflectance in addition to aerosol parameters when co-incident satellite measurements are available. The method was applied to observations of smoke and desert dust over the Mongu (Zambia) and Solar Village (Saudi Arabia) AERONET sites respectively. The AERONET data were complemented by available observations from the MISR, MODIS, and POLDER-2 satellite sensors. The retrieved bidirectional reflectance factor (BRF) and surface albedo comparison shows good agreement between results obtained using observations from different satellites. The robustness of the method is tested by analyzing surface albedo time series retrieved during periods of high aerosol optical depth variability and low seasonal changes in surface reflectance. The analysis shows that the performance of retrieval algorithm is stable under different aerosol loadings. It is shown that much of the observed surface albedo temporal variability could be attributed mostly to the combined uncertainty in satellite radiometric calibration and aerosol vertical distribution for Mongu and to differences in satellite angular sampling on different days for Solar Village. The sensitivity of surface retrievals to assumptions on aerosol vertical distribution and aerosol particle shape are analyzed. It is found that the maximum error in retrieved surface albedo at 0.44 μm is 0.035 for aerosol optical depth 0.85 at 0.44 μm. For aerosol optical depths lower than ∼ 0.7 the error in retrieved surface albedo is less than 0.02. Analysis of particle shape assumptions on surface retrievals showed that aerosol particle non-sphericity significantly affects the angular shape of BRF, but not the surface albedo. Finally, the sensitivity of AERONET aerosol retrievals to uncertainty in assumed surface reflectance is analyzed by comparing aerosol retrievals obtained with different surface assumptions. It is found that the uncertainty in surface reflectance model employed in the version 1 AERONET operational algorithm is larger than was previously assumed in [Dubovik, O., Smirnov, A., Holben, B. N., King, M. D., Kaufman, Y. J., Eck, T. F., & Slutsker, I. (2000), Accuracy assessment of aerosol optical properties retrieved from AERONET sun and sky radiance measurements. Journal of Geophysical Research, 105, 9791–9806] and may have more significant effect on the retrieved aerosol properties than was documented in that work. In particular, larger errors were encountered for the real part of the refractive index (∼ 0.05–0.07 increase) and maximum of the particle size distribution (∼ 20% decrease) retrievals for the Mongu case, when the aerosol optical depth was relatively small (∼ 0.4 at 0.44 μm). The retrieved single scattering albedo uncertainties were within the error bars (0.03) estimated in [Dubovik, O., Smirnov, A., Holben, B. N., King, M. D., Kaufman, Y. J., Eck, T. F., & Slutsker, I. (2000), Accuracy assessment of aerosol optical properties retrieved from AERONET sun and sky radiance measurements. Journal of Geophysical Research, 105, 9791–9806], with the exception of the 0.44 μm retrievals for the desert dust case when they increased by ∼ 0.09 and 0.07 for low and high aerosol loadings respectively.

Irregular shape of particles and the Martian aerosols' properties

The influence of the irregular shape of aerosol particles on the scattering phase function is studied, with emphasis on the interpretation of Mars spectrophotometry data (the KRFM experiment of the Phobos mission). An early application of Mie theory to this problem has resulted in some estimations of the Fe 2O 3 content in Martian surface material (Moroz et al., Planet. Space Sci. 41, 569–585, 1993). The present work supposed the conclusion that such absorption is necessary, even if the particles are not spherical. However, the abundance of the absorber can be lower if irregularly shaped particles are considered, rather than spherical ones.

Selective interaction of a train of laser pulses with an aerosol medium

Stimulated scattering of light by oscillations of the shape of aerosol particles, excited resonantly by a train of high-intensity laser pulses, was detected and investigated. The excitation of natural oscillations of fog drops could be used to determine their size, to induce selective fragmentation, and to generate monochromatic microwave radiation in the presence of an external electric field.