Size Distribution Of Dust Aerosols Research Articles

Modelling the 2021 East Asia super dust storm using FLEXPART and FLEXDUST and its comparison with reanalyses and observations

The 2021 East Asia sandstorm began from the Eastern Gobi desert steppe in Mongolia on March 14, and later spread to northern China and the Korean Peninsula. It was the biggest sandstorm to hit China in a decade, causing severe air pollution and a significant threat to human health. Capturing and predicting such extreme events is critical for society. The Lagrangian particle dispersion model FLEXPART and the associated dust emission model FLEXDUST have been recently developed and applied to simulate global dust cycles. However, how well the model captures Asian dust storm events remains to be explored. In this study, we applied FLEXPART to simulate the recent 2021 East Asia sandstorm, and evaluated its performance comparing with observation and observation-constrained reanalysis datasets, such as the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) and CAMS global atmospheric composition forecasts (CAMS-F). We found that the default setting of FLEXDUST substantially underestimates the strength of dust emission and FLEXPART modelled dust concentration in this storm compared to that in MERRA-2 and CAMS-F. An improvement of the parametrization of bare soil fraction, topographical scaling, threshold friction velocity and vertical dust flux scheme based on Kok et al. (Atmospheric Chemistry and Physics, 2014, 14, 13023–13041) in FLEXDUST can reproduce the strength and spatio-temporal pattern of the dust storm comparable to MERRA-2 and CAMS-F. However, it still underestimates the observed spike of dust concentration during the dust storm event over northern China, and requires further improvement in the future. The improved FLEXDUST and FLEXPART perform better than MERRA-2 and CAMS-F in capturing the observed particle size distribution of dust aerosols, highlighting the importance of using more dust size bins and size-dependent parameterization for dust emission, and dry and wet deposition schemes for modelling the Asian dust cycle and its climatic feedbacks.

Accounting for dust aerosol size distribution in radiative transfer

AbstractThe impact of size distribution of mineral dust aerosol on radiative transfer was investigated using the Aerosol Robotic Network‐retrieved aerosol size distributions. Three methods for determining the aerosol optical properties using size distributions were discussed. The first is referred to as a bin method in which the aerosol optical properties are determined for each bin of the size distribution. The second is named as an assembly mean method in which the aerosol optical properties are determined with an integration of the aerosol optical parameters over the observed size distribution. The third is a normal parameterization method based on an assumed size distribution. The bin method was used to generate the benchmark results in the radiation calculations against the methods of the assembly mean, and parameterizations based on two size distribution functions, namely, lognormal and gamma were examined. It is seen that the assembly mean method can produce aerosol radiative forcing with accuracy of better than 1%. The accuracies of the parameterizations based on lognormal and gamma size distributions are about 25% and 5%, respectively. Both the lognormal and gamma size distributions can be determined by two parameters, the effective radius and effective variance. The better results from the gamma size distribution can be explained by a third parameter of skewness which is found to be useful for judging how close the assumed distribution is to the observation result. The parameterizations based on the two assumed size distributions are also evaluated in a climate model. The results show that the reflected solar fluxes over the desert areas determined by the scheme based on the gamma size distribution are about 1 W m−2 less than those from the scheme based on the lognormal size distribution, bringing the model results closer to the observations.

Size distribution of dust aerosols observed over the Horqin Sandy Land in Inner Mongolia, China

Abstract Particle size distributions (psds) of airborne dust (PM 20 ) during different dust emission events are investigated in this study, using data obtained from a dust-event monitoring station in the Horqin Sandy Land in Inner Mongolia, China. The results show that for a weak saltation-bombardment and aggregate-disintegration dust emission (SADE) event (0.44 u ∗ −1 ) on 7 April 2012, dust aerosols ⩽1 μm in diameter ( d ) accounted for 80% for all dusts measured. While for a strong SADE event (0.85 u ∗ −1 ) on the same day, large dust aerosols ( d ⩾ 2.5 μm) increased significantly, with the largest proportion (40%) located in 4–7 μm, which agreed with the airborne dust psds observed during another two strong SADE events (mean u ∗ = 0.78 and 0.68 m s −1 ) on 13–14 April 2013. However, for a convective turbulent dust emission (CTDE) event (mean u ∗ = 0.31 m s −1 ) on 17 April 2013, the mean proportion of dust aerosols u ∗ , but they remain unchanged when u ∗ doesn’t change too much. In addition, the dust psds for a non-local dust event on 19 April 2012 appeared smoother because of the mixing of dust aerosols through the processes of dust advection and deposition.

Optical properties and size distribution of dust aerosols over the Tengger Desert in Northern China

Abstract A field experiment was performed during 1 April–30 September 2001 in the southeast Tengger Desert in Northern China to measure the solar radiant flux by a solar direct radiometer and a multi-wavelength sun-photometer. The observation and research results are as follows. On fine days, dust aerosols attenuate the direct solar radiant flux by 2.6–47.0%, with an average of 16.9%. On dusty days, dust aerosols attenuate the direct solar radiant flux by 10–90%, with an average of 38%. The mean atmospheric turbidity for broadband (300–4000 nm) flux is 0.26 for fine days and 0.74 for dusty days. Under the typical background, floating dust, and dust storm weather conditions, the aerosol optical depths (AODs; at 550 nm) are about 0.1, 0.9, and 2.0, and the Angstrom exponents are about 2.0, 0.38, and −0.24, respectively. The mean AOD of the examples is 0.66, and 0.87 for the Angstrom exponents. On dusty days, the aerosol number concentration is 2–10 times higher than that on fine days. The aerosol size distribution is a multi-normal distribution during dusty conditions, while the aerosol size distribution is a logarithmic normal distribution during fine weather.

Roles of saltation, sandblasting, and wind speed variability on mineral dust aerosol size distribution during the Puerto Rican Dust Experiment (PRIDE)

Recent field observations demonstrate that a significant discrepancy exists between models and measurements of large dust aerosol particles at remote sites. We assess the fraction of this bias explained by assumptions involving four different dust production processes. These include dust source size distribution (constant or dynamically changing according to saltation and sandblasting theory), wind speed distributions (using mean wind or a probability density function (PDF)), parent soil aggregate size distribution, and the discretization (number of bins) in the dust size distribution. The Dust Entrainment and Deposition global model is used to simulate the measurements from the Puerto Rican Dust Experiment (PRIDE) (2000). Using wind speed PDFs from observed National Centers for Environmental Prediction winds results in small changes in downwind size distribution for the production which neglects sandblasting, but it results in significant changes when production includes sandblasting. Saltation‐sandblasting generally produces more large dust particles than schemes which neglect sandblasting. Parent soil aggregate size distribution is an important factor when calculating size‐distributed dust emissions. Changing from a soil with large grains to a soil with smaller grains increases by 50% the fraction of large aerosols (D >5 μm) modeled at Puerto Rico. Assuming that the coarse medium sand typical of West Africa dominates all source regions produces the best agreement with PRIDE observations.

Mineral dust aerosol size distribution change during atmospheric transport

Airborne mineral dust can impact visibility, climate, biogeochemical processes, and possibly human health. The magnitude of the impact of dust depends on particle size. We measured the size distributions of airborne mineral dust over the Canary Islands during July 1995 and Puerto Rico during July 2000. Dust size distributions do not appear lognormal. Stokes gravitational settling overestimates losses of large dust particles during atmospheric transport from North Africa over the tropical North Atlantic and Caribbean. Normalized mineral dust size distributions of particles smaller than 7.3 μm over the Canary Islands and Puerto Rico were indistinguishable, indicating these particles were not preferentially removed during atmospheric transport. However, mineral dust aerosols larger than 7.3 μm were preferentially removed during atmospheric transport. Larger particles were more efficiently removed. A simple empirical model setting the vertical velocity of dust particles equal to the Stokes gravitational settling velocity minus an upward velocity of ∼0.33 cm s−1 accurately predicts changes in dust size distribution during atmospheric transport. Thus it appears some atmospheric process(es) partially counteracts gravitational settling.

Climatology of dust aerosol size distribution and optical properties derived from remotely sensed data in the solar spectrum

Simultaneous spectral remote observations of dust properties from space and from the ground create a powerful tool for the determination of ambient dust properties integrated on the entire atmospheric column. The two measurement methods have a complementary sensitivity to variety of dust properties. The methodology is demonstrated using spectral measurements (0.47–2.21 μm) from Landsat TM over the bright Senegalian coast and dark ocean, and Aerosol Robotic Network (AERONET) radiances measured in several locations. We derive (1) the dust size distribution, showing a dominant coarse mode at 1–5 μm and a secondary mode around 0.5 μm effective radius; (2) dust absorption, which is found to be substantially smaller than reported from previous measurements; (3) the real part of the refractive index which varies within the range 1.53–1.46; and we show that (4) the effect of the dust nonspherical shape on its optical properties is not significant for scattering angles <120°.

Size distributions of dust aerosol measured during the Soviet-American experiment in Tadzhikistan, 1989

Size distribution data obtained during the U.S.S.R.-U.S. dust experiment make it possible to propose a general conception about the size distribution of dust aerosols within the size range 0.005–100 μm. The microstructure for the optically active fraction of arid aerosol is approximated in the form of a log-normal distribution with parameters D = 3.5−6 μ m, δ 2 = 0.5−0.8, which can be used when estimating radiative calculations.