Nonsphericity Of Aerosols Research Articles

Simulation and evaluation study of atmospheric aerosol nonsphericity as a function of particle size

Aerosol nonsphericity causes great uncertainty in radiative forcing assessments and climate simulations. Although considerable studies have attempted to quantify this uncertainty, the relationship between aerosol nonsphericity and particle size is usually not considered, thus reducing the accuracy of the results. In this study, a coupled inversion algorithm combining an improved stochastic particle swarm optimization algorithm and angular light scattering is used for the nonparametric estimation of aerosol nonsphericity variation with particle size, and the optimal sample selection method is employed to screen the data. Based on the verification of inversion accuracy, the variation of aerosol aspect ratio with particle size based on the ellipsoidal model in global regions has been obtained from Aerosol Robotic Network (AERONET) data, and the effect of nonsphericity on radiative forcing and dry deposition has been studied. The results show that the aspect ratio increases with particle size in all regions, with the maximum ranging from 1.4 to 1.8 in the desert, reflecting the differences in aerosol composition at different particle sizes. In radiation calculations, considering aerosol nonsphericity makes the aerosol cooling effect weaker and surface radiative fluxes increase, but hardly changes the aerosol absorption, with maximum differences of 9.22% and 22.12% at the bottom and top of the atmosphere, respectively. Meanwhile, the differences in radiative forcing between aspect ratios as a function of particle size and not varying with particle size are not significant, averaging less than 2%. Besides, the aspect ratio not varying with particle size underestimates the deposition velocity of small particles and overestimates that of large particles compared to that as a function of particle size, with maximum differences of 7% and 4%, respectively.

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.

Optical Modeling of Sea Salt Aerosols: The Effects of Nonsphericity and Inhomogeneity

AbstractThe nonsphericity and inhomogeneity of marine aerosols (sea salts) have not been addressed in pertinent radiative transfer calculations and remote sensing studies. This study investigates the optical properties of nonspherical and inhomogeneous sea salts using invariant imbedding T‐matrix simulations. Dry sea salt aerosols are modeled based on superellipsoidal geometries with a prescribed aspect ratio and roundness parameter. Wet sea salt particles are modeled as coated superellipsoids, as spherical particles with a superellipsoidal core, and as homogeneous spheres depending on the level of relative humidity. Aspect ratio and roundness parameters are found to be critical to interpreting the linear depolarization ratios (LDRs) of NaCl crystals from laboratory measurements. The optimal morphology parameters of NaCl necessary to reproduce the measurements are found to be consistent with data gleaned from an electron micrograph. The LDRs of wet sea salts are computed based on inhomogeneous models and compared with the measured data from ground‐based LiDAR. The dependence of the LDR on relative humidity is explicitly considered. The increase in the LDR with relative humidity at the initial phase of deliquescence is attributed to both the size increase and the inhomogeneity effect. For large humidity values, the LDR substantially decreases because the overall particle shape becomes more spherical and the inhomogeneity effect in a particle on the LDR is suppressed for submicron sea salts. However, the effect of inhomogeneity on optical properties is pronounced for coarse‐mode sea salts. These findings have important implications for atmospheric radiative transfer and remote sensing involving sea salt aerosols.

Enhanced Bottom-of-the-Atmosphere Cooling and Atmosphere Heating Efficiency by Mixed-Type Aerosols: A Classification Based on Aerosol Nonsphericity

The current understanding of the climate effects of mixed-type aerosols is an open question. The optical and radiative properties of the anthropogenic, mixed-type, and dust aerosols were studied using simultaneous observations of a sun photometer and a depolarization lidar over the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL), northwestern China. The aerosol radiative effect was calculated using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model and was in good agreement with the Aerosol Robotic Network (AERONET) product. The anthropogenic, mixed-type, and dust aerosols were identified mainly based on the lidar-measured depolarization ratio, which was supported by the airmass back trajectories. The mixed-type aerosols exhibit lower (higher) extinctions below (above) 1.5 km above the ground, indicating anthropogenic pollution from the atmospheric boundary layer and dust aerosols above. The dust aerosols exhibit the highest absolute radiative effect because of the highest aerosol loading. However, the mixed-type aerosols are effective in both scattering and absorbing solar radiation, leading to the highest cooling efficiency at the bottom of the atmosphere (BOA), 7.4% and 6.5% higher than those of the anthropogenic and dust aerosols, respectively. The mixed-type aerosols exhibit the highest warming efficiency in the atmosphere (ATM), 20.8% and 28.2% higher than the anthropogenic and dust aerosols, respectively. The mixed-type aerosols also show the lowest cooling efficiency at the top of the atmosphere (TOA). The results suggest the necessity of carefully characterizing the mixed-type aerosols in atmospheric numerical models to more precisely assess the energy budget of the Earth–atmosphere system.

Importance of aerosol non-sphericity in estimating aerosol radiative forcing in Indo-Gangetic Basin

Aerosols are usually presumed spherical in shape while estimating the direct radiative forcing (DRF) using observations or in the models. In the Indo-Gangetic Basin (IGB), a regional aerosol hotspot where dust is a major aerosol species and has been observed to be non-spherical in shape, it is important to test the validity of this assumption. We address this issue using measured chemical composition at megacity Delhi, a representative site of the western IGB. Based on the observation, we choose three non-spherical shapes - spheroid, cylinder and chebyshev, and compute their optical properties. Non-spherical dust enhances aerosol extinction coefficient (βext) and single scattering albedo (SSA) at visible wavelengths by >0.05km−1 and >0.04 respectively, while it decreases asymmetry parameter (g) by ~0.1. Accounting non-sphericity leads top-of-the-atmosphere (TOA) dust DRF to more cooling due to enhanced backscattering and increases surface dimming due to enhanced βext. Outgoing shortwave flux at TOA increases by up to 3.3% for composite aerosols with non-spherical dust externally mixed with other spherical species. Our results show that while non-sphericity needs to be accounted for, choice of shape may not be important in estimating aerosol DRF in the IGB.

Factors for inconsistent aerosol single scattering albedo between SKYNET and AERONET

AbstractSKYNET and Aerosol Robotic Network (AERONET) retrieved aerosol single scattering albedo (SSA) values of four sites, Chiba (Japan), Pune (India), Valencia (Spain), and Seoul (Korea), were compared to understand the factors behind often noted large SSA differences between them. SKYNET and AERONET algorithms are found to produce nearly same SSAs for similarity in input data, suggesting that SSA differences between them are primarily due to quality of input data due to different calibration and/or observation protocols as well as difference in quality assurance criteria. The most plausible reason for high SSAs in SKYNET is found to be underestimated calibration constant for sky radiance (ΔΩ). The disk scan method (scan area: 1° × 1° area of solar disk) of SKYNET is noted to produce stable wavelength‐dependent ΔΩ values in comparison to those determined from the integrating sphere used by AERONET to calibrate sky radiance. Aerosol optical thickness (AOT) difference between them can be the next important factor for their SSA difference, if AOTs between them are not consistent. Inconsistent values of surface albedo while analyzing data of SKYNET and AERONET can also bring SSA difference between them, but the effect of surface albedo is secondary. The aerosol nonsphericity effect is found to be less important for SSA difference between these two networks.

The Asian Dust and Aerosol Lidar Observation Network (AD-NET): Strategy and Progress

We have operated a ground-based lidar network AD-Net using dual wavelength (532, 1064nm) depolarization Mie lidar continuously and observed movement of Asian dust and air pollution aerosols in East Asia since 2001. This lidar network observation contributed to understanding of the occurrence and transport mechanisms of Asian dust, validation of chemical transport models, data assimilation and epidemiologic studies. To better understand the optical and microphysical properties, externally and internally mixing states, and the movements of Asian dust and airpollution aerosols, we go forward with introducing a multi-wavelength Raman lidar to the AD-Net and developing a multi-wavelength technique of HSRL in order to evaluate optical concentrations of more aerosol components. We will use this evolving AD-Net for validation of Earth-CARE satellite observation and data assimilation to evaluate emissions of air pollution and dust aerosols in East Asia. We go forward with deploying an in-situ instrument polarization optical particle counter (POPC), which can measure size distributions and non-sphericity of aerosols, to several main AD-Net sites and conducting simultaneous observation of POPC and lidar to clarify internally mixed state of Asian dust and air pollution aerosols transported from the Asian continent to Japan.

Observation and simulation study of atmospheric aerosol nonsphericity over the Loess Plateau in northwest China

Aerosol nonsphericity, which is not well depicted in model calculations, seriously affects aerosol optical properties and subsequently alters the radiative forcing of the earth–atmosphere system. Based on aerosol backscattering linear depolarization ratio data observed by a polarization lidar at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) from September 2009 to August 2012 and numerical computations, the spatial and temporal distribution of the aerosol depolarization ratio, parameterization of the derived aspect ratio and influence of water vapor on aerosol nonsphericity were investigated. Aerosol nonsphericity varied considerably by season, with a pronounced maximum in the spring, when more nonspherical aerosols were transported upward to the free troposphere; moreover, the column-averaged lidar depolarization ratios were 0.13, 0.09, 0.08 and 0.10 for the spring, summer, autumn and winter, respectively. The derived aerosol aspect ratio, a simplified parameter that describes the particle nonsphericity, ranged from 1.00 to 1.30 and peaked at approximately 1.06. A modified log-normal function, which was fitted to the frequency distribution of the derived aspect ratios, yielded a log-normal distribution parameterization for this parameter and provided a better shape input for the aerosol optical modeling. The monthly averaged aspect ratios reached a maximum of 1.13 during the spring and a minimum of 1.04 in autumn. The depolarization ratios decreased significantly with column-integrated increasing precipitable water in summer when there was sufficient precipitable water.

Impact of particle nonsphericity on the development and properties of aerosol models for East Asia

In this paper, the effects of aerosol nonsphericity information on the classification of aerosol models and the associated radiative properties over East Asia are investigated. The radiance measurements and inversions of the Aerosol Robotic Network (AERONET) are used. Four aerosol models over East Asia are obtained by adding the shape information to the clustering analysis. These four aerosols are identified on the basis of their optical properties. Compared to the results without sphericity parameter, adding the sphericity parameter in the clustering process contributes to the extraction of a strongly absorbing aerosol. Furthermore, the effect of the physical and optical properties of the aerosol on the top of atmospheric (TOA) total reflectance and polarized reflectance are investigated. The results indicate that the addition of the sphericity parameter in the clustering process leads to a change in the total reflectance by up to 16% and a change in the polarized reflectance by up to 100%.

Quantitative evaluation of uncertainties in satellite retrieval of dust-like aerosols induced by spherical assumption

It has been frequently pointed out the necessity to consider non-sphericity of dust-like aerosols in modeling their optical properties. However, most existing aerosol retrieval algorithms still utilize the Mie theory in defining aerosol models and creating look up tables (LUTs) for simplicity. Neglecting non-sphericity can severely influence aerosol optical depth (AOD, τ) retrieval in case of dust-like aerosols—largely due to the difference of derived phase functions under spherical and non-spherical assumptions—whereas this uncertainty has rarely been quantitatively studied. This paper aims at a better understanding of uncertainties on AOD retrieval caused by aerosol non-sphericity. From a dust aerosol model, we adopt different methods to simulate and compare aerosol optical properties on the basis of spherical and non-spherical assumptions respectively. Consequently we generate two LUTs under varieties of observing conditions, aerosol loadings, and surface brightness using radiative transfer (RT) code. From the obtained LUTs we thoroughly evaluate and analyze the differences of TOA reflectance (ΔρTOA) and AOD retrieval errors (Δτ) induced by straightforwardly utilizing the Mie theory in dust-like aerosol retrieval. Errors may be positive or negative, depending on the specific geometry. For low aerosol loading (τ~0.25) and black surface, |ΔρTOA| could be greater than 0.06, with maximum |Δτ| of ~0.12. Moreover, this error can increase more than ten times and become even irregular as aerosol loading gets higher and surface gets brighter. Mean Δτ statistics show that this uncertainty significantly influences derived aerosol climatology as well. Therefore we conclude that the neglect of non-sphericity introduces substantial errors on RT simulation and AOD retrieval, and a representative aspheric aerosol model other than Mie calculation is recommended for inversion algorithms related with dust-like non-spherical aerosols.

MISR Dark Water aerosol retrievals: operational algorithm sensitivity to particle non-sphericity

Abstract. The aim of this study is to theoretically investigate the sensitivity of the Multi-angle Imaging SpectroRadiometer (MISR) operational (version 22) Dark Water retrieval algorithm to aerosol non-sphericity over the global oceans under actual observing conditions, accounting for current algorithm assumptions. Non-spherical (dust) aerosol models, which were introduced in version 16 of the MISR aerosol product, improved the quality and coverage of retrievals in dusty regions. Due to the sensitivity of the retrieval to the presence of non-spherical aerosols, the MISR aerosol product has been successfully used to track the location and evolution of mineral dust plumes from the Sahara across the Atlantic, for example. However, the MISR global non-spherical aerosol optical depth (AOD) fraction product has been found to have several climatological artifacts superimposed on valid detections of mineral dust, including high non-spherical fraction in the Southern Ocean and seasonally variable bands of high non-sphericity. In this paper we introduce a formal approach to examine the ability of the operational MISR Dark Water algorithm to distinguish among various spherical and non-spherical particles as a function of the variable MISR viewing geometry. We demonstrate the following under the criteria currently implemented: (1) Dark Water retrieval sensitivity to particle non-sphericity decreases for AOD below about 0.1 primarily due to an unnecessarily large lower bound imposed on the uncertainty in MISR observations at low light levels, and improves when this lower bound is removed; (2) Dark Water retrievals are able to distinguish between the spherical and non-spherical particles currently used for all MISR viewing geometries when the AOD exceeds 0.1; (3) the sensitivity of the MISR retrievals to aerosol non-sphericity varies in a complex way that depends on the sampling of the scattering phase function and the contribution from multiple scattering; and (4) non-sphericity artifacts occur at those view-illumination geometries where dust aerosols are indistinguishable from certain types of cirrus particles. Based on these results, we suggest that interested parties use caution with the version 22 MISR Dark Water aerosol non-sphericity product in situations where cirrus may be present.

Estimating the direct aerosol radiative perturbation: Impact of ocean surface representation and aerosol non‐sphericity

AbstractAtmospheric aerosols are now actively studied, in particular because of their radiative and climate impacts. Estimations of the direct aerosol radiative perturbation, caused by extinction of incident solar radiation, usually rely on radiative transfer codes and involve simplifying hypotheses. This paper addresses two approximations which are widely used for the sake of simplicity and limiting the computational cost of the calculations. Firstly, it is shown that using a Lambertian albedo instead of the more rigorous bidirectional reflectance distribution function (BRDF) to model the ocean surface radiative properties leads to large relative errors in the instantaneous aerosol radiative perturbation. When averaging over the day, these errors cancel out to acceptable levels of less than 3% (except in the northern hemisphere winter). The other scope of this study is to address aerosol non‐sphericity effects. Comparing an experimental phase function with an equivalent Mie‐calculated phase function, we found acceptable relative errors if the aerosol radiative perturbation calculated for a given optical thickness is daily averaged. However, retrieval of the optical thickness of non‐spherical aerosols assuming spherical particles can lead to significant errors. This is due to significant differences between the spherical and non‐spherical phase functions. Discrepancies in aerosol radiative perturbation between the spherical and non‐spherical cases are sometimes reduced and sometimes enhanced if the aerosol optical thickness for the spherical case is adjusted to fit the simulated radiance of the non‐spherical case. Copyright © 2004 Royal Meteorological Society

Aerosol optical properties from columnar data during the second Aerosol Characterization Experiment on the south coast of Portugal

Measurements of aerosol optical properties over the atmospheric column (aerosol optical thickness, spectral angular sky radiance (sky brightness), and downwelling hemispheric flux) have been used to derive climate‐relevant aerosol parameters such as the phase function, the broadband single‐scattering albedo, and the refractive index. These parameters are needed to estimate the direct short‐wave radiative forcing by aerosols and to validate aerosol models in the satellite retrieval algorithms. Values of the broadband single‐scattering albedo obtained in this study range between ω0 = 0.98 (marine aerosols) and 0.90 (continental pollution aerosols). The columnar ambient broadband refractive index is found to be m = 1.39 ± 0.044 − i (<0.003) for marine conditions and m = 1.48 ± 0.058 − i (0.01 ± 0.003) for polluted continental aerosols. Nonsphericity is shown to be important in the case of marine aerosols. Moreover, aerosol nonsphericity gives an additional contribution to the negative short‐wave radiative forcing of marine aerosols under clear‐sky conditions, which can be estimated as being 30 up to 50% of the radiative forcing estimated for spherical marine aerosols. In the case of continental polluted aerosols the optical properties can be represented by spherical particles, and no additional shape effect has to be considered. However, the aerosol absorption leads to an increase of about 40% of the radiative forcing estimated for nonabsorbing aerosol of the same size distribution.

Case study of Raman lidar measurements of Asian dust events in 2000 and 2001 at Nagoya and Tsukuba, Japan

Vertical distributions of aerosol optical properties and relative humidity were measured with a Raman lidar at Nagoya and Tsukuba, Japan, during the Asian dust events in 2000 and 2001. The data obtained on 23 April 2001 showed a maximum of aerosol backscattering ratio ( R) of 3.5–4.0 at 532 nm at an altitude of 5 km over both measurement sites. Around that height the aerosol depolarization ratios ( δ a), which indicate the aerosol nonsphericity, were higher than 20% and the relative humidities (RH) were lower than 30%. The aerosol optical thickness between 4 and 7 km was 0.18±0.02 and the average aerosol extinction-to-backscatter ratio (lidar ratio) was 46±5 sr at Tsukuba. This aerosol layer was present for over 6 h and finally showed the highest δ a value of about 33% and the lidar ratio of 10±3 sr at the uppermost region, where the RH was almost saturated with respect to ice. The data obtained on 4 May 2000 at Nagoya showed relatively high R values (∼2.5) below 5 km . The values of δ a were higher than 15% between 3.5 and 8 km in which the maximum RH was about 60%. The values of δ a and RH showed a weak negative correlation below 3.5 km , where the RH varied between 30% and 70%. In the 5– 8 km region, the δ a values were correlated negatively with the wavelength exponent of the aerosol backscattering coefficient ( α), whereas they showed the lowest value (∼7%) at smaller α values (<0) in the 2– 5 km region. We hypothesized that these relations of δ a to α and RH are due to the mixing state of mineral dust, sea-salt, and sulfate-containing particles that are major aerosol constituents in the free troposphere during the Asian dust period and to the dependence of their shape and size on RH.