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

Abstract

The 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.

How the Inhomogeneity of Wet Sea Salt Aerosols Affects Direct Radiative Forcing

“…However, the inhomogeneous structure of sea salt exists during the phase conversion. The assumption is appropriate for the accumulation step (Zeng et al, 2013) and may cause a little bit uncertainty in specifying the refractive index of mantle (the impact on the optical properties is estimated to be small according to Figure 15 in Bi et al, 2018). Zeng et al (2013) divide the deliquescence process into three steps: the accumulating step (34-75%), bulk melting step (75-86%), and delay dissolving step.…”

“…The use of an effective refractive index to represent a refractive index distribution associated with the inhomogeneity may lead to large uncertainties on the optical properties (Bi et al, 2018). This approach is appropriate for totally dissolve sea salt particles but might be insufficient for inhomogeneous sea salt aerosols, that is, coated wet sea salt during phase conversion.…”

“…In particular, the impact of inhomogeneity on the asymmetry factor becomes significant when the size parameter is larger than 5. However, the shape of the solid core has little impact on the extinction efficiency and asymmetry parameter, although they could be important to the other optical properties (such as phase matrix and depolarization ratios) according to the study by Bi et al (2018). Thus, a coated sphere is justified to be a simplified yet effective model to represent the inhomogeneity of sea salts aerosols, and the computational efficiency is significantly improved.…”

Assessing Superspheroids in Modeling the Scattering Matrices of Dust Aerosols

“…For n > 2.0, the octahedron becomes concave and shows sharp edges. It should be noted that another branch of superellipsoids (n < 1; not shown here) was employed in the study by Bi, Lin, Liu, et al (2018) and Bi, Lin, Wang, et al (2018) to model cube-like sea-salt aerosols.…”

“…Second, we asked if it is possible to build a general framework of model particles with a single equation, which can be more flexible in modeling various nonspherical aerosol particles. Such a rationale also follows the most recent study concerning sea salts' optical properties (Bi, Lin, Wang, et al, 2018), which uses a branch of superellipsoids to model sea-salt aerosols. Furthermore, the superspheroidal model is expected to be promising, because it also retains the rule of simplicity when building aerosol models.…”