Abstract
Among many parameters characterizing atmospheric aerosols, aerosol mass extinction efficiency (MEE) is important for understanding the optical properties of aerosols. MEE is expressed as a function of the refractive indices (i.e., composition) and size distributions of aerosol particles. Aerosol MEE is often considered as a size-independent constant that depends only on the chemical composition of aerosol particles. The famous Malm’s reconstruction equation and subsequent revised methods express the extinction coefficient as a function of aerosol mass concentration and MEE. However, the used constant MEE does not take into account the effect of the size distribution of polydispersed chemical composition. Thus, a simplified expression of size-dependent MEE is required for accurate and conventional calculations of the aerosol extinction coefficient and also other optical properties. In this study, a simple parameterization of MEE of polydispersed aerosol particles was developed. The geometric volume–mean diameters of up to 10 µm with lognormal size distributions and varying geometric standard deviations were used to represent the sizes of various aerosol particles (i.e., ammonium sulfate and nitrate, elemental carbon, and sea salt). Integrating representations of separate small mode and large mode particles using a harmonic mean-type approximation generated the flexible and convenient parameterizations of MEE that can be readily used to process in situ observations and adopted in large-scale numerical models. The calculated MEE and the simple forcing efficiency using the method developed in this study showed high correlations with those calculated using the Mie theory without losing accuracy.
Highlights
The aerosol extinction coefficient bext is the key parameter that determines the aerosol optical depth by integrating over the light path; it is important for the calculation of aerosol radiative forcing and the criterion of air pollution
The MEEL for both ammonium sulfate (AMS) and ammonium nitrate (AMN) was approximated as MEEL = 2.5d−1 for all geometric standard deviations
It is shown that the Mie theory-based calculations of mass extinction efficiency (MEE) and those approximated using the method proposed in this study are comparable without much loss of accuracy
Summary
The aerosol extinction coefficient bext is the key parameter that determines the aerosol optical depth by integrating over the light path; it is important for the calculation of aerosol radiative forcing and the criterion of air pollution. Where bext is the extinction coefficient, Qext is the single particle extinction efficiency, dp is the particle diameter, and n(dp ) is the number particle size distribution. The Mie theory-based calculations use Equation (1) for the extinction coefficient computation by integrating the Mie scattering libraries (e.g., Qext ) over an observed particle size distribution. Equation (2) implies that the aerosol extinction coefficient is a product of aerosol mass concentration and mass extinction efficiency (MEE, see Figure 1), which is known as the reconstruction method (e.g., [1,2]). The strong dependence of both extinction efficiency (blue lines) and MEE (red lines) on the particle size and composition is revealed as reported in previous studies (e.g., [4,5])
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