Light absorption by aerosols contributes to solar radiative forcing through absorption of solar radiation and heating of the absorbing aerosol layer. Besides the direct radiative effect, the heating can evaporate clouds and change the atmospheric dynamics. Aerosol light absorption in the atmosphere is dominated by black carbon (BC) with additional, significant contributions from the still poorly understood brown carbon and from mineral dust. Sources of these absorbing aerosols include biomass burning and other combustion processes and dust entrainment. For particles much smaller than the wavelength of incident light, absorption is proportional to the particle volume and mass. Absorption can be calculated with Mie theory for spherical particles and with more complicated numerical methods for other particle shapes. The quantitative measurement of aerosol light absorption is still a challenge. Simple, commonly used filter measurements are prone to measurement artifacts due to particle concentration and modification of particle and filter morphology upon particle deposition, optical interaction of deposited particles and filter medium, and poor angular integration of light scattered by deposited particles. In situ methods measure particle absorption with the particles in their natural suspended state and therefore are not prone to effects related to particle deposition and concentration on filters. Photoacoustic and refractive index-based measurements rely on the heating of particles during light absorption, which, for power-modulated light sources, causes an acoustic signal and modulation of the refractive index in the air surrounding the particles that can be quantified with a microphone and an interferometer, respectively. These methods may suffer from some interference due to light-induced particle evaporation. Laser-induced incandescence also monitors particle heating upon absorption, but heats absorbing particles to much higher temperatures to quantify BC mass from the thermal radiation emitted by the heated particles. Extinction-minus-scattering techniques have limited sensitivity for measuring aerosol light absorption unless the very long absorption paths of cavity ring-down techniques are used. Systematic errors can be dominated by truncation errors in the scattering measurement for large particles or by subtraction errors for high single scattering albedo particles. Remote sensing techniques are essential for global monitoring of aerosol light absorption. While local column-integrated measurements of aerosol light absorption with sun and sky radiometers are routinely done, global satellite measurements are so far largely limited to determining a semi-quantitative UV absorption index.

Aerosol light absorption and its measurement: A review

http://www.ifuap.buap.mx/~upal/assets/159.pdf

Scattering of electromagnetic radiation by a multilayered sphere

Whispering gallery modes (WGMs) have been exploited for a broad range of sensing applications. However, the vast majority of WGM sensors consist of passive resonators, requiring complex interrogation systems to be employed, ultimately limiting their practicality. Active resonators containing a gain medium, allowing remote excitation and collection of the WGM-modulated fluorescence spectra, have emerged as an alternative to passive resonators. Although research is still in its infancy, recent progress has reduced the performance gap between the two paradigms, fueled by the potential for new applications that could not previously be realized. Here, recent developments in sensors based on active WGM microresonators are reviewed, beginning with a discussion of the theory of fluorescence-based and lasing WGMs, followed by a discussion of the variety of gain media, resonator architectures, and emerging sensing applications. We conclude with a discussion of the prospects and future directions for improving active WGM sensors.

Fluorescent and lasing whispering gallery mode microresonators for sensing applications

The new Mie-scattering calculation is a robust and efficient algorithm used to compute light scattering from spheres. It calculates the ratio between Riccati-Bessel functions instead of the complicated logarithmic derivative. The Kapteyn inequality is used to estimate the number of significant digits of the calculated Riccati-Bessel functions and their ratio. This new algorithm is stable and accurate for both large and small particles. The implemented C++ code yields the same accurate results for both small and large particles compared with Wiscombe's MIEV0 code in double precision. Suggestions are provided for the porting of the MIEV0 code.

Mie-scattering calculation.

During the year 2008, we have been commemorating, in several places, the hundredth anniversary of the famous 1908-paper by Mie describing the interaction between an electromagnetic plane wave and a homogeneous sphere defined by its diameter d and its complex refractive index m . Due to the existence of a prior version by Lorenz, Mie's theory may also be named as Lorenz–Mie theory (LMT). The generalized Lorenz–Mie theory (GLMT) stricto sensu deals with the more general case when the illuminating wave is an arbitrary shaped beam (say: a laser beam) still interacting with a homogeneous sphere defined by its diameter d and its complex refractive index m . The name “GLMTs” is generically used to designate various variants for other particle shapes when the method of separation of variables is used. The present paper provides a review of the work accomplished in this generalized field during the last decade (the third decade). As a convenient selection criterion, only papers citing the work of the group of Rouen have been essentially used, with ISIweb of knowledge providing a database.

Generalized Lorenz-Mie theories, the third decade: A perspective

An improved recurrence algorithm to calculate the scattering field of a multilayered sphere is developed. The internal and external electromagnetic fields are expressed as a superposition of inward and outward waves. The alternative yet equivalent expansions of fields are proposed by use of the first kind of Bessel function and the first kind of Hankel function instead of the first and the second kinds of Bessel function. The final recursive expressions are similar in form to those of Mie theory for a homogeneous sphere and are proved to be more concise and convenient than earlier forms. The new algorithm avoids the numerical difficulties, which give rise to significant errors encountered in practice by previous methods, especially for large, highly absorbing thin shells. Various calculations and tests show that this algorithm is efficient, numerically stable, and accurate for a large range of size parameters and refractive indices.

Wen Yang

Papers

Improved recursive algorithm for light scattering by a multilayered sphere.