Enhancement of atmospheric radiation by an aerosol layer

Abstract

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol-forming material into the stratosphere byvolcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of "photon trapping. Consider a stratospheric H2SO4 aerosol layer formed by a volcanic eruption. This can be modeled as a nearly conservatively scattering layer in a Rayleigh-scattering atmosphere (layer B in Figure 1). Intuition suggests that the introduction of the scattering would lead to an increase in the actinic flux above the aerosol layer (region A in Figure 1) due to back scattering, or within the aerosol layer (region B) due to multiple scattering. However, we might not expect increases in the actinic flux beneath the aerosol layer (region C), for the following reasons. In region C there is additional actinic flux due to multiple scattering by the aerosol layer B, but we can argue that this flux is originally in the direct solar beam. It has been removed by layer B, which redistributes the radiation into the backward direction (region A) and the forward direction (region C). Therefore it follows that the sum of the attenuated direct solar flux and the diffuse actinic flux in region C should not exceed the original actinic flux when the aerosol layer is absent. We might also anticipate that the absolute value of the irradiance would decrease at the surface with the introduction of the aerosol layer, reflecting the loss to space of backscattered photons. (In this paper, variation of the irradiance or its components will be discussed always with regard to absolute values.)We will show, however, that under certain circumstances the actinic flux below the aerosol layer can increase without violating conservation of photons and under special conditions the irradiance also can increase below the aerosol layer, with respect to the clear sky situation. Our new radiative transfer model is described and utilized to explore the effects of aerosols.

What do satellite backscatter ultraviolet and visible spectrometers see over snow and ice? A study of clouds and ozone using the A-train

“…This is due to light that penetrates through an optically thin cloud and undergoes enhanced scattering between the cloud and bright surface . This photon trapping effect has been well established in the literature (e.g., Michelangeli et al, 1992;Rozanov et al, 2004a,b). The effect can lead to higher retrieved scene pressures for optically thin clouds (τ < 1 − 8) over snow and ice as compared with values of P scene for a dark surface.…”

Effect of aerosols and NO&lt;sub&gt;2&lt;/sub&gt; concentration on ultraviolet actinic flux near Mexico City during MILAGRO: measurements and model calculations

“…Such changes in O 3 due to aerosol-induced UV perturbations are comparable to or larger than O 3 reductions currently practical with VOC and NO x emission regulations. The aerosolinduced AF perturbations have been characterized by many modeling studies under different conditions (e.g., Michelangeli et al, 1992;He and Carmichael, 1999;Jacobson, 1998;Liao et al, 1999;Yang and Levy, 2004) and have been parameterized in chemistry-transport models such as WRFChem (Grell et al, 2005), CMAQ (Byun and Ching, 1999), CAMx (ENVIRON, 2010), and MOZART (Tie et al, 2005). Quantitative comparisons between measured and simulated AF in the presence of aerosols are fewer, but generally show good agreement from ground stations (Früh et al, 2003) and from aircraft (Kelley et al, 1995;Volz-Thomas et al 1996;Meloni et al, 2003), although discrepancies at high dust loading were also noted (Junkermann et al, 2002).…”

Actinic fluxes in Titan's atmosphere, from one to three dimensions: Application to high‐latitude composition

“…This is the mean intensity at a given point in the atmosphere, which is different from the net vertical flux used for calculations of radiative heating [Michelangeli et al, 1992]. This actinic flux is computed with the radiative transfer Monte Carlo code developed by Toublanc et al [1995] and Toublanc [1996], modified to use fractal aerosol cross sections [Rannou et al, 1995].…”