Modeled and Observed Relationships Between Mass Absorption Cross Section (MAC) and Absorption Ångström Exponent (AAE)
MARJAN SAVADKOOHI, Aki Virkkula, Marco Pandolfi, Andrés Alastuey, Philip K. Hopke, Institute of Environmental Assessment and Water Research
Abstract Number: 86
Working Group: Aerosol Physics
Abstract
1. Introduction
Optical properties (MAC, SSA, and AAE) of BC particles coated with a light-scattering shell were calculated for unimodal size distributions (GMD range 50-500 nm, GSD 1.4, 1.6, 1.8, 2.0) for core volume fractions fc = 0.01 – 100% at the seven Aethalometer wavelengths. We present main results of those simulations and some comparisons with field measurement data.
2. Simulations
A significant correlation was observed between MAC and the AAE difference ΔAAE=AAE (470/950)-AAE (370/470). In a surface plot, they have their maxima and minima roughly at the same fc vs GMD "coordinates". The scatter plots of MAC vs ΔAAE show that they have approximately linear relationships.
3. Measurement data, MAC vs ΔAAE
The simulated relationships were compared with analogous relationships found from three European measurement sites, Barcelona, Athens, and Kosetice. April-July probably had significant biogenic oxidized organics forming a particle scattering coating. The vast majority of ΔAAE > 0, agreeing with the assumption that light scattering material coats the particles. December-February is a period when light-absorbing carbon (BC and BrC) particles emitted from heating are present. This source explains the large fraction of ΔAAE < 0 in the MAC vs ΔAAE scatter plots. In this period, there were no sharp increases in MAC values at ΔAAE > 0, likely due to coating of BC cores with light-absorbing material. In the simulations, the MAC peak was due to coating with non-absorbing material.
4. Conclusions
Comparison of the Mie-modeled MAC vs ΔAAE with the observations suggests that particles can be treated as particles when processing filter-based absorption photometer data, not just as uniform layers like paint on the filter. The observed MAC vs ΔAAE relationships may prove to be useful in developing accurate BC and BrC measurement methods, especially for Aethalometer data reprocessing. More laboratory experiments and model development are needed.