Calculation of black carbon optical properties
The refractive index of black carbon (BC) is currently taken from the OPAC database (Hess et al., 1998) and is based on Shettle and Fenn (1979): 1.74-0.44i at 550 nm. However, Bond and Bergstrom (Aerosol Sci. Technol., 2006) have argued that this value should be retired. Instead they propose a refractive index of 1.95-0.79i at 550 nm or any other choice from their Table 5. Stier et al. (ACP, 2007) applied both this highly absorbing value and a medium absorbing value of 1.85-0.71i, and found that the medium absorbing value produced better agreement with observations. As Bond and Bergstrom only give values at 550 nm, the same functional shape of the refractive index versus wavelength was assumed. This means that a simple scaling can be applied in the model.
The impact on aerosol optical depth (AOD) and absorption AOD at 550 nm of scaling the refractive index to the highly absorbing value given above is shown in the figures. Using this value seems to improve AAOD compared to measurements. For instance, the left panels of Fig. 14 in Bond et al. (JGR, 2013) show the contribution to AAOD from carbonaceous aerosols derived from AERONET. These observational data can be directly compared with the AAOD maps from the model, as in the current version the absorption by mineral dust is very low at visible wavelengths.
However, comparison with other AAOD data is inconclusive with regard to the refractive index value to use. For instance, the model is high compared to AAOD from MODIS Collection 6 Deep Blue product over land, but low compared to Parasol AAOD observations over sea. For the moment, we therefore adopt the medium absorbing refractive index value of 1.85-0.71i at 550 nm.
The simplest method of predicting absorption and scattering for aggregates like BC is the Rayleigh-Debye-Gans (RDG) theory, which assumes that multiple scattering and interactions between the primary spherules within the aggregate are negligible. By comparing RDG theory with more exact calculations, several authors suggest that it reasonably approximates scattering and absorption of combustion-generated particles at visible wavelengths. Other studies have shown that absorption at infrared wavelengths could be underestimated, and that interactions between spherules could increase absorption by about 30% (up to about 50%), depending on the number and size of primary spherules.
By comparing RDG theory with Mie theory for particles of equivalent solid volume of equivalent mass, it can be concluded that Mie theory give reasonable estimates of scattering and absorption for particle's solid volume lower than that of a 100-nm sphere. For larger particles, Mie theory can greatly underpredict both absorption and scattering.
Note that the refractive index that should be used in Mie or RDG theory is that of void-free black carbon (Bond and Bergstrom, p. 20), as is the case for the value given above.
Since in RDG theory the particles are treated as a collection of solid spherules and in Mie theory as a solid sphere of equivalent mass, the particle density in these optical calculations should be that of void-free black carbon as well. This is the reason for Bond and Bergstrom to emphasize that the use of 1.0 g/cm3 should be abandoned. The density used in TM5 has been reduced from 2.0 to 1.75 g/cm3, which is still representative of void-free material.
How then should we account for the more open, aggregate structure of the particles when describing emissions and scavenging by wet removal?
Given the fact that the size distributions in the internal aerosol mixtures in the model are described by M7, it doesn't seem straightforward to switch between open aggregates to describe emissions and scavenging, and solid spheres for the Mie calculations. So, it seems we have to make a compromise here.
One option could be to assume that the particles are emitted to the atmosphere as solid spheres from the beginning. This would imply that the size of the emitted particles should not be the characteristic size of the aggregates (radius of gyration), but the volume-equivalent radius. It is not 100% clear what the values recommended for the AeroCom project (Dentener et al., 2006) actually represent. Given the fact that the emission radii presented in the overview paper by Bond et al. (JGR, 2013) correspond to volume-equivalent radii, it seems likely that the numbers given by Dentener et al. (2006) are also given as volume-equivalant radii, in which case the current implementation is correct.
#2 Updated by Twan van Noije over 6 years ago
- File AOD_mp_bcri_2006.eps added
- File AOD_mp_bcri-mp_mtime6_radii_soa_pom_nh4_dens_oc2pom_newradii_2006.eps added
- File AAOD_mp_bcri_2006.eps added
- File AAOD_mp_bcri-mp_mtime6_radii_soa_pom_nh4_dens_oc2pom_newradii_2006.eps added
- Status changed from New to Resolved