Sunday, July 8, 2007

Carbon Aerosols in Climate Models

"All models have assumed that particles are spherical and have chosen single values for the refractive index. If underlying model assumptions are inappropriate, then scattering, absorption and radiative forcing estimates will be incorrect."

This is one the statement made by Tami Bond and Robert Bergstrom in the review paper titled ‘Light Absorption by Carbonaceous Particles: An Investigative Review’ published in Aerosol Science and Technology, 2006.

I had chance to present this paper as class seminar in my ‘Atmospheric Aerosol’ class teaching by Dr. Kirk Fuller during this summer. Let me tell you this, “this is one of the best written paper I ever read on carbon aerosols”. It reads very well and has almost all the information you need to know about atmospheric carbon aerosols and their optical properties. Personally, I congratulate and thanks Tami and Robert for putting together this wonderful paper. Below, I will briefly touch some important points from the paper.

Motivation:

Tabulation of optical properties needs update and measured absorptive properties demonstrate variability that has not been represented in climate models.

Key Conclusion:

Optical properties of Light Absorbing Carbon (LAC) are necessary to model its effects on climate.
Mass absorption cross-section is 7.5±1.2 m2/g for at λ=0.55 µm uncoated LAC.
Highest refractive index for strongly absorbing LAC is 1.95-i*0.79 with narrow range and refractive index values used in current climate models are in error.
Spherical aggregates treatment using realistic refractive indices under predict measured absorption by about 30%

Recommendations:

Mass absorption cross-section 7.5±1.2 m2/g at λ=0.55 µm for fresh LAC, this may increase due to coating and decrease due to particle coagulation.

Single scattering albedo: 0.2-0.3 for fresh LAC.

Absorption cross-section may be assumed to depend inversely on wavelength throughout the visible spectrum.

Refractive index (m=1.74-i0.44) commonly used by climate models should be retired and new values between 1.75 – i0.63 and 1.95- i0.79 should be used.

Mie calculation should not be used for fresh LAC but coated sphere type Mie calculation can be used for aged LAC.
Reference:
Light Absorption by Carbonaceous Particles: An Investigative Review TC Bond, RW Bergstrom - Aerosol Science & Technology, 2006, ISSN: 0278-6826.

5 comments:

Harish Gadhavi said...

Pawan,
It is good that you raised this subject in our blog. I read one paper written around mid eighties, statement of the author was that we know spherical assumption is wrong for most aerosols but it is good to have that instead of having none. There has been many papers later on to model the non-sphericity; however, real challenge is to have a model which can be integrated in the GCM or radiative transfer scheme without having large over-head of computations as well as required inputs should be observable easily. And there seems to exist none.

Pawan Gupta said...

Harish,

That is really good point that 'having spherical assumption is better than none' but we are talking about 80s ...computers are now much faster than that time but still proper RT schemes should be available

The entire paper is not not really about commenting on climate models but it makes some recommendation to modelers to improve their accuracies.
Paper is more about literature review and detailed analysis of available measurements associated with carbon aerosols, which can be easily used in models to improve their accuracies. Such as most of the climate models uses refractive index value for LAC is 1.74-i0.44, which is much less compared to actual measured values of 1.85-i0.71, similarly its density is assumed to be 1 gcm-3 but measured value is 1.7 to 1.9 gcm-3 also the mass absorption cross section has similar problem.

Mixing is another important aspect about which paper talks in great detail. For example, Jacobson (2000) calculated radiative forcing 0.27 wm-2 due to LAC considering external mixture, but when you assume a shell-core type model you get 0.54 wm-2, which is double and if you assume volume mixture type assumption it becomes 0.74 wm-2. So, Mixing alone can create lot of difference in radiative forcing itself.

...well these are just few more points from the paper ..which I could not include in the blog ..otherwise it would have been long blog, which I was avoiding...

Harish Gadhavi said...

I concur with your opinion that this is the best paper I have ever read about carbonaceous aerosol; particularly I liked the fact that it states conclusion boldly and unambiguously.

While writing my previous comment I wasn't commenting on this paper but writing about a general problem I am facing currently - regarding sphericity of particle. Two years back Dilip and I were calculating asymmetry parameter using observed size distribution and Mie theory -- it took more than one week to complete using a computer with four processors. At the same time we surveyed literature for similar work. Most scientist put caveat regarding spherical shape assumption but we didn't see any using shape other than sphere for such calculations. So in my opinion research on sphericity of particle and their optical modelling is an open question.

Rodrigo said...
This comment has been removed by a blog administrator.
Rodrigo said...
This comment has been removed by a blog administrator.