Numerical investigation on measurement errors of mixing states of fractal black carbon aerosols using single-particle soot photometer and the effects on radiative forcing estimation
Abstract. The mixing state of black carbon (BC) aerosols can be measured by the single-particle soot photometer (SP2). However, the measured mixing state contains errors, because the core-shell model and Mie scattering calculation are employed in the measurement principle of SP2, and the spherical core-shell structure seriously deviated the real morphology of coated BC. In this study, fractal models are constructed to represent thinly and heavily coated BC particles for optical simulations, the scattering cross-section are selected as reference to conduct optical retrieval of particle diameter (Dp) based on Mie theory, just like the measurement principle of SP2, and the diameter of BC core (Dc) are the same for fractal and spherical models. Then, the measurement errors of mixing state (Dp / Dc) of BC are investigated from numerical aspect, and the estimation accuracy of BC radiative forcing is analyzed through the simple forcing efficiency (SFE) equation with SP2 measurement results taken into consideration. Results show that SP2 measured Dp / Dc based on Mie theory underestimates the realistic mixing state of coated BC for most particle sizes, and the largest relative error for single-particle can be about 42 %. The retrieval errors of mixing state of thinly coated BC for both single-particle and particle groups are larger than these of heavily coated BC. In addition, evaluation errors of radiative forcing of coated BC caused by measurement errors of SP2 are up to about 76 % and 43 % at 1064 and 532 nm, respectively. This study provides meaningful referential understandings of the measured Dp / Dc of SP2.
Jia Liu et al.
Status: open (until 19 Jun 2023)
- RC1: 'Comment on amt-2023-53', Joshua Schwarz, 25 May 2023 reply
- RC2: 'Comment on amt-2023-53', Anonymous Referee #2, 27 May 2023 reply
- RC3: 'Comment on amt-2023-53', Anonymous Referee #3, 31 May 2023 reply
Jia Liu et al.
SP2 measurement error of BC mixing state and radiative forcing evaluation https://doi.org/10.5281/zenodo.7589824
Jia Liu et al.
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I was happy to see this submission out focusing on improving interpretation of data types such as obtained with a single particle soot photometer (SP2). As a specialist with this instrument, I can clearly understand (and appreciate) the value of this to the community of SP2 users. The SP2 measures a few quantities on a per-particle basis relevant to determining mixing state. First, it provides the refractory black carbon (rBC) mass content of a particle (within some range of mass). This is based on an optical measurement of thermal emission, and is quite robust. Secondly, with appropriate analysis and setup, the instrument *measures* the total particle optical size – it detects a scattered light signal, and quantifies it. This is also valid only over some range of particle optical size, and is specific to the geometry of detection of the SP2. Third, some groups use the optical size of *only* the rBC portion of the particle (which can often be measured after the detection technique evaporates non-rBC material). Finally, inspection of the evolution of scattered light as a particle interacts with the SP2 laser provides another indication of internal mixing of materials with rBC. These measurements have been dealt with at length in the literature. After a measurement one can say: this particle had XX femtograms of rBC content, scattered as much light as a YY nm-diameter polystyrene latex sphere (PSL) into the SP2 scattering detector, and showed (or did not) evidence of shrinking during heating. These measurements have statistical and systematic errors associated with them, but are independent of Mie or any other theory of light scattering from particles. Now, the point at which this submitted manuscript becomes relevant is in the interpretation of those measured quantities. With knowledge about the amount of rBC mass and the total particle scattering signal, how can we interpret these quantities to infer conclusions about the amount of non-rBC material and its impacts on light absorption?
Presently, the paper is presented as though dealing with “SP2 measurement errors”. This is not the case. Rather, it deals with assessing Mie-theory inadequacies for complex aerosols (which are highly relevant to SP2 analyses). This is a more general topic of interest to a wider slice of the community than SP2 users/interpreters, but has been addressed often in the literature in the broad sense. Hence, I recommend maintaining the focus on the SP2 community.
“The SP2 user community often relies on Mie theory to interpret SP2-measured particle scattering cross-section and rBC mass content for the amount of non-rBC material internally mixed with the particles [Schwarz et al., 2008]. Although there is mild experimental support for the SP2 determination of coating thickness via Mie-theory assuming a shell‐and-core morphology [Laborde et al, 2012], recent results hint at the uncertainties associated with this approach. Scarnato et al.  used discrete dipole approximations as well as Mie theory to explore morphology effects on scattering and absorption of bare and internally mixed BC. They show (their Figure 3) that there can be considerable differences (~2X) between the exact numerical methods and the Mie-‐theory approximation for light-‐scattering at 1000 nm wavelength (near the SP2 wavelength of 1064 nm). Moteki et al.  included comparison of SP2 light-‐scattering observations from near core-‐shell morphologies of BC coated with oleic acid via vapor deposition. They observed a bias up to 40% compared to Mie shell-‐and-‐core theory estimates constrained by total particle mass, rBC mass, and the (known) index of refraction of the oleic acid. Exploration of the validity of Mie theory approximations is beyond the scope of this manuscript, but is clearly relevant.”
To summarize – this is a very promising entry that could provide a lot of value to the SP2 community. Making sure that the calculations are as relevant as possible to the geometry of the SP2 is one requirement. Another is correcting the association of interpretation differences to instrumental error. The authors also have the opportunity to provide a data set that I suspect would be broadly used in SP2-science.
Thanks for submitting this manuscript –
Joshua (Shuka) Schwarz