|The authors have done a reasonably satisfactory job of addressing most of the questions and concerns raised during peer-review. With its focus on estimating only the glass transition temperature and viscosity, and not the actual diffusivity and equilibration timescales, I still do not find much scientific value to this approach. All that has really been accomplished is to extend their previous model to include larger MW organics. However, the manuscript has improved during review and is now of an acceptable quality to recommend publication, after they address the major criticism in my original review that they have tried to side-step in their rebuttal (please see below). Although I rate the significance and originality of this manuscript to be rather low for ACP standards, we can let the community decide over time if this is a useful advance in our understanding of organic aerosol particles.|
Their rebuttal that the (superior, in my opinion) functional group-based method for estimating viscosity by Rothuff and Petters cannot account for mixed organic aerosol systems is well taken. The ability of these author’s approach (extended here to higher molecular weights) to model mixtures is a positive attribute.
The critical issue which the authors tried to avoid directly addressing in their rebuttal and revisions concerns the real need for a better understanding of diffusivity and equilibration timescales in atmospheric particles. Viscosity and Tg, which is all this paper focuses on, is indirectly related to diffusivity, but does not provide the information required to understand equilibration timescales, or any of the other important processes the authors lay out in the paper’s introduction to motivate their work. Throughout their rebuttal in response to my questions regarding the importance of diffusivity they just state “estimations of bulk diffusivity and mixing timescales are beyond the scope of this study”. And this is why I do not find much merit to what is being reported in this manuscript. It is fine if the authors wish to restrict this paper to only consider Tg and viscosity, but then the authors MUST significantly modify their introduction and conclusions to make it clear to readers what exactly can and can NOT be learned from viscosity in relation to all the critical processes they outline in the introduction. Without making these necessary revisions, the authors are frankly being somewhat deceptive by presenting their viscosity-limited work as though it can directly advance our understanding of critical atmospheric physics and chemistry. To borrow from their introduction (from their revised manuscript), here are some of the processes they outline in such a way that the reader is led to believe their viscosity estimates will advance our understanding of these same processes (_my emphasis_):
Line 64: “The particle phase state has been shown to affect gas uptake and chemical transformation of organic compounds due to _kinetic limitations of bulk diffusion_”
Line 70: “_Water diffusion can be still fast_ even in an amorphous solid matrix under room temperature, but it can be hindered significantly under low temperatures (Mikhailov et al., 2009; Zobrist et al., 2011; Bones et al., 2012; Berkemeier et al., 2014; Price et al., 2014), _affecting homogeneous vs. heterogeneous ice nucleation pathways_”
Line 79: “Partitioning of semi-volatile compounds into viscous particles may result in _kinetically-
limited growth_ in contrast to _quasi-equilibrium growth_ (Perraud et al., 2012; Shiraiwa and
81 Seinfeld, 2012; Booth et al., 2014; Zaveri et al., 2014; Mai et al., 2015; Liu et al., 2016), which
also affects the _evolution of particle size distribution upon SOA growth_ (Shiraiwa et al., 2013; Zaveri et al., 2018). Chamber experiments probing _mixing timescales_ of SOA particles derived by oxidation of various precursors such as isoprene, terpene, and toluene have observed _strong kinetic limitations_ at low RH, but not at moderate and high RH (Loza et al., 2013; Ye et al., 2016; Ye et al., 2018). Gorkowski et al. (2017) did not observe significant _diffusion limitations_ for glycerol and squalene in α-pinene SOA. _Quasi-equilibrium versus kinetically-limited or non-equilibrium SOA growth_ remains an open issue and warrants further investigations.”
This is certainly a nice introduction that discusses many of these important and uncertain processes, with good references to recent findings. Unfortunately, estimating only the glass transition temperature (and thus phase state and viscosity) tells us very little about diffusivity, equilibrium timescales, heterogeneous ice nucleation, kinetic-limitations to particle growth, etc. Yet the introduction gives the reader the strong impression that the new results presented in this paper will in fact advance our understanding of these critical processes. To summarize, the authors need to be much more upfront regarding what can and what cannot be learned just from their estimates of the glass transition temperature, and the phase state and viscosity they estimate from Tg.