|I generally agree with the author’s stance on most of my comments and I think the change in the framing of the problem – one of focusing on the SOA formation from VOCs – responds to my major concern with this manuscript. There are however a few things I would like to point out from the response that caught my attention. |
1. For the response on comment #1, I don’t agree with the simple explanation added to the manuscript to justify the use of a non-volatile POA that ‘there is evidence for both semi-volatile and non-volatile OA’. The issue of volatility is nuanced and should be discussed accordingly. For instance, source measurements of combustion POA performed at different dilutions (e.g., May et al., 2013a,b,c) and thermodenuder measurements of laboratory-generated (e.g., Huffman et al., 2009a) and field OA (e.g., Huffman et al., 2009b) clearly demonstrate that OA is semi-volatile with some fraction of non-volatile material. However, certain experiments with model SOA systems exhibit delays in evaporation that could be a result of diffusion-linked limitations to evaporation (e.g., Vaden et al., 2009). Simply stating that there is evidence for both and then treating the POA as non-volatile suggests that the weight of the literature points towards POA being more non-volatile, which is definitely not the case.
2. The response in comment #2 makes it sound that we do not know a lot about the differences in VOC reaction rates, SOA mass yields, and speciation of anthropogenic and biomass burning emissions. I would argue that a lot has been understood over the past decade (e.g., motor vehicle VOC = May et al., 2014; Zhao et al., 2015; Zhao et al., 2016, biomass burning = Stockwell et al., 2015; Hatch et al., 2017) but that there is still a lot more to learn. Also, I disagree with some of the inferences made from the literature. For instance, the differences in SOA mass yields between different aromatic compounds can be explained if the aromatics are split into low and high yield categories (Odum et al., 1996). NOx effects on SOA formation from aromatics can be modeled through a competition of the RO2 radical with NO and HO2 (Henze et al., 2008). Furthermore, for a model such as that used in this work adding model species can be computationally expensive and need to be kept to a minimum. However, their reactivity and SOA mass yield could be informed by a weighted average of the mixture of species used to represent the model species.