|In this study the authors report the formation of organosulfates from the photooxidation of C10-C12 alkanes. The effect of acidity, humidity, and solvent extraction on the formation and quantification of organosulfates were investigated. The authors evaluated possible mechanisms for the formation of the assigned organosulfates. The manuscript went through the ACPD open discussion process and got mixed reviews, with a number of good suggestions from the more critical reviewers. Based on my examination of the authors’ responses to the reviewers’ comments, I believe the concerns of the reviewers have been adequately addressed. This revised manuscript is well written and provides evidence for the importance of anthropogenic precursors in the formation of organosulfates that have also been identified in field studies. The manuscript could be published as is, but I have a few minor comments that follow.|
1. The effect of filter solvent extraction on the quantification of organosulfates was investigated by using either methanol or an acetonitrile/toluene mixture. The extracts were dried and then reconstituted in solvents, but not in the same solvents as the ones used in the filter extraction step. Tao et al. 2014 saw a difference in quantified organosulfates between an acetonitrile/toluene mixture and an acetonitrile/water mixture that were used in extraction and direct MS analysis, which suggest very different solubility of organosulfates in these solvents. What was the rationale for using a different solvent for the extraction and re-dissolution? Is there a concern that the initially extracted compounds remained undissolved during the re-dissolution step (i.e., the less polar aliphatic species more soluble in the acetonitrile/toluene solvent that were initially extracted from the filters were not re-dissolved in the acetonitrile/water mixture)?
2. The authors did not appear to directly respond to previous comments from Referee #3 posted during the ACPD open discussion stage. I think more attention needs to be paid to these comments:
“Further, it is unclear if the conclusion that enhancement of OS yields are due to increased acidity of the seed aerosol is really due to acidity, rather than an effect of seeding the experiments with an atomized solution containing more sulfate.”
“Line 400: The authors claim that “the presence of acidic aerosols significantly increase OS formation in most cases”. However, is this just an effect of using an atomized solution with more sulfate (0.06M ammonium sulfate + 0.06M sulfuric acid) in the acidic case versus only 0.06M ammonium sulfate in the non-acidified case? It may be a concerted effect of more available sulfate in the “acidic” case as well as acidity.”
“Lines 484-486: Enhancement of OS due to acidified ammonium sulfate seed needs to be addressed with regard to the effect of just having introduced more sulfate into the experiments compared to the non-acidified case. See earlier Major Comment, 33.”
In response to these comments, authors cited additional references, but the main concern is that by adding sulfuric acid to acidify SOA, the effect of varying the acidity is not cleanly separated from the effect in the increase in sulfate concentration. Cited references also use sulfuric acid to change SOA acidity. It is important to note that no control was conducted to show that under the same initial conditions, but with increased sulfate concentration there is not a change in organosulfate formation. There is not a disagreement that acidity can increase organosulfate formation, but a question of whether an increased concentration of sulfate can affect organosulfate formation.
Tao, S., Lu, X., Levac, N., Bateman, A.P., Nguyen, T.B., Bones, D.L., Nizkorodov, S.A., Laskin, J., 870 Laskin, A., and Yang, X.: Molecular characterization of organosulfates in organic aerosols from Shanghai and Los Angeles urban areas by nanospray-desorption electrospray ionization high-resolution mass spectrometry, Environ. Sci. Technol., 48 (18), 10993-11001, 2014.