Answers to referees for “ Online measurements of water-soluble organic acids in the gas and aerosol phase from the photooxidation of 1 , 3 , 5-trimethylbenzene ” ( acp-2013-922 )

There is no need to define the abbreviation PSI here, since it is not used further in the abstract. The last sentence of the abstract should be a bit further explained in order to be meaningful. The PSI abbreviation has been removed from the abstract and the last sentence has been edited in order to clarify its meaning. Page 986 Line 25. At least one reference is needed to support this. Kroll and Seinfeld (2008) as well as Hallquist et al. (2009) are cited in the revised manuscript to support this statement. Page 987: References are needed in lines 2 (. many different compounds) and line 5. We now cite Kanakidou et al. (2005) and Williams et al. (2007) on line 2 and Saxena and Hildemann (1996) on line 5. line 16-: Are online methods not prone to artefacts? Later in the discussion of the present manuscript, it seems that online methods may also have such problems. The formulation of this paragraph was imprecise and partly incorrect (see comment from Anynymous Referee #2). Therefore it was rewritten taking into account available CIMS instruments and the fact that online methods, such as PTR-MS, can also show artefacts. line 22: "the method . allows selective collection of organic acids". I assume the authors mean water-soluble species, not just organic acids. It is true that all water-soluble species will be collected, but on the chromatographic column, only organic compounds with at least one carboxylic acid functional group as well as a few inorganic anions (nitrite, nitrate, and sulphate) will be separated and eventually detected. The original sentence was rewritten in order to clarify it: “The ion chromatography (IC) method presented here allows water soluble organic acids (as well as nitrite, nitrate and sulphate) to be separated prior to detection. ” Page 989 line 20: "hydroxy anion" should be changed to "hydroxide anion". What is the counterion (cation) in the eluent? The settings of ESI-MS should be clearly stated. The revised manuscript uses “hydroxide anion” and includes more details regarding eluent generation. (See also answer to comment 5 from K. Sato.) The counter ion is potassium (K). The capillary voltage of the ESI-MS was set to -1.5 kV. This information has been included as well in the manuscript. Page 991: line 7-9: Please provide more details on the sampling and extraction method. The formed secondary organic aerosol were collected on Teflon filters when the maximum aerosol mass was reached in the chamber. The filters were extracted with water and this solution has been directly injected via a capillary and electrospray to the high-resolution mass spectrometer. This is now mentioned in the revised manuscript. line 21: It seems that "aerosol and gas phase" should read "gas phase and aerosol". No. The authors use the definitions from the original paper from Pankow (1994), where Ai represents the gaseous concentration and Fi the aerosol (or “particulate-associated”) concentration. Page 994: Line 3: How can the particles formed be bigger? I assume you mean that they grow to larger aerosol diameters. Indeed, our statement was referring to the final particle diameter. This sentence has been rewritten in the revised manuscript. line 13: Please add that the black lights were additional sources of light. This clarification has been added to the manuscript. line 21: What do you mean by "at least one chromatographic peak was identified of twenty-five masses. "? Because we use mass spectrometry coupled to ion chromatography, we generate a chromatogram for each mass to charge ratio measured, also called “mass chromatogram”. We mean that for twenty-five of these chromatograms, at least one peak has been identified. To make it clearer, this sentence has been modified in the revised manuscript: “Twenty-five mass chromatograms showed one or more peak. The corresponding masses are listed in Table 1 and a chemical formula could be attributed to twelve of them with the help of the high resolution MS data. ” Line 25: unambigious identification did you have authentic standards for all compounds? Please also state in the experimental section, for which compounds you had authentic standards for quantification? Yes, unambiguous identification is based on authentic standards. The list of these compounds is now included in the experimental section of the manuscript. Page 995: Line 13: What were your detection limits? Because detection limits expressed in ng/m depends on the sampling flows and varies from experiment to experiment, we decided not to include detection limits in our manuscript. Additionally, calibration curves were derived from direct injection of 25μl of aqueous solutions. Therefore, the detection limits and converted signals units were expressed in ng and the gas phase and aerosol concentration expressed in ng/m, taking the sampling flow into consideration. The typical range for organic acids for the WEDD and the AC devices were ranging from 0.12 to 5.1 ng/m. (For nitrate and sulphate, the estimated detection limits ranged from 2.8 to 17 ng/m). We added this information to the section describing the calibration procedure. Line 25-28: This paragraph needs a little further explanation including references to relevant literature. We decided to remove this paragraph, as it does not help the discussion. Indeed, we are not able to quantify the different pathways, due to the impossibility of performing a complete mass balance (see next comment), because of the lack of available standards and the remaining unidentified compounds. Line 29-: Can you quantitatively compare functionalisation/oligomerisation pathways and fragmentation pathways without complete mass balance? As said, we agree with the referee that it is not possible to quantify the two pathways without complete mass balance and that is what our original manuscript said. Our statement was meant in a general way, but we removed it in the revised version of the manuscript. Page 996 line 17-18: Since glycolic acid does not seem to depend on precursor concentration, I am wondering if it could be related to background concentrations in your system. Did you investigate this e.g. in experiments with light but without TMB? We have been performing experiments without TMB, that are not included in this manuscript. Glycolic acid concentrations reported are slightly higher than the one measured during these socalled “blank” experiments. However, it is true that the concentrations are very close to the background values. Therefore, we soften our statement in the revised manuscript. (See also comment from Anonymous Referee #2.) Page 997: Lines 2-5: What is the uncertainty on these numbers? The uncertainty on the normalised acid concentration is mainly the uncertainty on the aerosol phase concentration, which is roughly 30%. Lines 12-16: The background for this statement is not clear to the reader. Please explain your reasoning. This paragraph has been edited and should be easier to understand. The reasoning has also been modified according to comment 16 from K. Sato. Line 17: Remove "also" in the beginning of the sentence. The whole paragraph needs editing to improve style and grammar. This paragraph has been completely edited to improve clarity, style and grammar. See also the comments by the other referees. Add a reference to Table 2 for the discussion of the unknown compounds. References have been added in the revised manuscript. page 998 line 29: Which source for the carboxylate ions do you suggest/expect? The source of carboxylate ions in the aerosol phase would for instance be hydrolysis of ester functionalities of formed oligomers due to the dissolution in water in the aerosol collector or decomposition of PANs. page 999 Line 2: "confirms" should read "suggests" due to the large associated uncertainties. We agree with the referee that our statement must take into account the large uncertainties and therefore, this sentence has been edited. Line 20-21: References are needed here. Sato et al. (2007), for instance, observed a similar effect due to water sampling. This reference is included earlier in the text. page 1000: The last statement only refers to the present method for determining Kp, correct? Yes, the last statement applies especially for the presented method and this has been clarified in the revised manuscript. However, our statement may also apply to other methods determining Kp from gas phase and aerosol measurements. Figure 3: "due to an instrumental limitation" is it just lack of data or? The instrumental limitation is the upper detection limit of the instrument. NO2 is derived from the subtraction of the NO concentration from the NOx concentration. At the beginning of the experiments with missing data, both NO and NOx signal saturates, resulting in an incorrect NO2 concentration (close to 0ppbv). We decided to exclude this data from the plots. Figure 5: Why were the data not corrected for wall losses? Wall losses are difficult to address properly and would not affect the main findings in our manuscript, as we are reporting normalised particle phase concentrations. However, the role of species lost on the smog chamber walls on partitioning coefficients is now briefly discussed. Figure 8: In my version, the legend is in the middle of the figure, which looks odd. As the experimental conditions are not immediately comparable between panel (b) and (d), we decided to use two different columns, while (a) and (c) differ only by the initial concentration of TMB (1200ppbv vs 600ppbv). Please explain that the data were normalized to the total aerosol mass from SMPS data. It is now explained in the caption of Figure 8.