In this study, authors investigate chemical composition of particulate matter in Beijing, China and contrast these results with the measured aerosol optical parameters. The results of the manuscripts shed new light on how chemical composition influences the optical properties of the aerosol particles and introduces a new method for analysing the collected particulate matter. The paper is overall well written and in scope of Atmospheric Chemistry and Physics journal but needs some corrections before publication.

Major comments:

Section 2.2 (Offline FIGAERO-CIMS analysis): Major part of this section is left out to be detailed in future publication. Even though I understand the reasoning behind the decision, I feel that too many details are missing. In this state I feel that I cannot assess the credibility of the method and hence the results shown in the paper. The authors should present the draft of this future publication showing the main details of the methodology, or the used methodology should be presented in more detail in the current manuscript or in its supplement.

Line 346: Recent paper (Yang et al., 2021) showed that many compounds with high DBE (>2) and oxygen number (>4) are prone to thermal decomposition during FIGAERO heating. Lot of compounds shown in Figure 4 fall into this category, and I would not be surprised that some of the observed compounds could be thermal decomposition products. Authors should consider this possibility and discuss its implications to their results and conclusions.

Minor comments:

Section 2: The information about the used CIMS should be more detailed, for example the model, mass resolving power, ect should be presented. Also details of the used I- nitrogen flow and IMR pressure are missing.

Line 95: Some estimation of collected particulate mass on the filters should be presented.

Line 96: Sampling dates for the three blanks are missing from the Figure 1 and Table S1.

Line 97: How long was the time between measurement and off-line analysis? Chemical reactions are possible overtime, even if the filters have been stored in freezer. Was the FIGAERO-CIMS analysis done in the same location as the measurements?

Line 101: Elaborate what does “pre-baked” mean.

Line 198: Episode periods could be marked more visibly in the Figure 1 plots, now its a bit guessing about the correct time periods.

Line 363: Indicate which panel of the Figure S4 is in question.

Line 364-366: This comparison could be more illustrative in a Table. Now it’s somewhat hard to follow.

Line 369: Indicate which panel is in question in Figure 4.

Line 398: Is there reference to a wrong figure here?

Line 401: Is the data shown in Figure S12 from the whole campaign? If so, it should be clarified in the caption so that reader does not think that different panels represent different episodes

Line 401: Acronym ”POA” is not defined in the text.

References:

Yang, L. H., Takeuchi, M., Chen, Y. and Ng, N. L.: Characterization of thermal decomposition of oxygenated organic compounds in FIGAERO-CIMS, Aerosol Sci. Technol., 1–22, doi:10.1080/02786826.2021.1945529, 2021.

In this manuscript, Cai and colleagues discuss their findings on the correlation between the chemical composition and optical properties of organic aerosols based on online Aethalometer and offline FIGAERO CIMS measurements. A discussion on different haze formation mechanisms observed in Beijing is also presented. In general, the manuscript is well written and the discussion is easy to follow. With some necessary clarifications and discussions as outlined below, the manuscript would be suitable for publication in ACP.

Offline FIGAERO-CIMS analysis: I agree with the other referee that more information is needed to validate the method. In addition to thermal decomposition, offline FIGAERO analysis would be susceptible towards sample handling and storage artefacts. For instance, acid-catalyzed reactions such as organonitrate hydrolysis (leaving CHO and HNO3) may occur on the filter. Would the small carboxylic acids desorb during storage? The “sandwich” technique for filter analysis may complicate quantification and/or volatility analysis due to increased vapor-filter interactions (there are now three filters as opposed to the single filter used for online FIGAERO).

Line 257-262: Do low-carbon compounds (e.g. C2-6 or C2-4) compounds have thermograms commensurate with their expected volatility? In other words, do they behave like “real” compounds, decomposition products, or a mixture of both during FIGAERO desorption? Some examples should be provided.

Aerosol water content, Line 146 -151: How sensitive are the ISRROPIA results to ACSM measurements, e.g. contribution to NO3 by orgnaonitrates? Furthermore, detection of HCl and to some extent HNO3 should be possible with the NO3- CI-APi-TOF, and can further constrain the AWC estimations.

Minor Comments

Line 71-79: References to online organic aerosol measurement using extractive electrospray ionization (EESI) technique (e.g. 10.5194/amt-12-4867-2019 and 10.5194/amt-14-1545-2021) should be added.

Line 105-108: A demonstration of IHNO3- time series would be helpful here.

Line 123-125: Because the aerosol composition was presented in rather semi-quantitative manners (i.e. percentage contribution by CHO vs. CHOX, or by individual elements), a note (even if somewhat qualitative) on potential sensitivity biases of I- CIMS should be added here. For instance, does I- CIMS respond equally well to organic acids, organosulfates, organonitrates, and reduced nitrogen species?

Line 141: How is NH3 detected by the NO3- CI-APi-TOF? Was there any consideration taken to minimize wall effects for the quantification of NH3, which can be quite “sticky”?

Line 186: “P_BC > 23%”. Should this be “P_BrC”?

Line 288: Does the TOF ACSM have enough mass resolving power for ion assignment? If not, the sentence should be revised to make the assumption here more explicit. Also, C2H4O+ would be found at m/z 44, not at m/z 60.

Line 315, 320, etc.: It would easier to write C6HxO5 (where x is a range of hydrogen atoms observed) instead of “C6 compounds with 5 oxygens”

Line 318: “C6HhO3Nn”. Does “h” stand for anything in particular? If not, it would be clearer to write down the ranges of hydrogen atoms observed, e.g. C6H4-10O3Na

Line 370-372: FIGAERO CIMS can only determine the elemental formula, not the molecular identity. The molecular identities (e.g. “malonic acid”, “succinic acid”, “glutaric acid”) should be presented in less definitive tones.

Line 401-405: How consistent (with respect to OA loading and sources) is the POA vs SOA estimation based on ACSM measurements? How sensitive is the conclusion regarding POA vs SOA vs SIA effects to uncertainties in the f44 vs. f57 parameterization?