Xu et al. examined the influence of solvent selection on brown carbon (BrC) absorption measurements and source analysis for ambient aerosols. Water, methanol, methanol-DCM mixtures, THF, and DMF were tested. Measurement results showed that DMF exhibited the highest extraction efficiency of ambient organic carbon (OC), particularly for low-volatile OC, and DMF extracts also had significant higher light absorption than other solvent extracts. Moreover, the comparison of sources between DMF and methanol extract absorption is very interesting and indicates that the methanol-extraction method will underestimate BrC contributions from non-combustion sources.

The authors suggested that DMF can extract more BrC than commonly used solvents. DMF might be an important solvent for investigating low-volatile OC in the near future. This manuscript provides very useful information for further studies on radiative forcing and sources of organic aerosols, and I recommend the publication of this manuscript in ACP, though I’d like the authors to address some minor specific comments below.

1. In this work, several solvent extracts of ambient OC were measured for light absorption, would the authors consider changing the title to “The dependence of brown carbon absorption on solvent selection and its impacts on source analysis”, or something similar to highlight the differences in different solvent extraction methods?

2. Line 31. “However, undissolved OC fractions will lead to underestimated BrC absorption.” What is the magnitude of this underestimation? Also, what about the mass? If the undissolved fraction has low light absorption, the underestimation might not be large, right?

3. Lines 41-42, “the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed distinct time series. Specifically, what is the difference, and do they have any common temporal patterns?

4. Lines 58-60, “The radiative forcing (RF) of the light-absorbing organic carbon, also termed “brown carbon” (BrC), is not well quantified due to the lack of its emission data and large uncertainties in in situ BrC measurements” The secondary formation will also add complexity on RF estimation of BrC. Please mention it.

5. Lines 261-262, “THF based on the two methods for rOC measurements (section 2.2) are compared in Figures S1 and S2.” Would the authors consider putting these two figures in the main text? They provide very useful information.

6. Section 3.1.2. Is the difference across solvent extraction methods related to the physicochemical properties of OC? If it is true, please state which factors have a substantial influence.

7. Page 13, lines 298–299. “This is because the light absorption of DMF extracts depends less on wavelengths than other solvent extracts (Å ~4.5, Table 2).”

Page 14, lines 339–341. “In comparison to Å_m (6.81± 1.64; Table 3), the lower average Å_d (5.25 ± 0.64, p < 0.01) supports that more-absorbing BrC had less spectral dependence than less-absorbing BrC.”  

In Tables 2 and 3, there seems to be a negative relationship between the MAE and Å values. To illustrate that strong BrC chromophores had less spectral dependence than weak ones, I would suggest showing the relationship visually by plotting MAE vs. Å.

8. Figures 2 and 3. I would suggest the authors to put Abs365, MAE365, and Å on the y-axis.

Publisher’s note: this comment was reposted as referee comment RC2.

This study compares the extraction of ambient PM_2.5 samples applying different solvents and the subsequent light absorption and determination of brown carbon (BrC). Authors find that the traditional approaches using MeOH or water extraction underestimate BrC absorption due to the insolubility of OC possessing larger chromophores and DMF exhibits the highest extraction efficiency among all the tested solvents. They suggest that using DMF instead of MeOH for BrC extraction and incorporate the results into receptor model will generate distinct source apportionment results. After PMF analysis, they conclude that the contributions of BrC from unburned fossil fuels and polymerization of aerosol organics are underestimated particularly. I do appreciate the interesting work and the information provides new insights into the radiative forcing of BrC. The work is well drafted, and I recommend publication in ACP before a few comments to be addressed as below.

Line 146-147. In the sampling setup, PUF is attached after two quartz filters to collect the gas phase polar and non-polar organic compounds. However, we do not see the subsequent treatment of the gas phase samples. Also, the absorption of vapors to quartz filter is substantial. In this regard, the sampling artifacts of this experimental design may be great concern and should be addressed.

Session 3.3 PMF analysis. Current discussion about the PMF is brief, and the following key information should be included, either in the main text or the SI. (1) the stability test of the final solution, as it indicates the robustness of the solution. A solution fails the robustness test is meaningless. (2) The change of the Q_robust/Q_exp with factor numbers should be examined.

Figure S5 UV-VIS spectra of 4-nitrophenol and 4-nitrocatechol. There is a strong light absorption at around 450 nm using DMF, which is not observed in other samples. It looks that unknown reactions occur, and the products introduce the unexpected light absorption. Considering that 4-nitrophenol and 4-nitrocatechol are representative tracers for biomass burning, readers may concern that DMF extracts would cause significant bias when investigate the BB BrC.

Line 317-318. The authors propose that the low-volatility OC fractions are possibly featured with PAH skeleton and DMF has higher dissolubility for those compounds than MeOH. Nevertheless, no light absorbance difference is observed in Figure S5 g-l. What are the 25 PAHs in the mixture solution and can you give some example structures that DMF have higher solubility than MeOH.

Line 283-284. As the author put it, the lower capability of MeOH in dissolving low-volatility OC fractions (OC3 and OC4) would lead to an underestimation of BrC absorption. Can you give an estimation of the underestimation so that the readers have intuitive knowledge?

Line 132. There should be a space before and after multiple sign.