The presented manuscript describes a new way how to measure size-resolved black carbon mass using a combination of two established aerosol instruments, aerodynamic aerosol classifier and aethalometer. Authors also provide a comparison of the new method to the measurement using SP2 and discuss the uncertainty of the new method. From the presented analyses, the new method seems promising

There are some issues that may be improved before an eventual publication:

The structure of the text may be slightly altered, for example 2. Methods, 2.1. Instrumental setup, 2.2.1. AE, 2.2.2. AAC, 2.2. Field measurement, 2.3. Size resolved calculations, 2.3.1. binned AC, 2.3.2. BCMSD? Also, the conclusions text is a summary rather than conclusions, and L291 to 296 is a summary of a summary and most of the text could be omitted.

In Fig. 1, and the connected text, some more information on the sampling system would be useful; in the plot, it looks as all the instrumentation was on one inlet. Is it so? What was the cut-off? What was the flow? How did authors solved sampling with instruments with very different flow rates (those could be also added to the picture, for an easier understanding of the sampling)? Was an isokinetic subsampling considered?

The agreement between the new method and SP2 data could be made stronger, for example:

• in Fig. S2, the two regressions are somehow confusing and not explained in the text? Could this be added?
• could the data from Fig. 2a be presented in a scatter plot with a linear regression to see the agreement?
• Would not be better to correlate the below 720 nm data with those 720 to 1500 nm data to see if these are connected, rather than regress them with data not measured in the size range? (L173 to L177)

In the uncertainty analysis, if the BCMSD was negative for below 23 % (L227), does it mean it is the limit of the method? If so, this should be somehow more explained and highlighted. If it is not so, what does it mean for the method if it brings negative values…? It seems a larger problem than the resulting uncertainty in Fig. 4b? Similarly, what is the reason of the incomplete parametrization? (L232) Would it be applicable to all measurements? Can the parametrization be improved?

Technical corrections:

L101 „a“ should be omitted?

L112 c in Cunningham should be in capital?

L118 a verb is missing?

L119 why not to be specific and state that AE33 did not need any correction, instead of „the instrument downstream“

L134 Why the constant MAC is stated when not used in the calculations? L134 and 135 may be omitted.

L143 one „bin“ may be omitted?

L164, could be the pollution episodes somehow highlighted in the plot?

In Fig. 3, the dotted line does not denote mean +- std, only std…

L205 S2b should be S3b?

L230 is the 1 % uncertainty or difference?

L253 and 254 What was the correlation between the two lines?

L264 limiting instead of limited?

The paper presents a new method to quantify elemental carbon mass size distribution using size-resolved absorption coefficient measured by an aerodynamic aerosol classifer in tandem with an aethaelometer. It aims to demonstrate the feasibility of the method by comparing the measurement results with that of refractory black carbon mass size distribution measured by a differential mobility analyzer in tandem with a single particle soot photometer (DMA – SP2) during a field measurement in Yangtze River Delta. The impact of different assumptions in calculation processes on EC mass size distribution is also presented.

The method presented in the paper is of general interest to the scientific community with application in the climate and air quality sectors. A complete understanding of the black carbon cycle and radiative impacts requires knowledge of processes related to the temporal and spatial evolution of BC size distribution. Although a variety of techniques are available to measure BC size distribution, several limitations of these techniques relates to time resolution, instrument cost and complex data analysis.

Some improvements are needed before the manuscript can be accepted for publication.

1. The authors should follow the recommendation by Petzold et al. (2013) to properly clarify the terms used for BC. Due to their design the various BC measuring techniques used in the paper are based on different physical and chemical BC properties. This generates difficulties when comparing data from the different techniques because it may provide different BC concentrations. Since there is no recognized technique to quantify BC in the atmosphere, Petzold et al. (2013) established terminology recommendations for reporting BC results. The terms “refractory BC” (rBC) and “equivalent BC” (eBC) should be used to refer to the BC mass concentration quantified using SP2 and aethalometer, respectively. Internally mixed BC particles composed of both BC and non-BC material should be designated as “BC-containing particles”.
2. The significance of the study should be strengthened. The main motivation of the proposed new method is to extend the detection range of BC size distribution up to 1.5 µm, while the widely used SP2 instrument can’t measure rBC above 600 nm. The mass median diameter of fresh BC is normally distributed within the range of 30−200 nm. If they exist in the atmosphere, I guess superlarge-sized BC may be formed by massive coating or superaggregation. What is the state of the art on these coarse BC particles? Have they already been observed in the atmosphere using other measurement techniques? Are they formed in the atmosphere by massive coating or superaggregation ? What is their contribution to the total BC concentration, BC radiative forcing and CCN ability?
3. There is lack of information on the measurement setup. There is no information on the aerosol sampling (inlet used for aerosol sampling, cut-off diameter of the sampling inlet, aerosol conditioner to dry,…), which need careful consideration for interpreting the results. In addition the authors did not provide any information regarding the calibration of the instruments. Have each instrument been calibrated individually using spherical monodisperse particles? Which detector was calibrated and used to analyze SP2 measurements? The accuracy of the rBC mass concentration measured by SP2 is strongly dependent on the calibration material chosen. This should be mentioned in the paper. Without these information, the reliability of the data is questionable.
4. There are some methodology and assumptions in calculation processes of rBC mass size distribution measured by the DMA-SP2 that are not discussed. Just as in a typical SMPS data analysis, the rBC mass size distribution need to be calculated from the raw observations of rBC number (or mass) versus voltage using an inversion code. This inversion depends on particle charging efficiencies and the DMA transfer function. The authors should clearly explain the method they used to correct the size distribution measured by the DMA-SP2 to output the information for singly charged particles only. In addition the particle size selected by the DMA is an electrical mobility diameter, which is highly sensitive to the particle shape. If the authors neglected the dependence of rBC asphericity, they shoud mention it in the paper. The authors could also analyze in section 4 the possible uncertainty in deriving rBC diameters from DMA-SP2 owing to the dynamic shape factor they used.

Specific comments :

L80-85 : Why did not you use the SP2 alone to obtain rBC mass size distribution ? Please add the size range of the DMA-SP2.

L98 : Where was C measured in Zhao et al. (2020b)?

L205: Fig. S3b instead of S2b