Comments:

General comments:

The authors presented 4-year optical properties of atmospheric aerosols collected over two size factions (PM1 and PM10). The analysis includes scattering and absorbing coefficients, along with derived values of SSA, AAE, and RFE. The manuscript is submitted as a ‘Measurement Report”. Four-year continuous datasets from a highly populated city like Beijing, China, is beneficial.  

Major comments:

According to the authors, the policies implemented to improve the air quality in Beijing are the main reason for the reduction in optical properties magnitude from 2018-2021. The policies effectively reduced the absorbing aerosols, which reduced the absorption coefficients. Does the same reduction happen in the scattering coefficient also? The absorbing aerosols like black carbon, BrC, and Dust also scatter in nature. So, for a complete picture, it will be better to see the fate of scattering coefficients over the years.

Figure 1 shows the overall reduction in PM_2.5. So, it is safe to assume that the PM1 and PM10 mass concentrations were also proportionally reduced. The general decrease in PM will automatically lessen the optical properties irrespective of the type of aerosol controlled.

Minor comments:

Line 88-89 – The term σ_ap is not defined.

Figure 5 – Use a different color for the backscattering ratio (b) frequency line.

The author contribution list is incomplete.

Review of “Measurement Report: Rapid decline of aerosol absorption coefficient and aerosol optical properties effects on radiative forcing in urban areas of Beijing from 2018 to 2021” by Xinyao Hu and coauthors.

This manuscript presents 4 years of aerosol optical properties in PM10 and PM1 size fractions at an urban site in Beijing. The dataset is interesting, and the manuscript is well written. I’m not sure if it is completely appropriate for ACP since the scope of the results is quite limited.

General comment: The authors have to make clear that talking about trends require long-term data, and apply statistical tests to verify if the decrease observed is statistically significant or not (most likely it is, but need verification). Another concern is related to atmospheric conditions. Changes in meteorological conditions over time, as well as the COVID-19 restrictions could be behind the observed “trend”. However, none of these causes are discussed in the manuscript. There have been many publications about the decrease of pollutants in 2020 during COVID-19 lockdown, but this is not even mentioned in this manuscript. Furthermore, meteorological variables such as temperature, wind, precipitation, should be proven to be similar from year to year, so the results can be more conclusive.

L50: variation-> variable

L59: for reference in-situ SSA values refer better to in-situ techniques. Consider the following references which are more appropriate:

Laj et al. 2020, Atmos. Meas. Tech., 13, 4353–4392, 2020

Pandolfi et al. 2018, Atmos. Chem. Phys., 18, 7877–7911, 2018

L54: What about the impact of backscatter fraction in the radiative forcing efficiency? That’s another important aerosol property to take into account.

L110: Measurements were performed at a single measurement site. Change the title to “… an urban area…”, otherwise it is misleading.

L112: together with the coordinates, indicate the altitude of the site.

L114: This is not clear, measurements are taken at 54m agl?

L125: A whole paragraph is dedicated to the TAP instrument, but nothing is said about the nephelometer. How often zero checks were done? How often was the instrument calibrated?

L178: Here it is introduced the backscatter coefficient. State that before when talking about the neph, and also in the introduction as an important variable for RFE calculations.

L180: Make it clear that the RFE calculations are referred to dry conditions (as the optical properties are measured) and do not refer to ambient conditions. Previous studies have demonstrated the influence of RH on the RFE (see for example Titos et al., 2021, Atmos. Chem. Phys., 21, 13031–13050, 2021, their figure 5).

L214: It doesn’t make sense to compare Beijing with Hyytiälä, of coarse values are higher at Beijing… Consider other studies performed in urban areas:

Lyamani et al. 2010, Atmos. Chem. Phys., 10, 239–254, 2010

Titos et al. 2012, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, D04206, doi:10.1029/2011JD016671, 2012

Pandolfi et al. 2018, Atmos. Chem. Phys., 18, 7877–7911, 2018

L225: You could use the reference Collaud-Coen et al. 2020, Atmos. Chem. Phys., 20, 8867–8908, 2020 for comparison of differences in the scattering and absorption trends. The study of Collaud-Coen et al., is a long-term trend analysis and the results are different but I think it could be mostly due to the difference in the time period considered.

General comment: make clear that the time period considered is not long enough to establish statistically significant trends. For trend analysis, at least 10 years of data are needed. On the other hand, the authors could apply statistical tests to check if the decrease between 2018 and 2021 was statistically significant.

L293: There are many assumptions behind the calculation of RFE and the impact of this assumptions should be discussed in more detail. The RH impact in RFE should be included here, so the results are relative to ambient conditions and not dry conditions.

Section of cluster analysis: not sure if this section is very informative relative to the main objective of the paper. Further discussion on the changes observed over the years is difficult to draw.