General comments:

The paper titled “Measurement report: On the contribution of long-distance transport to the secondary aerosol formation and aging” by Zhong et al. present detail analysis in  physio-chemical properties of aerosol transported from major pollution regions in North China at a regional receptor site located at the junction of the North China Plain and Fenwei Basin. The chemical composition of non-refractory PM_2.5 were measured by a Tof-ACSM and organic aerosol source apportionment were resolved and analyzed with measured black carbon, gas-phase pollutants and meteorological parameters to explore the secondary inorganic/organic formation during the transport. This study provide useful information on understanding the influence of long-distance transport of aerosol on the atmospheric environment. The manuscript was well written and presented, but some issues needed to be clarified. Therefore I recommend the publication of Zhong et al. work after replying the following comments clearly.

Specific and technical comments:

1. Line 26-27, three or four pollution transport sectors? It seems four sectors as it descibe that Beijing-Tianjin- Hebei (BTH), urban Guanzhong Basin (GZB), northern China and one clean transport sector from rural Guanzhong Basin region were identified. Please comfirm it and revised it properly.
2. Line 45-47. Some recent studies about the air pollution in China are missed here, such as “Atmos. Chem. Phys., https://doi.org/10.5194/acp-18-8849-2018”.
3. Line 157, the height of 200 m is above the ground? As the obsevation site is on the rooftop of a 33-floor tall building which is already 200m above the ground, the height of backtrajectories would be better than 200m. Please clarify it
4. Line189-199. It is interesting to note that the average PM₅ concentration is higher in the northern China transport sector than that in the urban GZB transpport sector, the latter secor usually has more anthropogenic emissions and more polluted than the former. Maybe the much higher frequcy of pathway in urban GZB transpport sector (60%) resulted in the lower PM2.5 value. Please clarify it.
5. Line 235-240. It is a good idea to compare the chemcial composition between the receptor site and the transpor sector. However, it should keep in mind that the difference in the chemical compostion would be also originated by the different observation period, not only due to the transformation during the long range transport. I note that the observation period in Beijing urban area by Xu et al., 2019 was almost the same as the present study, but the observation period in Xi’an urban area by Duan et al. 2021 (not Duan et al., 2020 in Figure 4) was different. In addition, as the BTH transport sector and GZB transport sector only contribued a part of the full observation period, it is better to select the same period to do the comparision. Thus, more information should be provided here.
6. Line 267-268 I do not understand the explain for the less oxidized SOA observed in the rural transport sector. The atmospheric oxidation of anthropogenic VOCs is prevail over the biogenic VOCs? Please clarify it.
7. Line 274-278. To backup the much higher O/C ratios observed in the transport sector, the O/C ratio results observed in the background area in North China would be suitable to explain the long range transportation from source region to the receptor site. Such as the results reported in Li et al., 2021 (Atmos. Chem. Phys., https://doi.org/10.5194/acp-21-4521-202) conducted at a mountainous site in North China Plain in summer 2019.
8. Line 278-281. The comparisons of O/C and SOA fractions between the receptor site (this study) and the source regions (previous studies) should be also notice the difference would be caused by the different observation period. More discussion should be provided before analysis the comparison results.

This study performed in-situ measurements at a regional receptor site with the height of ~200 m above the ground, which locates in the junction of the North China Plain and Fenwei Basin, to investigate the influences of region-to-region transport on aerosol chemical compositions and secondary formations. The authors compared the characteristics of secondary aerosol species among the four transport sectors, finding that long-distance transport largely enhanced the SIA formation, the OA oxidation and aging. The manuscript is generally well written. I recommend for its publication after addressing the following comments:

1. Line 31-32, which aspect the MO-OOA played a dominant role in?
2. Line 78-79, “we performed a two-months observation”, but only one month data points were shown in Figure 1. Please confirm that.
3. Line 100, SMPS is the abbreviation of scanning mobility particles sizer, and differential mobility analyzer should be abbreviated as DMA. In addition, the results of SMPS measurements are not given in the manuscript, please clarify it.
4. Line147, “m/z from 1~120” should be from12~120？
5. Line 224, valve?
6. Line 219-221, the authors wanted to illustrate the high fraction of sulphate in the BTH transport sector was caused by high concentrations of SO₂ and sulphate in the BTH region, however, the citing reference here can not sufficiently support this conclusion. The more information should be given to prove the higher concentration level of SO₂ and sulphate in the BTH region than other sectors.
7. Line 239-241, some recent studies on evolutions of aerosol physicochemical properties during the transport processes, suggesting the mass loss of semi-volatile aerosol species driven by the evaporation process when aerosols are exposed to a cleaner environment, such as “Atmos. Chem. Phys., https://doi.org/10.5194/acp-21-14749-2021”. It would be suitable to explain the decreased nitrate faction in the BTH transport pathway.
8. Line 265-268. Is the statement here means the LO-OOA at this site were mainly contributed by biogenic VOCs oxidation? Please provide more information to clarify it.