Wang et al. reported a five-year data containing organic carbon and elementary carbon in the Shanghai, one of the biggest cities in the world. The measurement of such long-term is very valuable for policy evaluation and numerical modelling. The manuscript is well written although subjected to a few linguistic typos. The data were presented in a concise but detailed way and the main conclusion from the manuscript is clear that secondary organic aerosol (SOA) is becoming more and more important for mitigating air pollution. I believe the manuscript is suitable for publication in ACP from addressing the following issues, most of them are minor:

1. The “ambient” in the title can be removed.
2. The introduction is a bit lengthy; I understand the carbon aerosol is a large topic and there are many existing knowledge, but you don’t need to introduce them all, e.g. what’s the point of citing all the carbonaceous aerosol studies from line 63 to line 67.
3. Line 48. You mention carbonate carbon only once, what’s the point using an abbreviation
4. Line 55. Termed as
5. Delete “in issues varying”
6. Line 68 what is the relationship between this nationwide measurement with your study?
7. Line 73 you don’t need a reference to introduce Shanghai?
8. Line 96 in PM2.5, not fraction.
9. Section 2 in Figure S1, please add a scale on the map.
10. Section 3 as far as I understand, the concentrations is not normally distributed, the data should be reported in median (and 25th to 75th ranges)
11. Line 172 what is the possible cause of the abrupt change of POC in 2020?
12. Line 213 why is the weekend -weekday pattern important?
13. The manuscript was focused on secondary organic aerosol, but a large fraction in 3.1 was given to primary organic aerosol.
14. Section 3.2.1 It is intriguing to see the SOC increased with temperature, the authors represent a detailed analysis on the effect of temperature on SOC concentrations, but it is unclear how temperature impacts SOC. I understand that temperature can boost biogenic VOC in boreal area that act as SOC precursors, but in mega city in Shanghai it is uncommon. A possible reason should at least be given.
15. Line 311. “The concentration of Ox”
16. Line 392. Does this suggest that SOC observed in Shanghai is mainly originated from regional transport?

The manuscript entitled "Measurement report: characterization and sources of the ambient secondary organic carbon in a Chinese megacity over five years from 2016 to 2020" conducted a long-term field campaign at a regional site in the YRD region from 2016 to 2020 and aimed to investigate the characteristics of carbonous aerosol pollution and their seasonal and diurnal variations, as well as the relationship between the meteorological factors and carbonaceous aerosol concentrations. This study enhanced the understanding of the variation and sources of SOC in the PM_2.5 fraction, and was in favor of evaluation of the effectiveness of the current air pollution control policies. The manuscript is overall well organized, and can be read easily. I broadly agree with the discussions and findings of this manuscript. I therefore recommend a minor revision of this manuscript before final publication in ACP.

1. In the conclusion, it is difficult for me to find research findings with strong regularity or regional characteristics of the Yangtze River Delta. Therefore, it is suggested to condense the conclusion.
2. In 3.2.3, The discussion on the formation of photochemistry should have become one of the highlights of the paper, but unfortunately, the reviewer found that the author basically stayed at the level of the discussion on the correlation between ox and SOC, and lacked in-depth analysis of radiation intensity and liquid phase processes, suggesting further in-depth discussion.