The author conducted chemical composition measurement at high altitude, expanding our understanding on aerosol chemistry at mid-upper PBL. I suggest major revision for the manuscript prior to be finally published in ACP.

1. The collection efficiency was chosen as 0.5. In fact, the composition dependent CE was more precise. The author should compare these two methods and evaluate whether default CE influence NR-PM₁ species quantification.
2. PMF source apportionment was performed for entire study, with two-factor solution being resolved. Considering that the emissions sources could be different in different seasons, PMF should be done separately during each season. Did the author try to do ME-2 analysis with constrained POA profiles to improve results?
3. Simultaneous measurements of chemical compositions at Shanghai tower and the ground benefit the comparisons of vertical differences. In line 140-145, the author compared nitrate at SHT with previous studies. The question is that meteorology significantly impacts PM concentrations. The author simply compared the annual average nitrate contribution with surface measurements, without considering the sampling sites, seasons and meteorology. Another question is why higher nitrate aloft owed to lower temperature. Is it possible that other pathways also contribute to nitrate formation?
4. The author said throughout the study that SHT is close to the top of PBL but no PBL data was shown. The PBL is dynamically varied during daytime and nighttime, and the PBL height might lower than 600 m under severe haze pollutions.
5. In line 155-165, it is confused that the author described the maximum and minimum temperature and RH in detail solely. In fact, the meteorology was linked to chemical compositions. Discussing the influence and interaction between meteorology and PM is more charming. Why the author showed the daily average values in Table 1?
6. In Sec. 3.2.2, please give the definition of anomaly. Is it calculated by comparing with annual average or history records? Why were they anomaly?
7. In line 202-203, the author state that extra aerosol productions contributed to higher PM_2.5 concentrations at SHT than surface. Please elaborate the conclusion.
8. In line 205-210, the author said that exchange between SHT and surface only exits in daytime. If nocturnal PBL is higher than SHT, nighttime exchange can also occur. Nighttime PM_2.5 at SHT was not independent from the ground level.
9. In Sec. 3.2.4 and figure 4, the author can also plot and discuss mass fractions of NR-PM₁ during daytime and nighttime separately.
10. In figure 6, NO₂ at SHT increased by 21.8-61.4% from 08:00 to 12:00, while they were reduced at ground level during this time. Thus, vertical mixing could be the explanation rather than vehicles. In fact, the peak at morning at surface was attributed to traffic during morning rush hour.
11. In line 62, “vatical distribution” might be a typo.
12. Please uniform the subscripts of sulfate, nitrate and ammonium throughout the study.

In this manuscript, the authors gave a very details analysis of observed one year continuous PM2.5 and its chemical components at the top of 632 m high Shanghai Tower (SHT). The data collected were precious, and the topic is of great interesting to recognize vertical PM2.5 characteristics and its formation processes related to emission, chemical production and boundary layer (BL) etc. The analysis is mostly sound, but some details need clarify. I recommend a minor revision and my specific comments listed below.

Specific comments:

1. My primary concern is that the study address the PM2.5 and its chemical components at SHT dominating by vertical mixing from surface (most in daytime) and chemical production therein from surface precursors, while omitted the PM originating from transport outside Shanghai. The seasonal winds induced by Asia monsoon are quite difference in upstream (ocean or land, most natural or anthropogenic in background) and could impact much at SHT than on the surface. I suggest the authors should refer to this factor or indicting for future research.
2. nitrate (NO3-) and sulfate (SO42=) should be correctly present in the manuscript.
3. In line 69, Shanghai is not only one of the most densely populated megacities in China, but also in the world.
4. In this study, the heights of BL were important. Please give a brief introduction of BLHs in different season and day and night in Shanghai.
5. In line 176-178, the inference is not very exact. The seasonal variations of BLH could be key factor for the similar monthly variations of PM2.5 at SHT and SUR, and related to regional transport, vertical diffusions etc. And I am happy to find you mentioned of regional transport, while did not raise this in conclusion, abstract and other paragraphs.
6. In 188-190, the anomalies may reflect the seasonal variations of BLHs.
7. In 210-211, “completely” is not very exact because in some synoptic conditions, the mass and energy exchange between free troposphere and within the BL could occur.
8. What were the definitions of POA, OOA, and HOA, and their chemical components in this study?
9. In figure 5, why there was the largest difference of PM2.5 between SHT&SUR in summer, while the largest difference of NOR in winter and spring in figure 7?
10. In line 370, latitude should be altitude.
11. In line 374, “since the SO2 level was relatively lower than the other seasons.”. or also because the favorable diffusion and wet scavengingcondition of atmosphere in summer.