This study reports unique long-term CIMS measurements of HIO_x as well as NPF events in two urban areas, Beijing and Nanjing, China. The source analysis of iodic acid indicated potential long-range transport from marine and terrestrial sources rather than local anthropogenic emissions. They also estimated the contribution of HIO_x and H₂SO₄ to nanoparticle growth rate and survival probability of nanoparticles. This paper shows that, although iodic acid does not play a major role in nanoparticle growth rate, the additional growth by iodic acid can enhance the survival of sub-3 nm particles in Beijing during NPF events. This suggests iodic acid contributes to NPF events in urban conditions with support from field measurements. However, some technical details on the measurement and analysis need to be clarified. Therefore, I recommend the manuscript for publication after revisions to address my comments below.

Main comments:

1. Eq. (9) & (11): What was the range of temperature/size correction factor applied to GR(HIO₃) in eq. (9)? Also, please explain why GR(H₂SO₄) in eq. (11) does not need temperature correction. In eq. (5), GR is corrected only to account for growth by condensation. And it is unclear in the text whether GR(HIO₃) and GR(H₂SO₄) in those equations are growth rates only by condensation (excluding coagulation).
2. Line 273-274: “We define SPtot as the particle survival probability calculated using the measured GRs (in Beijing) or the expected growth rate considering growth contributions from both H₂SO₄ and HIO₃ (in Nanjing).” Please explain how using different inputs for SP_tot calculation would affect EF in each site.
3. Line 305-306: “The calibrated HIO_x concentration is above the detection limit during almost the entire measurement periods…” However, I can not find more details on the calibration, except for the general description of nitrate CIMS in section 2.1.2. Therefore, the quantification method of HIO_x (as well as H₂SO₄) needs to be shown and the sources of uncertainty need to be discussed since HIO_x concentrations are being used for the growth rate and survival probability calculations. How and how often were the instruments calibrated for HIO_x? Do authors expect losses in the sampling line or variations of HIO_x sensitivity due to changes in temperature/relative humidity?
4. Line 426: “implies that marine iodine sources could be important” Could it be emissions from industrial areas along with the back trajectory path?
5. Line 428: That is the opposite direction of continental outflow.
6. Line 445: Even if the HIO₃ emission (or secondary formation) is higher in winter (when PM_2.5 loading is higher), still the concentration of HIO₃ can be lower due to potentially higher CS. Thus the y-axis in Fig.3 can be normalized by CS to see if the source of HIO₃ is correlated to PM_2.5 pollution.
7. Line 522-529: Survival parameter (P) is newly introduced here and it is unclear why P is being used for AHL/BUCT case while survival probability (SP) is used for the rest of the analysis. Please compare the survival parameter from Kulmala et al. (2017) and survival probability (SP) from Lehtinen et al. (2007) and explain why using P is useful here.
8. Line 567: “a fixed GR enhancement” What does “fixed” mean?
9. Line 570-575: Are the MOD, APT-x, and APT-y methods equally credible? Then the differences in those methods represent the uncertainty of using MOD fitting method? Or do APT-x and APT-y represent upper/lower limit of MOD fitting? 
10. Line 586-588: “This is based on the observed consistency between the gaseous H₂SO₄ concentration and its significant contribution to the sub-3 nm particle growth rate in Beijing (Deng et al., 2020b).” However, in this study (Table S4), GR_1.5-3 was generally higher than GR(H₂SO₄) in Beijing indicating contributions from other species.
11. Fig. 5: In my understanding of the text, although HIO₃ is not the major contributor in the measured GR, the sub 3-nm SP is so sensitive to additional GR from HIO₃ that the additional HIO₃ enhances SP of sub 3-nm significantly (as shown in fig. 7). And fig. 5 is a nice place to visually indicate that sensitive regimes that correspond to the data in fig. 7.
12. Line 606-608: “As depicted in Fig. S8(c), SP enhancements in percentage are generally one order of magnitude higher than the GR contribution in percentage and HIO₃ can result in as high as 2-fold enhancement on SP in sub-3 nm particle growth.” How important is the ~100% (or ~40% on average) enhancement of SP from HIO₃ in the occurrence of NPF event? In Table S4, during NPF events in Beijing, the survival probability (“P1”) of sub 3-nm particles spans over 3 orders of magnitude from 6.1E-4 to 2.1E-1. Also, that wide range of SP needs to be directly mentioned in the main text.

Technical comments: 

1. Line 94: “survival probability (Kulmala et al., 2017)...” According to the rest of the text, the survival parameter is from Kulmala et al. (2017) and the survival probability is from Lehtinen et al. (2007).
2. Line 250: GR’(HIO₃) -> GR(HIO₃).
3. Line 269: Define dpinitial and dpfinal.
4. Line 594: “IA” and ”SA” not defined previously (supposedly HIO₃ and H₂SO₄?). Use consistent names in the main text and SI unless otherwise needed.
5. Fig. 1&6: Make the panels bigger.
6. Table S8 and S9 caption typo: SOREPS -> SORPES. 
7. Table S2-S7 & S9: Please use the consistent notations for survival probability (SP) and enhancement factor (EF) rather than “P” and “E”. Especially, “P” was used for the survival parameter in the main text and Fig. S6.
8. Fig S4: The back trajectory color scheme is not consistent. Also, specify the meaning of (%) that the colors represent in the caption.

Please see attached file.