General Comments:

Heterogeneous and multiphase reactions in the atmosphere can affect the formation and transformation of pollutants and determine the composition of gases and aerosol. Kinetic study is used to evaluate the impact of heterogeneous and multiphase reactions on the atmosphere environment. This manuscript developed a method to measure gas uptake coefficients, explained the principle of the method in detail and evaluated the measurement results. This study has made a good attempt; however, some concerns still remain. So, I think it needs some revisions before publication.

Some concerns and suggestions:

1. The introduction procedure of reactant gas and sample may not be suitable. When the lid is opened to place the sample, the reactor is filled with high concentration of reactant gas. At this time, the sample has been in contact with these high concentration reactant gases for a period of time during the placement, resulting in the occupation of its surface-active sites and thus affecting the measurement of initial uptake coefficients. This method may be effective for the measurement of steady-state uptake coefficient, however, there are very few heterogeneous reactions that can take place in a steady-state. So, its application may be limited.
2. The influence of gas diffusion in the reactor on the kinetics was not sufficiently evaluated. Due to the defects of the sample placement mentioned above, the selection of exposure time seems not rigorous. In addition, the effect of the types of samples and reaction gases on the time to reach steady-state is not known. The wall loss of reaction gas under different humidity conditions also needs to be characterized.
3. The derivation of the formula should give the dimension or unit like in formula 6.

In the work submitted, Li et al. presented the design, construction, and validation of a dynamic chamber system, which can be used to measure uptake coefficient on bulk solid-phase sample. Also, author have shown a relatively good agreement between flow tube, reference, and this study. The technique they developed is very important, and they also carried out tests for operational parameters and validation experiments very comprehensively. The paper is also well-written, and I recommend the manuscript to be published after corrections.

General issue

1. Page 3 line 25-30: The detailed calculation for uptake coefficient has shown here. The wall-loss effect on the uptake coefficient determination is supposed to discussed in detail. Also, wall-lost correction is necessary to described in 2.1.1. section.
2. As author explained why outlet position C is used to represent the average concentration due to existence of vertical concentration gradient inside the chamber. However, the smaller horizontal concentration gradient is observed in Fig. 4 and Fig. S2 due to mixing fan. If another mixing fan is placed at both sides of chamber or buffer flask for completing mixing, the vertical concentration gradient might be minimized.
3. Surface morphology of KI coating sample on V_t have shown no significant effects on the deposition velocity. If possible, different preparation on KI sample is better to use optical microscope or SEM/TEM to characterize their morphology.
4. Regarding to Fig. 7, there is a clear boundary for mass transport and surface reaction. Could author explain how to define “surface-reaction-limited”, “transition regions”, and “transport-limited region”