This paper presents a long-term and high-resolution PSD dataset at urban Lanzhou in northwest China, within a valley, to evaluate the emission control impacts to air quality using some statistical methods. This paper focused on an interesting topic, with novel results that are important and significant for formulating air pollution control plan by the policymakers. I suggest this manuscript can be accepted for publication by ACP after minor revision.

Minor comments:

1. Climate and environmental effects of aerosol particles largely depended on their optical properties, and thus the variations in optical parameters during the measurement campaign should be explained or referred to some the published important works in Introduction. AOD and Alpha, two important optical parameters of aerosol particles, were analyzed during the campaign. However, the statement of relevant instrument should be given in the section of “Data and methods” of the revised manuscript.
2. The limitations on used data such as only a sampling site and used analyzing method should be given, and future works should be preliminarily introduced by 1-2 sentences in the last paragraph of the conclusions.
3. All abbreviations should be defined when they firstly appear throughout the manuscript, such as NPF, particle number in different size bins (N_13-25, N_25-100, N_100-800).
4. The sampling site should can represent urban background. Please explain that in the section of “Data and methods”.
5. The formation and evolution of air pollution may be more complex inside the basin as compared to that at the plain due to terrain and PBL impacts. Therefore, difference of aerosol pollution between inside the basin and the plain and the climate and environmental effects should be introduced in detail in the section of Introduction.
6. Only some novel findings should be clearly listed in the conclusions, and thus that should be revised to provide some key conclusions.
7. All figures in the manuscript should be revised and largely improved for seeing more clearly by readers.

This manuscript presents an analysis of long-term (7.5 years) particle size distribution, size-dependent number concentrations, and other related parameters at an urban background site (Lanzhou, West China) to study the influence of emissions and meteorology on the various particle size-modes. The long-term timeseries (Sept 2012–Aug 2019) also provides the opportunity to study the impact of China’s ambitious efforts to reduce air pollution over the last decade. The authors use k-means clustering to classify hourly-averaged data from a scanning mobility particle sizer (13–800 nm) into six categories defined by specific sources and other influencing factors. Furthermore, the authors use a customized Sen-Theil trend estimator as described in Sun et al. (2020) to quantify the long-term trends of particle modes and other parameters. Overall, the findings suggest that accumulation mode particles decreased because of the clean air action. However, ultrafine particle concentrations increased as new particle formation clusters became more prominent. These findings have relevance for Lanzhou and other similar cities in China and around the world and highlight the importance of long-term measurements.

My specific comments:

1. The analysis of long-term analysis of NPF and its frequency is incomplete without calculating and discussing the changes in the condensation (and coagulation) sink during this period — I highly recommend including this analysis. Furthermore, the authors could also calculate H₂SO₄ proxy (Dada et al., 2020) to estimate the changes in the NPF precursors over the study period.
2. The authors separate the study into two contrasting periods (before and after Jan 2016). However, based on the timeseries in Figure 2, it seems that 2013 is an unusually polluted year even for 2012–2015 period. I think presenting the average particle size distribution surface plots for each year (by season) can be instructive for highlighting the overall similarities and differences of each year of the study period (perhaps in the SI).
3. Many of the graphics in the manuscript are well made and explain the central themes of the study well (e.g., I think Figure 4 is excellent). However, I think the readability of manuscript can be improved if some of the graphics that are supplementary to the analysis are removed or moved to the SI (e.g., Figure 3). Furthermore, some more details should be provided in the captions of the figures as they should be interpretable independent of the text (e.g., Figure 9 was not clear to me).
4. Page 9, Line 4 (“The nucleation mode particles also can grow…”). I do not think this sentence is correct or required for this discussion. The timescales for such a transport (from ground to the sampling inlet) is likely to be much shorter than for the particles to grow from nucleation to Aitken mode.
5. Page 17, Line 15 (“NPF events predominantly occurred under dry and sunny weather conditions”). This should be discussed with more nuance based on more recent literature. For example, according to a relatively recent review paper on NPF, “The observed factors that favor the occurrence of regional NPF include a high intensity of solar radiation, low RH, high gas-phase sulfuric acid concentration, and low pre-existing particle loading, i.e. low CS and CoagS” (Kerminen et al., 2018).
6. For figures in manuscript or SI: (i) Include units of all parameters (where applicable); (ii) Avoid using captions such as “Same as Figure X, but for…”; (iii) Use continuous colorbars when using surface plots (Fig. 17 and Fig. S7).

References:

Dada, L., Ylivinkka, I., Baalbaki, R., Li, C., Guo, Y., Yan, C., Yao, L., Sarnela, N., Jokinen, T., Daellenbach, K. R., Yin, R., Deng, C., Chu, B., Nieminen, T., Wang, Y., Lin, Z., Thakur, R. C., Kontkanen, J., Stolzenburg, D., Sipilä, M., Hussein, T., Paasonen, P., Bianchi, F., Salma, I., Weidinger, T., Pikridas, M., Sciare, J., Jiang, J., Liu, Y., Petäjä, T., Kerminen, V.-M., and Kulmala, M.: Sources and sinks driving sulfuric acid concentrations in contrasting environments: implications on proxy calculations, 20, 11747–11766, https://doi.org/10.5194/acp-20-11747-2020, 2020.

Kerminen, V.-M., Chen, X., Vakkari, V., Petäjä, T., Kulmala, M., and Bianchi, F.: Atmospheric new particle formation and growth: review of field observations, Environ. Res. Lett., 13, 103003, https://doi.org/10.1088/1748-9326/aadf3c, 2018.

Sun, J., Birmili, W., Hermann, M., Tuch, T., Weinhold, K., Merkel, M., Rasch, F., Müller, T., Schladitz, A., Bastian, S., Löschau, G., Cyrys, J., Gu, J., Flentje, H., Briel, B., Asbach, C., Kaminski, H., Ries, L., Sohmer, R., Gerwig, H., Wirtz, K., Meinhardt, F., Schwerin, A., Bath, O., Ma, N., and Wiedensohler, A.: Decreasing trends of particle number and black carbon mass concentrations at 16 observational sites in Germany from 2009 to 2018, 20, 7049–7068, https://doi.org/10.5194/acp-20-7049-2020, 2020.