Dear authors
There are a few issues that still need to be addressed.

(1)Selection of uptake coefficients.
I agree with the reviewers. The fact that “the parameterization of heterogeneous reaction of SO2 on soot, organics, and SNA aerosols are not well established” can’t justify the selection. In fact, there are established kinetic parameters available in literature, e.g., uptake of NO2 on soot, organics, etc (Kalberer et al., 1999). Strictly speaking, the uptake coefficient of SO2 is determined through model-observation comparison rather than from Wang et al 2012. Therefore, I would suggest the authors to clearly mention that the selection of uptake coefficients from Wang et al. 2012 is arbitrary.

(2)Double counting of heterogeneous reactions
After including new mechanisms, the reactions of NO2 are represented in both R14 and R20. This will result in a double counting if both reactions happen on the same aerosol surfaces.

(3)Aerosol hygroscopic growth
Do the aerosols n Table 1 refer to dry aerosol particles? Was aerosol hygroscopic growth taken into account and how?

(4)Abrupt increase in PM2.5/sulfate concentration
Zheng et al. 2014 suggests an important role of regional transport for the abrupt increase/decrease in January 12. In Fig. 3, the modelled wind speed seems to be lower than the observations. If so, it may explain the underestimated the peak concentration and overestimated minimum on 12 January (Fig. 4). To get a better idea, I would suggest the authors to provide comparison for other sites (e.g., Chengde, Shijiazhuang and Langfang).

(5) Format and figure resolution
Please comply with the reference style of the journal using Zhao et al., 2013a or 2013b (instead of X., J. Zhao et al., 2013 ). Please check the following link for details: http://www.atmospheric-chemistry-and-physics.net/submission/manuscript_preparation.html
Please use higher resolution for Figs. 6-8. Make sure that all labels are legible.
Please use consistent date format (Fig. 1, 3, 4). A mixture of year and hour is hard to follow.


Reference:
Kalberer, M., Ammann, M., Arens, F., Gaeggeler, H. W., and Baltensperger, U.: Heterogeneous formation of nitrous acid (HONO) on soot aerosol particles, J. Geophys. Res., 104, 13825, 1999.
Wang, K., Zhang, Y., Nenes, A., and Fountoukis, C.: Implementation of dust emission and chemistry into the Community Multiscale Air Quality modeling system and initial application to an Asian dust storm episode, Atmos. Chem. Phys., 12, 10209–10237, doi: 10.5194/acp-12-10209-2012, 2012.
Zheng, G. J., Duan, F. K., Ma, Y. L., Cheng, Y., Zheng, B., Zhang, Q., Huang, T., Kimoto, T., Chang, D., Su, H., Pöschl, U., Cheng, Y. F., and He, K. B.: Exploring the severe winter haze in Beijing, Atmos. Chem. Phys. Discuss., 14, 17907-17942, 10.5194/acpd-14-17907-2014, 2014.

Dear authors
Now I accept this paper for publication in ACP though the format issue still remains. Please carefully check my previous comments. I don't believe that all coauthors would have missed it. Therefore please get their agreement before submitting your final responses and revised manuscript.

Best regards

Dr. Hang Su
Editor
Atmospheric Chemistry and Physics