In this manuscript, A BC-tagged method in WRF-Chem model  was used to investigate the orgins of BC peaks at the altitude of  4km in Beijing, China.  It is important to better evaluate the radiative effect of aerosols and its effect on boundary layer in polluted regions. I I would recommend publishing in ACP after addressing the following comments.

1.  Meteorological and chemical observations in Beijing was used to evaluate the model performance. It is not enough for evaluation on regional transport. More observations in North China are needed, particulaly, HN,SX (source regions).

2.  Usually, high BC is related to impact of biomass burning because these emissions are easilly lifted to high altitudes in high temperature without atmospheric dynamic uplifting. I suggested that authores investigate the impact of biomass burning in this study. At least, fire counts from satellites.

3.  Section 3.2.1  more dynamic conditions are suggested to conduct. For exapmle, the convergence of BC/PM2.5 at different altitudes in each phase  of this episode.  It's better to provide a comprehensive dynamic drivling forces of regional transport. 

4. 3.3.1  what is the input data pf HYSPLIT? 

5.LIne 169 China central seems to be cenrtal China.