In your manuscript, a new variable, i.e., the ratio of ΔBC to ΔCO, was introduced and proposed as a possible indicator for aging and removal of BC. However, there was little discussion either quantitatively or qualitatively on how this variable would be linked to those that are employed conventionally to characterize atmospheric BC aging. For example, BC aging is typically reflected by the coating thickness, which is related to the mixing state and morphological variation (Khalizov et al., J. Geophys. Res. 114, D05208, doi:10.1029/2008JD010595, 2009; Pagels et al., Aerosol Sci. Tech. 43, 629, 2009). Such a change under atmospheric conditions is commonly associated with variations in the optical and cloud-forming properties of BC particles (Zhang et al., Proc. Natl. Acad. Sci. USA 105, 10291, 2008; Khalizov et al., J. Phys. Chem. 113, 1066, 2009). You concluded in the abstract “Therefore, the ΔBC/ΔCO ratio and correlation coefficient are possible indicators for the aging and removal of BC”, but provided little insight on how this quantity was correlated with those related to the mixing state, morphology, and optical and hygroscopic properties of BC particles, which are ultimately employed for assessing the impacts of BC particles in atmospheric models. In addition, some discussions on the plausible mechanisms leading to rapid BC aging in China would be beneficial, such as the processes detailed by Guo et al. (PNAS, 2014).

I would also recommend a careful proofread of your manuscript to improve its readability. I provide a few examples below. Abstract, line 11, change “campaign” to “campaigns”. Line 17, replace “The slopes, i.e. ΔBC/ΔCO ratios derived from their relationship were” by “The slopes, i.e., the ratio of ΔBC to ΔCO derived from their relationship, were”. Line 19, replace “ratios” by “values”. Line 21, replace “the ΔBC/ΔCO ratio” by “the quantity of ΔBC/ΔCO”.