Abstract: In order to explore the variations of carbon componentsincluding organic carbon (OC) and element carbon (EC) in PM_2.5 at the urban area of Nanjing during the breakout of COVID-19 period, PM_2.5, OC and EC mass concentrations were measured by online monitoring from January to February 2020. Compared to the concentrations before the emergency response, the PM_2.5 concentration after the emergency response was reduced by 41.2% and EC reduced by 57.9%. These results indicate that the shutdown significantly reduced PM_2.5 and EC concentrations, but the OC concentration increased. Before the emergency response, the OC and EC concentrations were low in daytime and high at night and early morning. After the emergency response, the average concentration of EC only had a slight change within a day and maintained at low concentration level, OC showed a curve trend which was higher in the afternoon and lower in morning and evening. The significant decrease of PM_2.5 concentration after the emergency response period led to the increasing of light radiation intensity, which promoted the formation of secondary organic carbon (SOC). The SOC concentration increased when the PM_2.5 and EC concentrations decreased, and the correlation between OC and EC also decreased significantly.