Bicarbonate scavenges (OH)-O-center dot to form CO3 center dot- that enhances the bromate formation by ozonation. However, the role of CO3 center dot- in the bromate formation during ozonation has never been quantitatively investigated. Herein, we establish a quantitative approach for evaluating the role played by CO3 center dot- based on the detection of CO3 center dot--involved bromate and CO3 center dot- exposure. Experiments demonstrated that the CO3 center dot--involved bromate was responsible for 33.7-69.9% of the total bromate formed with bicarbonate concentrations from 0.5 mM to 4 mM. The CO3 center dot- exposure was two orders of magnitude higher than the corresponding (OH)-O-center dot exposure during ozonation. These results demonstrate that CO3 center dot- plays a comparable or even more pronounced role in the oxidation of bromine during bromate formation than (OH)-O-center dot. A model was developed based on the ratio of bromine oxidized by CO3 center dot-, which could predict the CO3 center dot--involved bromate formation well. Modeled and experimental results illustrated that the contribution of the CO3 center dot--involved bromate to the total bromate decreased with increasing pH or initial bromide, but almost remained unchanged at different ozone dosages. Moreover, the presence of humic acid led to an increase in this contribution during ozonation. The results of this study provide a more in-depth understanding of the mechanism of bromate formation during ozonation.