Study of Different Carbon Bond 6 (CB6) Mechanisms by Using aConcentration Sensitivity Analysis

Abstract. Since the year 2010, different versions of the Carbon Bond 6 (CB6) mechanism have been developed, to accurately estimate the contribution to the air pollution by the chemistry. However, the discrepancies in simulation results brought about by the modifications between different versions of the CB6 mechanism are still not fully understood. Therefore, in the present study, we investigated the behavior of three different CB6 mechanisms (CB6r1, CB6r2 and CB6r3) in simulating ozone (O3), nitrogen oxides (NOx) and formaldehyde (HCHO) under an urban condition, by applying a concentration sensitivity analysis in a box model. The results show that when the surface emission is excluded, the O3 level predicted by CB6r1 is approximately 6 % and 8 % higher than that predicted by CB6r2 and CB6r3, specifically due to the change in the sink of CXO3 in the mechanism. In contrast, the levels of NOx and HCHO estimated by these three CB6 mechanisms are mostly similar, when the surface emission is turned off. After adding the surface emission, the simulated profiles of O3, NOx and HCHO obtained by CB6r2 and CB6r3 are similar. However, the deviation between the O3 levels provided by CB6r1 and the other two CB6 mechanisms (i.e. CB6r2 and CB6r3) is enlarged, because of the weakening of the ozone dependence on the emission of isoprene in CB6r1. Moreover, HCHO predicted by CB6r1 is found larger than that predicted by CB6r2 and CB6r3, which is caused by an enhanced dependence of HCHO on the emission of isoprene in CB6r1. Regarding to NOx, it was found that CB6r1 gives a higher value during the daytime and a lower value during the nighttime than the other two mechanisms, which is caused by the relatively stronger connection between the NOx prediction and the local chemistry in CB6r1, so that more NOx is consumed and converted to PANX (peroxyacyl nitrate with three and higher carbons) in the nighttime and more NOx is reformed by the photolysis of PANX in the daytime.