A regional three-dimensional chemical transport model is used to simulate the contributions of various sources to the NOx and O3 distributions in the troposphere over China during a summertime period. O3 tracers are added in the model to simulate the distribution of O3 that originates from stratosphere-troposphere exchange (STE), lateral boundary import (LBI) and in situ photochemistry via NO, respectively. Tracers for reactive nitrogen species are further incorporated into the model to simulate the distributions of NOx that originate from surface emissions, aviation, lightning, STE and LBI, respectively. Meanwhile, the O3 formed by chemical reactions via the NO tracer originating from these sources is also tagged. The model results indicate that surface emissions are the dominant contributor to the NOx and hence O3 concentrations in the polluted boundary layer of eastern China. At higher altitudes the O3 produced via surface-emitted NOx accounts for 10--20% of the O3 concentrations over these polluted regions. The impact of aircraft NOx emissions on tropospheric O3 over China is small, contributing up to 2% of the O3 concentrations at cruise altitudes. The net O3 production via surface-emitted NOx over the region is estimated to be 132 Gg d-1 in the boundary layer and 33 Gg d-1 in the free troposphere. Because there is much more O3 transported into than out of the region and high relative humidity, in situ photochemistry acts as a sink for O3 in the troposphere over the region.