Summertime tropospheric ozone over China simulated with a regional chemical transport model 2. Source contributions and budget

A regional three‐dimensional chemical transport model is used to simulate the contributions of various sources to the NO x and O 3 distributions in the troposphere over China during a summertime period. O 3 tracers are added in the model to simulate the distribution of O 3 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 NO x that originate from surface emissions, aviation, lightning, STE and LBI, respectively. Meanwhile, the O 3 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 NO x and hence O 3 concentrations in the polluted boundary layer of eastern China. At higher altitudes the O 3 produced via surface‐emitted NO x accounts for 10–20% of the O 3 concentrations over these polluted regions. The impact of aircraft NO x emissions on tropospheric O 3 over China is small, contributing up to 2% of the O 3 concentrations at cruise altitudes. The net O 3 production via surface‐emitted NO x 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 O 3 transported into than out of the region and high relative humidity, in situ photochemistry acts as a sink for O 3 in the troposphere over the region.