Temporal characterization of NO_x and its relationship with O₃ at a riverine site of Foshan

Nitrogen dioxide (NO 2 ) is a crucial precursor of tropospheric O 3 . This study undertakes continuous measurements of nitric oxide (NO), nitrogen dioxide (NO 2 ) and ozone (O 3 ) at Shishan Port (SSP), a riverine site in Foshan, China. Oxides of nitrogen (NO x = NO + NO 2 ) exhibit a distinct seasonality, with high concentrations in winter and low concentrations in summer. The daily average concentration of NO x is 30.5 ppb, with high concentrations in the evening and morning and low concentrations in the afternoon. The variation of the total oxidant (O x = O 3 + NO 2 ) with the levels of NO x was examined to infer the atmospheric sources of O x as the sum of a local contribution dependent on NO x and a regional contribution independent of NO x . The local contribution predominates during low wind speed and calm weather in winter. The case analysis of a typical NO 2 pollution episode affirms that the increase of NO 2 at night in SSP is significantly influenced by the exhaust emissions of passing ships, atmospheric diffusion conditions and the O 3 concentration generated during the day. Due to the chemical coupling of NO x and O 3 , along with regional transport, the levels of NO x and O 3 are inextricably linked yet complicated.