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

Abstract

Nitrogen dioxide (NO2) is a crucial precursor of tropospheric O3. This study undertakes continuous measurements of nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3) at Shishan Port (SSP), a riverine site in Foshan, China. Oxides of nitrogen (NOx = NO + NO2) exhibit a distinct seasonality, with high concentrations in winter and low concentrations in summer. The daily average concentration of NOx is 30.5 ppb, with high concentrations in the evening and morning and low concentrations in the afternoon. The variation of the total oxidant (Ox = O3 + NO2) with the levels of NOx was examined to infer the atmospheric sources of Ox as the sum of a local contribution dependent on NOx and a regional contribution independent of NOx. The local contribution predominates during low wind speed and calm weather in winter. The case analysis of a typical NO2 pollution episode affirms that the increase of NO2 at night in SSP is significantly influenced by the exhaust emissions of passing ships, atmospheric diffusion conditions and the O3 concentration generated during the day. Due to the chemical coupling of NOx and O3, along with regional transport, the levels of NOx and O3 are inextricably linked yet complicated.