Flux measurements of NO_x, O₃ and O_x: Emission characteristics, inventory validation and titration effects

As the precursor of ozone and secondary particles, the incomplete knowledge of NO_x emission dynamics constrains our ability to fully elucidate air pollution formation. Flux measurements offer new insights into NO_x emissions and titration effects. Here, we present eddy covariance flux measurements of NO_x, O₃ and O_x in urban Beijing during the summer of 2023. The measured NO_x flux was positive with the 24-h average of 10.4 ± 10.7 nmol/m²/s, which is at the lower end of the flux reported in urban regions. The low NO_x emission strength in urban Beijing may be related to the successful control measures of diesel vehicles, while confirming significant dependence of NO_x flux with traffic flow. The discrepancies by a factor of 1.3–14.5 between measured NO_x flux and estimates in emission inventories are observed, indicating most emission inventories may fail to accurately characterize NO_x emissions in Beijing. Better agreements between flux measurements and satellite retrieval are obtained. In contrast to NO_x, predominantly downward fluxes were observed for ozone (O₃) and O_x (=O₃ + NO₂). A multiple linear regression (MLR) method is developed to examine the impacts of NO_x emissions on the ozone downward flux, revealing that NO_x emissions induce 54 ± 53 % of ozone downward flux average over the observation period. This study demonstrates valuable information on emission strength and chemical transformation provided by eddy covariance flux measurements of NO_x, O₃ and O_x.