Flux measurements of NOx, O3 and Ox: Emission characteristics, inventory validation and titration effects.

As the precursor of ozone and secondary particles, the incomplete knowledge of NOx emission dynamics constrains our ability to fully elucidate air pollution formation. Flux measurements offer new insights into NOx emissions and titration effects. Here, we present eddy covariance flux measurements of NOx, O3 and Ox in urban Beijing during the summer of 2023. The measured NOx 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 NOx emission strength in urban Beijing may be related to the successful control measures of diesel vehicles, while confirming significant dependence of NOx flux with traffic flow. The discrepancies by a factor of 1.3-14.5 between measured NOx flux and estimates in emission inventories are observed, indicating most emission inventories may fail to accurately characterize NOx emissions in Beijing. Better agreements between flux measurements and satellite retrieval are obtained. In contrast to NOx, predominantly downward fluxes were observed for ozone (O3) and Ox (=O3 + NO2). A multiple linear regression (MLR) method is developed to examine the impacts of NOx emissions on the ozone downward flux, revealing that NOx 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 NOx, O3 and Ox.