Measurement of NOy during Campaign of Air Quality Research in Beijing 2006 (CAREBeijing-2006): Implications for the ozone production efficiency of NOx

Total reactive nitrogen oxides (NOy) are among the key components in the chemistry of ozone production. In order to improve the understanding of the formation mechanisms of high-ozone events in Beijing, China, an intensive experiment on the reactive nitrogen oxides was carried out at the observatory of Peking University during the Campaign of Air Quality Research in Beijing and surrounding areas in 2006 (CAREBeijing-2006) campaign. In this study, analysis focusing on the data of high O-3 episodes was performed to examine the relationship between the concentration of total oxidant and the composition of NOy. High levels of NO and NOy were observed in the morning rush hours, indicating the influences of fresh emissions from local traffic. However, the ratio of [NO] to [NOy] was only 11-60% in the morning, implying that there was a substantial amount of aged air pollutants remained overnight in the stagnant air mass. Significant increases in the NOz mixing ratio (= [NOy]-[NOx]) were observed during the period from the morning toward early afternoon, consistent with the increasing oxidant level. Ozone production efficiency of NOx (OPEx), which was derived from the NOz-O-x regression, was found to range from 3.9 to 9.7 mol/mol in Beijing. Furthermore, it was revealed that the daily [NOz] maximum was proportional to the NOx peak level in the morning, and that OPEx decreased with the increases of [NOz] in a hyperbolic form. According to the results, abatement in NOx emission would not be effective toward reducing ozone concentrations in Beijing.