Elucidating the quantitative characterization of atmospheric oxidation capacity in Beijing, China

Highlights: • Newly developed indexes were used to elucidate the atmospheric oxidation capacity (AOC). • Well-defined seasonal and diurnal patterns of AOC were found in urban and suburban Beijing. • AOC was dominated by oxidation of CO and alkenes in summer and of CO and NO₂ in winter. • The dominate contributor to AOC was OH and ozone during the daytime and nighttime, respectively. • The heterogeneous oxidation processes contributed substantially to the underestimation of AOC. The atmospheric oxidizing capacity (AOC) is the essential driving force of tropospheric chemistry, but its quantitative representation remains limited. This study presents the detailed evaluation of AOC in the megacity of Beijing based on newly developed indexes that represent the estimated oxidative capacity from the prospective of oxidation products (AOIe) and the potential oxidative capacity considering the oxidation rates of major reactants by oxidants (AOIp). A comprehensive suite of data taken from summer and winter field campaigns were used to create these two indexes and in the calculation of AOC. The AOC showed a clear seasonal pattern, with stronger intensity in summer compared to winter. The gaseous-phase oxidation products (O₃ and NO₂) dominated AOIe (~80%) during summertime at both sites, while the contribution of particle-phase oxidation products (sulfate, nitrate, and secondary organic aerosol) to AOIe increased in winter (~30%). As for AOIp in summer, the dominant contributor was alkenes (31.0%, urban) and CO (38.5%, suburban), whereas CO and NO₂ dominated AOIp at both urban (68.8%) and suburban (61.0%) sites during wintertime. As expected, the dominant oxidant contributor to AOIp during the daytime was OH, while O₃ was the second most important oxidant at both sites. The diurnal variations of normalized AOIe and AOIp were examined, revealing that they share the same daytime peak but showed significant bias during the nighttime. To explore the possible deviation in sources between AOIe and AOIp, a constrained photochemical box model and a constrained multiphase chemical box model were used to evaluate AOC budgets and their source apportionment. Our results suggest that unmeasured OVOC (oxygenated volatile organic compound) species and missed heterogeneous oxidation processes in the calculation of AOIp contributed substantially to the underestimation of AOC by this index, which should be taken into consideration in future studies of AOC.