Reproducing Strong Urban Oxidation Capacity in Beijing Using an Empirical Parametrization

Atmospheric oxidation processes drive the conversion of primarily emitted compounds to secondary pollutants, with the hydroxyl radical (OH) as the predominant oxidant. In urban areas, the OH concentration is extremely useful to determine the formation rate of secondary pollutants (i.e., ozone, particulate nitrate, secondary organic aerosol), but the in situ measurement of OH is extremely difficult due to its high reactivity and low concentration. In this study, we presented OH measurements in a megacity, Beijing, for two seasons. Though the chemical conditions were highly variable, we found that more than half of the OH variability could be attributed to changes in photolysis frequencies. We developed an empirical parametrization that successfully reproduces observed OH concentrations across different seasons in Beijing by considering photolysis rates and NO₂ concentrations. This parametrization demonstrates sustained OH production even during winter haze, which is critical for understanding oxidation processes under low solar radiation. Our findings provide a valuable tool for improving air quality models in polluted urban environments and offer insights into effective pollution control strategies.