<p><strong>Abstract.</strong> The impact of volatile organic compound (VOC) emissions to the atmosphere on the production of secondary pollutants, such as ozone and secondary organic aerosol (SOA), is mediated by the concentration of nitric oxide (NO). Polluted urban atmospheres are typically considered to be <q>high-NO</q> environments, while remote regions such as rainforests, with minimal anthropogenic influences, are considered to be <q>low-NO</q>. Policy to reduce urban air pollution is typically developed assuming that the chemistry is controlled by the high-NO regime. However, our observations from central Beijing show that this simplistic separation of regimes is flawed. Despite being in one of the largest megacities in the world, we observe significant formation of gas and aerosol phase oxidation products associated with the low-NO <q>rainforest-like</q> regime during the afternoon. This is caused by a surprisingly low concentration of NO, coupled with high concentrations of VOCs and of the atmospheric oxidant hydroxyl (OH). Box model calculations suggest that during the morning high-NO chemistry predominates (95&#8201;%) but in the afternoon low-NO chemistry plays a greater role (30&#8201;%). With increasing global emphasis on reducing air pollution, the modelling tools used to develop urban air quality policy need to adequately represent both high- and low-NO regimes if they are to have utility.</p>
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