Ground-gridded observations of high concentrations of ambient limona ketone suggest overlooked limonene oxidation in subtropical megacities.

Limonene significantly influences atmospheric oxidation and secondary pollution, yet its transformation in polluted subtropical megacities remains unclear. This study combined large-scale gridded observations and intensive diurnal measurements of its oxidation product, limona ketone (LK), in the Pearl River Delta (PRD) region. Unusually high concentrations of LK during specific spring and autumn ozone (O3)-pollution episodes were revealed. Using LK as a constraint, a parameterized inversion suggested potentially overlooked limonene precursor abundance and revealed a pronounced nighttime enhancement. LK abundance covaried with regional O3 pollution, while hydroxyl radical (OH) dominated modeled LK production and the major LK loss pathway shifted from OH during daytime to nitrate radical at night. Under highly oxidative conditions, primary limonene signals are rapidly obscured, whereas measured LK provides a complementary constraint on preceding limonene oxidation. The nocturnal enhancement remained across plausible parameter ranges and was consistent with nighttime accumulation and/or sustained emissions. Our episode-specific studies highlight the need for long-term, multi-season observations of limonene chemistry in photochemically active subtropical megacity regions, and suggest that the potential atmospheric and health implications of rapidly formed oxidation products such as LK warrant further investigation.