Impact of NOx titration on nighttime isoprene in coastal South China: Preserving ozone precursors via suppressed oxidation.

Volatile organic compounds (VOCs) emissions and their subsequent secondary transformation processes drive ozone (O3) formation. However, observed ambient concentrations (OBS-VOCs) only represent the net residual levels following atmospheric chemical consumption, imposing considerable uncertainty in identifying key species. To address this limitation, this study calculated the photochemical initial concentrations (PIC-VOCs) for 70 VOC species observed in the Greater Bay Area, incorporating kinetic parameters for their reactions with major atmospheric oxidants. Results showed a net VOC consumption of 2.6 ± 0.5 ppb attributable to photochemistry, varying by individual species reactivity. This consumption led to substantial alterations in the ranking of key species based on ozone formation potential (OFP) when calculated from PIC-VOCs compared to OBS-VOCs, with OFP derived from PIC-VOCs exceeding that from OBS-VOCs by 40.4 μg/m3. OFP assessments based on OBS overestimated the contribution of oxygenated VOCs (OVOCs) by 29.7%, which are predominantly of secondary origin, while underestimating the highly reactive potential of primarily emitted biogenic VOCs (BVOCs) by 65.4% in near-source regions. Notably, this study provides quantitative field evidence of the persistent nocturnal isoprene presence driven by anthropogenic-biogenic interactions. On 62% of observation days, isoprene concentration remained above 0.3 ppb during the day-to-night transition following a midday peak. Under high-NO conditions (NO > 3 ppb), suppressed NO3 oxidation reduces isoprene consumption, extending its chemical lifetime by a factor of 2-3. Underestimating nocturnal NO leads to overestimating NO3 levels and thus underestimated isoprene. Ozone precursor control strategies should account for the photochemical loss of VOCs, the preservation of reactive BVOCs due to suppressed nocturnal oxidation by NOx, and the optimization of vehicle emissions.