VOC evolution during petrochemical abnormal operations and vertical characteristics during shutdown: Evidence from ground and tower observations.

Petrochemical shutdown, maintenance, and start-up operations emit substantial amounts of reactive volatile organic compounds (VOCs). However, the vertical distribution of these emissions within the lower boundary layer remains poorly characterized. This study combined full-cycle online monitoring with shutdown-stage multi-height tower sampling (10, 50 and 90 m) and receptor-site observations to investigate VOC evolution, vertical profiles, and spatial transport in a large petrochemical park. After meteorological normalization, mean TVOC concentrations during shutdown, maintenance, and start-up were 97.59, 106.62 and 85.36 ppbv, respectively. VOC composition showed more pronounced stage-dependent changes. Alkenes increased by 53.69% during maintenance, with ethene reaching 32.13 ppbv. Propylene and 1,3-butadiene increased markedly during start-up. During shutdown, TVOC showed a non-monotonic vertical profile and peaked at 146.29 ppbv at 50 m, 1.52 times that at 10 m. The difference between 10 and 50 m was significant (p < 0.001). VOC-derived OH reactivity (LOH) and ozone formation potential (OFP) also peaked at 50 m, reaching 32.07 s⁻1 and 1160.73 μg m⁻3, respectively. After normalization, LOH/TVOC was similar at 10 and 50 m, while OFP/TVOC increased with height. Near-ground observations within the park and at the tower during the shutdown stage showed reduced VOC concentration fluctuations during transport, accompanied by a shift from alkane- and alkene-dominated composition toward greater OVOC contributions. Combined near-ground and multi-height tower observations during shutdown highlighted the potential limitations of single-height near-ground monitoring in characterizing vertical variations in elevated plumes. These measurements support the value of height-resolved observations for characterizing reactive VOCs during petrochemical shutdown.