The impact of reducing industrial emissions of volatile organic compounds (VOCs) on ozone (O 3) pollution is of wide concern particularly in highly industrialized megacities. In this study, O 3 , nitrogen oxides (NO x) and VOCs were measured at an urban site in the Pearl River Delta region during the 2018 Chinese National Day Holidays and two after-holiday periods (one with ozone pollution and another without). O 3 pollution occurred throughout the 7-day holidays even industrial emissions of VOCs were passively reduced due to temporary factory shutdowns, and the toluene to benzene ratios dropped from ∼10 during non-holidays to ∼5 during the holidays. Box model (AtChem2-MCM) simulations with the input of observation data revealed that O 3 formation was all VOC-limited, and alkenes had the highest relative incremental reactivity (RIR) during the holiday and non-holiday O 3 episodes while aromatics had the highest RIR during the non-pollution period. Box model also demonstrated that even aromatics decreased proportionally to levels with near-zero contributions of industrial aromatic solvents, O 3 concentrations would only decrease by less than 20% during the holiday and non-holiday O 3 episodes and ozone pollution in the periods could not be eliminated. The results imply that controlling emissions of industrial aromatic solvents might be not enough to eliminate O 3 pollution in the region, and more attention should be paid to anthropogenic reactive alkenes. Isoprene and formaldehyde were among the top 3 species by RIRs in all the three pollution and non-pollution periods, suggesting substantial contribution to O 3 formation from biogenic VOCs. [Display omitted] [ABSTRACT FROM AUTHOR]