Estimating ozone formation potential from ambient volatile organic compound observations introduces bias by ignoring photochemical aging. Reconstructed initial emissions for a Shanghai suburban site reveal that primary oxygenated compounds and unmeasured species constitute 40.8 % and 12.6 % of total emissions, respectively. These overlooked precursors exhibit ozone formation capacity comparable to non-methane hydrocarbons, highlighting the need to include them in mitigation strategies.