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Current atmospheric quality models usually underestimate the level of ambient secondary organic aerosol (SOA),one of the possible reasons is that the precursors at different concentrations may undergo different oxidation processes and further affect SOA formation.Therefore,there is a need to perform more chamber studies to disclose the influence.In this work,SOA formation over a wide range of initial precursor concentrations (tens of ppb to hundreds of ppb levels) was investigated in a 30 m3 indoor smog chamber,and mainly through the analysis of multiple generations of VOCs detected from HR-ToF-PTRMS to expound the difference in the oxidation process between low and high precursor concentrations.Compared to high initial concentrations,gas-phase intermediates formed at low concentrations had a higher intensity by about one order of magnitude,and the lowvolatility compounds also had a higher formation potential due to the competition between semi-volatile intermediates and precursors with oxidants.In addition,the formed SOA was more oxidized with higher f44 value (0.14 ± 0.02) and more relevant to real atmosphere than that formed at high concentrations.This work should help to deeply understand SOA formation and improve the performance of air quality models for SOA simulation.