Photochemical Aging of China VI Gasoline Vehicle Exhaust Drives Nonlinear Secondary Aerosol Formation

China VI vehicle emission standards reduce primary exhaust pollutants, but impacts on secondary air pollution are less understood. We studied photochemical aging of hot-start exhaust from a China VI gasoline vehicle using smog-chamber experiments, focusing on semi- and intermediate-volatility organic compounds (S/IVOCs) and oxygenated volatile organic compounds (OVOCs) under variable oxidative conditions. Over OH exposures equivalent to 0.6-5.8 days, nonmethane organic gases (NMOGs) rapidly depleted, while low-molecular-weight OVOCs formed continuously. Secondary aerosol formation exhibited nonlinear dependence on oxidative capacity. Under low OH, secondary organic aerosol (SOA) dominated particle mass, whereas at high OH, secondary inorganic aerosol─mainly ammonium nitrate─accounted for 55-85% of total secondary aerosol mass, promoted by enhanced oxidation and ammonia availability. S/IVOCs contributed 54-88% of predicted SOA despite low NMOG fractions, while VOC/OVOC-based predictions underestimated SOA under high oxidation. Increasing OH exposure decreased SOA carbon oxidation state, reflecting a transition from functionalization to fragmentation chemistry. Health risk assessment indicated that photochemical aging increases noncarcinogenic risk via OVOC formation but decreases carcinogenic risk as aromatics oxidize. Overall, China VI vehicles remain efficient sources of secondary particulate pollution under oxidant- and ammonia-rich conditions, highlighting the importance of including secondary products in air quality and health impact assessments.