Secondary organic aerosol (SOA) significantly impacts radiative forcing, climate, and human health. A key process in SOA formation is the production of highly oxygenated molecules (HOMs) through the atmospheric oxidation of volatile organic compounds. These HOMs, formed by autoxidation of organic peroxy radicals (RO2), are low in volatility and can condense or nucleate, contributing to SOA. Despite extensive laboratory studies, the formation of HOMs under atmospheric conditions and their role in SOA formation are not yet fully understood. Laboratory conditions often lead to high RO2 concentrations, emphasizing RO2 cross reactions and potentially resulting in overestimated SOA yields. In contrast, atmospheric conditions involve complex mixtures where HOM-RO2 predominantly react with the hydroperoxyl radical (HO2) or nitric oxide (NO).This study investigates HOM formation in the -pinene photooxidation system using steady-state experiments in the SAPHIR-STAR reaction chamber. We shifted the chemical reaction regime from RO2-dominated to more atmospherically relevant conditions by increasing the concentrations of HO2 and NO, both separately and in mixture, while maintaining constant primary -pinene oxidation conditions. Ammonium sulfate particles were added to gain direct insight into the volatility of the HOM product distribution.The atmospheric fate of HOM-RO2 is defined by reactions with RO2, HO2, and NO, and the overall reactivity of their surroundings. Our experiments systematically varied these parameters. The results show that laboratory experiments with dominance of RO2+RO2 cross reactions lead to overestimations of the SOA formation when the according SOA yield is directly transferred to the atmosphere. By mechanistic considerations, we show how the HOM product distribution depends on the HO2 and NO concentrations and explain the reduced SOA formation by changes in the HOM products and their volatility.