Naphthalene, the most abundant atmospheric gas-phase PAH and a known Secondary Organic Aerosol (SOA) precursor, is emitted from combustion and asphalt production and paving alongside other hydrocarbons, NO_x and SO₂. We investigated the influence of reaction conditions on the formation of SOA from the OH-oxidation of naphthalene in the Irish Atmospheric simulation Chamber (IASC). Benzene, NO_x and SO₂ were added to the chamber both individually and in combination, resulting in a total of eight reaction mixtures (RM). SOA formation was observed in all RM, with the base case of 30 ppbv of naphthalene producing 40 <em>&mu;</em>g m^&minus;3 after 1 hour of photooxidation. The different co-reactants significantly influenced the formation and evolution of SOA: the addition of benzene delayed the onset of particle formation, whereas NO_x accelerated particle formation by promoting faster generation of oxidation products and enhanced the total particle mass concentration, likely through enhanced OH radical recycling. The addition of SO₂ accelerated particle formation by increasing the condensability of organic vapours without altering the underlying chemistry. The combined presence of NO_x and SO₂ produced the strongest effect, both accelerating oxidation product formation and enhancing partitioning into the particle phase. Our results demonstrate that studies of individual precursor systems may overlook secondary pollutant formation from mixture effects that are relevant to real-world environments. Air pollutant formation depends not only on SOA precursor emissions but also on co-oxidation with other gaseous pollutants. Reducing Volatile Organic Compounds (VOC), NO_x, and SO₂ emissions may therefore improve air quality and reduce occupational exposure.