Gas-particle partitioning, molecular weight, and yield of organic nitrate under different urban VOC, NO_x, and oxidation conditions during SAPHIR-CHANEL campaign

Abstract. Oxidation of volatile organic compounds (VOCs) involving hydroxyl radicals (OH^.), nitrogen oxides (NO_x), or nitrate radicals (NO₃^.) forms organic nitrates that undergo gas-particle partitioning, changing the lifetime of nitrogen compounds and their deposition on ecosystems. In urban areas, VOC composition is complex, with contributions from traffic, cooking, volatile chemical products (VCPs), and biogenic emissions. Secondary organic aerosol (SOA) formation from urban VOC mixtures was investigated using chamber experiments during the SAPHIR-CHANEL campaign under realistic VOC-NO_x and oxidation conditions. The yield of total organic nitrates is higher for precursor mixtures with a higher percentage of unsaturated VOCs, such as those from traffic and cooking sources (11–21 %), compared to VCPs and complex urban emission replicas (2–7 %). Enhanced particle-phase partitioning is observed under nighttime oxidation (by NO₃^.) versus daytime oxidation (by OH^.). Particulate organic nitrates have a higher average molecular weight under nighttime conditions (331 ± 13 g mol^-1) than under daytime conditions (258 ± 24 g mol^-1) due to increased oligomerization. Similarly, the mass fraction of the total organic aerosol that is organic nitrate is 2.6–4.5 times higher under nighttime than daytime conditions, likely due to higher molecular weight and lower temperatures. Although gas-phase organic nitrate composition varies substantially between precursor mixtures, bulk organic nitrate volatility is generally similar to that of modeled oxidized monoterpene nitrates (10⁻⁴–10⁻² m³ μg⁻¹ at 18–40 °C). These findings improve understanding of bulk organic nitrate sources and properties in a complex urban environment, allowing better simulations of air quality and nitrate deposition.