Monoterpenes amplify the oxidative potential of biomass burning secondary aerosols

The interactions between different emission sources have significant impacts on secondary organic aerosol (SOA) formation and related environmental effects. Biomass burning and biogenic emissions could interact with each other; however, the formation, composition, and health effects of SOA from such mixtures remain poorly understood. Here, we investigated the interactions and atmospheric evolution of mixed biomass burning-derived guaiacol and biogenic α-pinene using an oxidation flow reactor (OFR), coupled with aerosol size distribution, offline iodometry-mass spectrometry, and dithiothreitol (DTT) assays. The results show that the mixed SOA yield shifted from suppression (at low hydroxyl radicals (OH) exposure) to enhancement (at high OH exposure), driven by low-volatility organics and subsequent enhanced gas-particle partitioning of intermediates. Fresh guaiacol SOA exhibited higher oxidative potential (OP) due to redox-active C₇ quinones and methoxy-hydroquinones, whereas the declining total peroxide of α-pinene SOA was attributed to the oxidative degradation. The OP of aged mixed SOA surpassed predicted values, which may be linked to the C₁₅-C₁₇ quinone-terpenoid adducts and C₇ toxic products stabilizing semiquinone radicals, thereby facilitating sustained reactive oxygen species (ROS) generation. This study revealed that biogenic emissions amplify the health risks of biomass-burning emissions.