Investigating the Role of Ammonia in Enhancing Secondary Organic Aerosol Formation from the Co-photooxidation of Anthropogenic and Biogenic VOCs

Abstract. Ammonia (NH₃) plays a crucial role in the complex physicochemical processes occurring in the atmosphere, but the mechanisms governing secondary organic aerosol (SOA) formation from NH₃-involved interactions between mixed anthropogenic and biogenic organic compounds remain poorly understood, thereby limiting the predictive capacity for air quality and climate. Previous studies have demonstrated that NH₃ can alter the oxidation pathways of single aromatic hydrocarbon precursor, promoting particle formation and growth. However, its role in mixed organic precursor systems has not been systematically explored. This study aims to elucidate these mechanisms through photooxidation experiments conducted in a large outdoor photochemical smog chamber, investigating mixtures of n-heptylcyclohexane (anthropogenic) and α-pinene (biogenic) under varying NH₃ conditions. The results indicated that NH₃ obviously accelerated VOC degradation and significantly contributed to SOA enhancement through facilitating nucleation and participating in particle-phase reactions in the mixed system. The presence of NH₃ could not only promote the generation of intermediates such as aldehydes and ketones, but also lead to an increase in SOA mass and number concentration, particularly nitrogen-containing light-absorbing substances like imidazoles. This research can provide a scientific basis for systematically assessing how NH₃ affects the co-oxidation of ambient anthropogenic and biogenic gases, and deepen the understanding of its role in SOA generation, particularly light-absorbing aerosols, in the AVOC-BVOC mixed system.