Large Contributions of Gas-Particle Partitioning and Heterogenous Processes to Particulate Nitroaromatic Compounds at a Mountain Site Revealed by Observation-Based and Multiphase Modeling

Particulate nitroaromatic compounds (NACs), among the major atmospheric components of light-absorbing brown carbon, have garnered increasing attentions due to their impacts on atmospheric environment and ecological health. However, there is a scarcity of comprehensive understanding on their origins and formation pathways. In this study, 14 particulate NACs were measured at high time resolution at the summit of Mount Tai over North China Plain in winter with an average concentration of 30.5 ng m⁻³. The relatively higher concentrations and distinct diurnal profiles of NACs suggest the occurrence of strong secondary formation driven from polluted urban plumes transported by mountain-valley breezes, elevated NO_x and precursor levels, and strong oxidation capacity. An observationally constrained multiphase chemical box model was developed and employed to elucidate the origins and formation pathways of NACs. They mainly came from the air mass transport from the polluted urban regions to the mountaintop during the day, while the nocturnal NACs formation is dominated by the partitioning of gaseous NACs to the particle phase and heterogenous processes on aerosols. This work provides observational evidence of elevated NACs levels at high mountain site as well as modeling reference for multiphase processes of particulate NACs and the contributions from different formation pathways to a large degree addressing the problem of underestimation in traditional models.