Atmospheric water cluster-catalyzed formation of nitroaromatics as a secondary aerosol source.

Nitroaromatic compounds (NACs) are an abundant class of compounds in atmospheric particulate matter, exerting considerable impacts on air quality, climate, and public health. They are typically derived from secondary formation, which is generated through atmospheric chemical reactions involving precursor compounds, such as phenolic compounds. However, information on NAC formation mechanisms remains limited. Here, we found that particulate-phase NAC concentrations were notably affected by relative humidity (RH) in field observations. Smog chamber experiments indicated that RH could influence NAC formation via changes in gaseous water concentration, rather than solely through aerosol liquid water content. Theoretical calculations further clarified that water clusters (WCs), formed from gaseous water, can notably lower reaction energy barriers along the NAC formation pathway, rendering this pathway competitive under atmospheric conditions. Hence, we propose a potential mechanism in which WCs catalyze NAC formation, offering a previously unidentified perspective on atmospheric secondary aerosol formation.