New Perspectives on Atmospheric Nitrogen Oxides Chemistry: The Role of the Air–Water Interface

Aerosols, suspensions of fine particles in air, play central roles in ozone depletion, cloud formation, and secondary product formation. Their climatic and air-quality impacts are significant, yet processes at the gas–particle interface remain poorly understood. A major gap concerns interfacial redox chemistry, which underpins the (photo)chemistry and interconversion of reactive nitrogen species (RNS: HNO₃/NO₃⁻, NO₂, NO, HONO/NO₂⁻, etc.). RNS regulate atmospheric OH radical levels and ozone chemistry, yet current knowledge cannot fully explain their observed abundances. For instance, unexpectedly high photochemical HONO yields in mixed NO₃⁻ aerosols point to mechanisms beyond gas-phase and bulk-aqueous photochemistry.The working hypothesis posits that, at the air–water interface, ion solvation is incomplete and an electric field is present, thereby favoring rapid, spontaneous reactions and enabling RNS interconversion.In this work, we combined three experimental approaches to investigate RNS chemistry, ranging from single levitated droplets in an optical tweezer apparatus to nebulized water droplets in a flow tube, and bulk aqueous photochemistry experiments. We demonstrate that:• NO₂⁻ is oxidized to NO₂ and NO₃⁻ in synthetic air in the absence of light and catalysts• NO₃⁻ is reduced to HONO/NO₂⁻ in synthetic air in the absence of light and catalysts• The NO₂ production rate from NO₃⁻ photochemistry is significantly reduced in high-ionic-strength aqueous environmentsThe results are discussed from both physical-chemistry and environmental perspectives, with the impact of air–water interface chemistry on atmospheric oxidation capacity compared with previous studies and other radical sources.