Nitric Acid-Amine Chemistry in the Gas Phase and at the Air-Water Interface

Gas-phase simulations of nitric acid-amine chemistry suggest that the fundamental acid-base interaction between HNO₃ and NH₃ results in a variety of HNO₃-NH₃-based complexes, such as (HNO₃)₃·(NH₃)₂, (HNO₃)₃·(NH₃)₃, and (HNO₃)₄·(NH₃)₃, that can be formed. The formation of these complexes in the gas phase follow different growth mechanisms under different relative humidity conditions. On the other hand, at the air-water interface, Born-Oppenheimer molecular dynamics simulations suggest that the formation of the fundamental NO₃^-··(R₁)(R₂)NH₂⁺ [for NH₃, R₁ = R₂ = H; CH₃NH₂, R₁ = H, R₂ = CH₃; and (CH₃)₂NH, R₁ = R₂ = CH₃] ion pairs require the formation of the HNO₃··(R₁)(R₂)NH complexes in the gas-phase prior to their adsorption on the water surface. Ion-pair formation at the water surface involves proton transfer from HNO₃ to (R₁)(R₂)NH and occurs within a few femtoseconds of the simulation. The NO₃^-··(R₁)(R₂)NH₂⁺ ion pairs preferentially remain at the interface over the picosecond time scale, where they are stabilized via hydrogen bonding with surface water molecules. This offers a novel chemical framework for understanding gas-to-particle partitioning in the atmosphere. These results not only improve our understanding of the formation of nitrate particulates in polluted urban environments, but also provide useful guidelines for understanding particle formation in forested or coastal environments, in which organic acids and organosulfates are present in significant quantities and their exact role in particle formation remains elusive.