Partitioning of atmospheric fixed nitrogen species among gas phase, fine particles, and coarse particles

To improve our knowledge about the factors controlling the partitioning of ammonia/ammonium and nitric acid/nitrate among the gas phase, fine particles, and coarse particles, a year-round observation of these species in the gaseous and particulate phases was conducted at an inland rural site in Japan. The partitioning of ammonia/ammonium between the gaseous and particulate phases is strongly controlled by the amount of acidic species that can potentially react with ammonia gas. An excess amount of the acidic species over the sum of ammonia/ammonium facilitates the shift of gaseous ammonia to particulate ammonium. Although the ambient temperature affects the partitioning of nitric acid/nitrate between the gas phase and fine particles, the amount of coarse sea-salt particles dominantly control the partitioning between the gaseous and particulate phases; a large amount of coarse sea-salt particles facilitates the partition of nitrate into the coarse mode range, whereas it prevents the partition of nitric acid into the gas phase. Sea-salt particles would have a critical role for the partition of nitric acid/nitrate among the gas phase, fine particles, and coarse particles, even in the inland area with lower amounts of sea-salt particles. An excessive amount of ammonia/ammonium for complete neutralization of the particulate non-sea-salt sulfate is also important for the partitioning and its lack is unfavorable for the generation of nitrate in the fine particles even under lower temperature conditions.

► The amount of acid species that can react with NH3 controls the phase partitioning of NH3/NH4+. ► The amount of coarse sea-salt particles controls the phase partitioning of HNO3/NO3−. ► The amount of NH3/NH4+ also affects the phase partitioning of HNO3/NO3−. ► Temperature is not critical for the partitioning of HNO3/NO3− under the condition with low NH3/NH4+.