Contributions of ammonia to high concentrations of pm_2.5 in an urban area

Atmospheric ammonia (NH₃ ) plays a critical role in PM_2.5 pollution. Data on atmospheric NH₃ are scanty; thus, the role of NH₃ in the formation of ammonium ions (NH₄⁺ ) in various environments is understudied. Herein, we measured concentrations of NH₃, PM_2.5, and its water-soluble SO₄²⁻, NO₃⁻, and NH₄⁺ ions (SNA) at an urban site in Jeonju, South Korea from May 2019 to April 2020. During the measurement period, the average concentrations of NH₃ and PM_2.5 were 10.5 ± 4.8 ppb and 24.0 ± 12.8 µg/m³, respectively, and SNA amounted to 4.3 ± 3.1, 4.4 ± 4.9, and 1.6 ± 1.8 µg/m³, respectively. A three-dimensional photochemical model analysis revealed that a major portion of NH₃, more than 88%, originated from Korea. The enhancement of the ammonium-to-total ratio of NH₃, NH_X (NHR = [NH₄⁺ ]/[NH₄⁺ ] + [NH₃ ]) was observed up to ~0.61 during the increase of PM_2.5 concentration (PM_2.5 ≥ 25 µg/m³ ) under low temperature and high relative humidity conditions, particularly in winter. The PM_2.5 and SNA concentrations increased exponentially as NHR increased, indicating that NH₃ contributed significantly to SNA formation by gas-to-particle conversion. Our study provided experimental evidence that atmospheric NH₃ in the urban area significantly contributed to SNA formation through gas-to-particle conversion during PM_2.5 pollution episodes.