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.

Ammonia (NH₃) plays an important role in air quality and atmospheric chemistry, yet studies on the characteristics and impacts of NH₃ are limited. Herein, we revealed the spatial distribution of atmospheric NH₃, as measured by passive samplers, at three different sites (R1, R2, and R3) in the rural area (livestock environment) of Jeongeup, South Korea, from September 2019 to August 2020. At site R1, the boundary of a large-scale pig farm, dramatically high daily mean concentrations of NH₃ were observed (118.7 ppb), whereas sites R2 and R3, located ~1 km from R1, exhibited lower concentrations of 18.2 and 30.4 ppb, respectively. In the rural environment, the monthly NH₃ variations showed a peak in June (34.2 ppb), which was significantly higher than in the urban and remote areas. To examine the impact of NH₃ from the rural area on a nearby urban area in June 2020, simultaneous measurements were performed using a real-time instrument in Jeonju. When high NH₃ events occurred in the urban area in June, the results for the NH₃ concentrations and observed meteorological conditions in the rural and urban areas showed that the rural area influenced the NH₃ levels in the adjacent urban area.