Characteristics of Air-Pollutant Concentration Variations Across Rice Cultivation Stages at Four Paddy Sites in Korea

Agricultural activities emit ammonia (NH3) and particulate matter (PM), yet Korean air-quality monitoring is concentrated in urban areas. We analyzed daily PM10, PM2.5, NO2, SO2, and NH3 at four rice-paddy sites (March 2023–June 2025) across five calendar-defined cultivation stages, which are confounded with season. We therefore separated winter from early spring, calculated increments relative to paired urban stations, and compared recorded operation days with other days of the same month. PM10 was highest in the preparation stage (46.08 vs. 23.97 μg/m3 in growth management; η2 = 0.092), and the March–April maximum persisted after winter was separated and after meteorological adjustment. However, at the three sites without a persistent local coarse-particle source, the early-spring PM10 increment over urban stations was the lowest of all periods (−7.62 μg/m3), while local PM10 and PM2.5 increments peaked in July–September. In exploratory contrasts, the PM2.5 increment was 2.3 μg/m3 higher on tillage/leveling days (p = 0.005; q = 0.063). NH3 was governed mainly by site (partial η2: site 0.256, site × stage 0.145, stage 0.033; site means 8.6–37.0 ppb). The calendar-stage PM contrast largely reflects the regional seasonal cycle, whereas agricultural influences appear as operation day and warm-season local increments and as site-specific NH3 levels.