Agricultural activities emit ammonia (NH₃) and particulate matter (PM), yet Korean air-quality monitoring is concentrated in urban areas. We analyzed daily PM₁₀, PM_2.5, 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 PM_2.5 increments peaked in July–September. In exploratory contrasts, the PM_2.5 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.