Field measurements for trace-level chemical species in the gas and particulate phases were conducted at a rural site in westernmost Japan to better understand formation of secondary pollutants in transboundary air. A comparison of photochemical ages estimated by chemical clock with ozone concentrations or fractions of carboxylate in organic aerosol showed proportional relationships, and their slopes were 3.48 × 10⁻⁷ ppbv molecule⁻¹ cm³ h⁻¹ and 1.05 × 10⁻⁹ molecule⁻¹ cm³ h⁻¹, respectively.