Abstract. To better understand the secondary air pollution in transboundary air over westernmost Japan, ground-based field measurements of the chemical composition of fine particulate matter ( ≤  1 µm), mixing ratios of trace gas species (CO, O₃, NO_x, NO_y, i-pentane, toluene, and ethyne), and meteorological elements were conducted with a suite of instrumentation. The CO mixing ratio dependence on wind direction showed that there was no significant influence from primary emission sources near the monitoring site, indicating long- and/or mid-range transport of the measured chemical species. Despite the considerably different atmospheric lifetimes of NO_y and CO, these mixing ratios were correlated (r² = 0.67). The photochemical age of the pollutants, t[OH] (the reaction time  ×  the mean concentration of OH radical during the atmospheric transport), was calculated from both the NO_x ∕ NO_y concentration ratio (NO_x ∕ NO_y clock) and the toluene ∕ ethyne concentration ratio (hydrocarbon clock). It was found that the toluene / ethyne concentration ratio was significantly influenced by dilution with background air containing 0.16 ppbv of ethyne, causing significant bias in the estimation of t[OH]. In contrast, the influence of the reaction of NO_x with O₃, a potentially biasing reaction channel on [NO_x] / [NO_y], was small. The t[OH] values obtained with the NO_x ∕ NO_y clock ranged from 2.9  ×  10⁵ to 1.3  ×  10⁸ h molecule cm⁻³ and were compared with the fractional contribution of the m∕z 44 signal to the total signal in the organic aerosol mass spectra (f₄₄, a quantitative oxidation indicator of carboxylic acids) and O₃ mixing ratio. The comparison of t[OH] with f₄₄ showed evidence for a systematic increase of f₄₄ as t[OH] increased, an indication of secondary organic aerosol (SOA) formation. To a first approximation, the f₄₄ increase rate was (1.05 ± 0.03)  ×  10⁻⁹  ×  [OH] h⁻¹, which is comparable to the background-corrected increase rate observed during the New England Air Quality Study in summer 2002. The similarity may imply the production of similar SOA component, possibly humic-like substances. Meanwhile, the comparison of t[OH] with O₃ mixing ratio showed that there was a strong proportional relationship between O₃ mixing ratio and t[OH]. A first approximation gave the increasing rate and background mixing ratio of ozone as (3.48 ± 0.06)  ×  10⁻⁷  ×  [OH] ppbv h⁻¹ and 30.7 ppbv, respectively. The information given here can be used for prediction of secondary pollution magnitude in the outflow from the Asian continent.