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To better understand the secondary air pollution in transboundary air overwesternmost Japan, ground-based field measurements of the chemicalcomposition of fine particulate matter ( ≤  1 µm), mixing ratios oftrace gas species (CO, O3, NOx, NOy, i-pentane, toluene, andethyne), and meteorological elements were conducted with a suite ofinstrumentation. The CO mixing ratio dependence on wind direction showedthat there was no significant influence from primary emission sources nearthe monitoring site, indicating long- and/or mid-range transport of themeasured chemical species. Despite the considerably different atmosphericlifetimes of NOy 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 atmospherictransport), was calculated from both the NOx ∕ NOy concentrationratio (NOx ∕ NOy clock) and the toluene ∕ ethyne concentration ratio(hydrocarbon clock). It was found that the toluene / ethyne concentrationratio was significantly influenced by dilution with background aircontaining 0.16 ppbv of ethyne, causing significant bias in the estimationof t[OH]. In contrast, the influence of the reaction of NOx withO3, a potentially biasing reaction channel on [NOx] / [NOy],was small. The t[OH] values obtained with the NOx ∕ NOy clock rangedfrom 2.9  ×  105 to 1.3  ×  108 h molecule cm−3and were compared with the fractional contribution of the m∕z 44 signal to thetotal signal in the organic aerosol mass spectra (f44, a quantitativeoxidation indicator of carboxylic acids) and O3 mixing ratio. Thecomparison of t[OH] with f44 showed evidence for a systematic increase off44 as t[OH] increased, an indication of secondary organic aerosol (SOA) formation. To a firstapproximation, the f44 increase rate was (1.05 ± 0.03)  ×  10−9  ×  [OH] h−1, which is comparable to thebackground-corrected increase rate observed during the New England AirQuality Study in summer 2002. The similarity may imply the production ofsimilar SOA component, possibly humic-like substances. Meanwhile, thecomparison of t[OH] with O3 mixing ratio showed that there was a strongproportional relationship between O3 mixing ratio and t[OH]. A firstapproximation gave the increasing rate and background mixing ratio of ozoneas (3.48 ± 0.06)  ×  10−7  ×  [OH] ppbv h−1and 30.7 ppbv, respectively. The information given here can be used forprediction of secondary pollution magnitude in the outflow from the Asiancontinent

Photochemical age of air pollutants, ozone, and secondary organic aerosol in transboundary air observed on Fukue Island, Nagasaki, Japan