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Transboundary Secondary Organic Aerosol in Western Japan: An Observed Limitation of the f44 Oxidation Indicator

To obtain evidence for secondary organic aerosol formation during the long range transport of air masses over the East China Sea, we conducted field measurements in March 2012 at the Fukue atmospheric monitoring station, Nagasaki, in western Japan. The relative abundance of m/z 44 in fine organic aerosol mass spectra (f44) was measured by an Aerodyne aerosol chemical speciation monitor. The stable carbon isotope ratio (d13C) of low volatile water soluble organic carbon (LV-WSOC) in the daily filter samples of total suspended particulate matter was also analyzed using an elemental analyzer coupled with an isotope ratio mass spectrometer. Additionally, in situ measurements of NOx and NOy were performed using NOx and NOy analyzers. The measurements showed that, unlike the systematic trends observed in a previous field study, a scatter plot for d13C of LV-WSOC versus f44 indicated a random variation. Comparison of f44 with the photochemical age estimated by the NOx to NOy ratio revealed that the f44 values distributing around 0.2 likely reached a saturation level already and the f44 values were significantly lower than the observed f44, approximately 0.3, at Hedo in the previous study. These findings imply that the saturation point of f44, and the use of f44 as an oxidation indicator, is case dependent.