  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.
