Seasonal variation of enhancement ratios of trace gases observed over 10 years in the western North Pacific

We analyzed seasonal and geographical variations of the enhancement ratios of trace gases of the Asian continental pollution (ACP) events observed over the western North Pacific region. The ACP events were identified by using continuous carbon monoxide (CO) measurements taken at three monitoring stations of Minamitorishima (MNM), Yonagunijima (YON), and Ryori (RYO) in Japan for more than 10 years. The frequency of ACP events at MNM and YON increased in the winter and early spring seasons, but not at RYO. The enhanced concentrations of CO (Delta CO) associated with the ACP events showed a clear seasonal variation at MNM and YON, but not at RYO. Other trace gases of methane (CH4), carbon dioxide (CO2), and ozone (O-3) were simultaneously enhanced for many of the ACP events, and were highly correlated with Delta CO. The frequency distributions of the enhancement ratios of Delta CH4/Delta CO, Delta CO/Delta CO2, and Delta O-3/Delta CO depended largely on the geographical locations of the stations. Distinct seasonal variations in the enhancement ratios showed a decrease in Delta CH4/Delta CO but an increase in Delta CO/Delta CO2 in the winter and early spring at all three stations, although their seasonal amplitudes were different each station. These results indicate that the chemical compositions associated with the ACP events were considerably influenced by the seasonal variations in the trace gas emission ratios of the Asian continental source regions, with increased fluxes of CH4 in summer and CO in winter. Contributions to the Delta CH4/Delta CO and Delta CO/Delta CO2 ratios from the photochemical evolution during atmospheric transport were estimated to be not significant, while the seasonal changes in the Delta O-3/Delta CO ratio were controlled significantly by the photochemistry. (C) 2011 Elsevier Ltd. All rights reserved.