ABSTRACT

An anomalously low carbon monoxide (CO) episode was observed at Cape Ochi-ishi located in northern Japan in summer 2004. The low CO episode persisted for as long as twelve days from July 27 to August 7, 2004, and the CO levels at Cape Ochi-ishi during this episode were comparable to at Hateruma Island, a southernmost island of Japan. We explored regional distributions of CO and roles of synoptic-scale meteorology in this episode by using a regional chemistry-transport model. The model reproduced the observed levels and day-to-day variations of CO very well, and indicated that the low CO episode widely occurred around Japan. The model simulations revealed the presence of steep gradient in central Hokkaido, explaining the observed differences in CO mixing ratios between Cape Ochi-ishi and Rishiri Island. The eleven-year model results and backward trajectories suggested that the low CO event observed in 2004 was due to rapid transport of pristine air masses from the Pacific Ocean, driven by anomalously stronger easterly flows in the summer of 2004. This mechanism well explained the anomalous patterns observed for methane, carbon dioxide, and molecular hydrogen associated with low CO.