Seasonal variations of black carbon observed at the remote mountain site Happo in Japan

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Abstract

[1] The emission of black carbon (BC) from East Asia and its long-range transport strongly influence the mass concentration of BC (MBC) over the western Pacific. However, reliable and long-term BC data are still limited in this region, especially at elevated altitudes. In this study, we present accurate measurements of MBC using a continuous soot monitoring system at Happo, a remote mountain site at an altitude of about 1.8 km in Japan, from August 2007 to August 2009. The annual average MBC at Happo was about 0.26 ± 0.18 (1σ) µg m−3. The monthly average MBC values exhibited similar seasonal variations during both years, with minimum values in winter. Around 40% of the air sampled at the site was of free tropospheric (FT) origin, with about 10% originating in North China (NC) origin, respectively. The MBC values for FT (0.24 µg m−3) and NC (0.23 µg m−3) air were representative of the MBC values (0.26 µg m−3) at 1.8 km height in the western Pacific, which are strongly influenced by BC emissions in North China. The MBC values calculated using a regional-scale model reproduced well the MBC observed at Happo. The model predicted that BC transported from northern China alone contributed ~53% to the measured MBC, consistent with trajectory analysis. The comparison of model-calculated and observed MBC values indicates that the minimum values of MBC in winter were caused by the suppressed upward transport of BC over the Asian continent. Biomass burning in Siberia substantially increased MBC in the spring of 2008.

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