Abstract

Abstract. Long-term (2009–2015) observations of atmospheric black carbon (BC) mass concentrations were performed using a continuous soot-monitoring system (COSMOS) at Fukue Island, western Japan, to provide information on wet removal rate constraints and the emission strengths of important source regions in East Asia (China and others). The annual average mass concentration was 0.36 µg m−3, with distinct seasonality; high concentrations were recorded during autumn, winter, and spring and were caused by Asian continental outflows, which reached Fukue Island in 6–46 h. The observed data were categorized into two classes, i.e., with and without a wet removal effect, using the accumulated precipitation along a backward trajectory (APT) for the last 3 days as an index. Statistical analysis of the observed ΔBC ∕ ΔCO ratios was performed to obtain information on the emission ratios (from data with zero APT only) and wet removal rates (including data with nonzero APTs). The estimated emission ratios (5.2–6.9 ng m−3 ppb−1) varied over the six air mass origin areas; the higher ratios for south-central East China (30–35° N) than for north-central East China (35–40° N) indicated the relative importance of domestic emissions and/or biomass burning sectors. The significantly higher BC ∕ CO emission ratios adopted in the bottom-up Regional Emission inventory in Asia (REAS) version 2 (8.3–23 ng m−3 ppb−1) over central East China and Korea needed to be reduced at least by factors of 1.3 and 2.8 for central East China and Korea, respectively, but the ratio for Japan was reasonable. The wintertime enhancement of the BC emission from China, predicted by REAS2, was verified for air masses from south-central East China but not for those from north-central East China. Wet removal of BC was clearly identified as a decrease in the ΔBC ∕ ΔCO ratio against APT. The transport efficiency (TE), defined as the ratio of the ΔBC ∕ ΔCO ratio with precipitation to that without precipitation, was fitted reasonably well by a stretched exponential decay curve against APT; a single set of fitting parameters was sufficient to represent the results for air masses originating from different areas. An accumulated precipitation of 25.5 ± 6.1 mm reduced the TE to 1∕e. BC-containing particles traveling to Fukue must have already been converted from hydrophobic to hydrophilic particles, because the behavior of TE against APT was similar to that of PM2.5, the major components of which are hydrophilic. Wet loss of BC greatly influenced interannual variations in the ΔBC ∕ ΔCO ratios and BC mass concentrations. This long-term data set will provide a benchmark for testing chemical transport/climate model simulations covering East Asia.

Highlights

  • Black carbon (BC) particles suspended in the atmosphere induce positive radiative forcing by absorbing sunlight in the atmosphere and by reducing ice/snow albedo once deposited on such surfaces (IPCC, 2013; Bond et al, 2013; Kondo, 2015; and references therein)

  • Fukue Island is located in the East China Sea and receives air masses from China, Korea, and Japan, with typical traveling time from these source areas ranging from 10 to 60 h

  • The observed ratios for air masses originating from CEC and Korea were significantly smaller than the BC / CO emission ratios used in the inventory, at least by factors of 1.3 and 2.8, respectively, pointing to the necessity of revising the emission database

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Summary

Introduction

Black carbon (BC) particles suspended in the atmosphere induce positive radiative forcing by absorbing sunlight in the atmosphere and by reducing ice/snow albedo once deposited on such surfaces (IPCC, 2013; Bond et al, 2013; Kondo, 2015; and references therein). The increased mass concentrations of BC in the atmosphere since pre-industrial times have directly induced warming of the Earth’s climate. The atmospheric residence time of BC particles is only about a week (IPCC, 2013), much shorter than that of CO2, which is a major long-lived greenhouse gas. BC is regarded as an important type of short-lived climate pollutant. Reductions in their emissions could effectively and promptly slow down global temperature increases in the near future Besides the relevance to climate change, the World Health Organization warns about the health effects of BC (Janssen et al, 2012)

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