Black carbon (BC) aerosols are produced from incomplete combustion of fossils fuels and biomass burning. The concerns for the BC aerosols at the global levels have been increased in recent past due to its active involvement in climate change. The radiative forcing (a measure of climate change) caused by BC is about 55% of that of carbon dioxide (CO2). Also, BC interferes in microphysical properties of clouds by changing the radius and lifetime of clouds. BC emissions are increasing in Asia due to rapid commercial and economic developments. Asian BC is not only impacting Asian climate, but also being transported to Arctic region, causing decreasing of snow albedo and increasing of Arctic haze. Climatic impacts of BC depends on the concentrations exist in atmosphere. The variations of ambient BC levels depends on the strength of emissions and local meteorology at source regions, while, transport and level of removal at remote places. These aspects of BC are poorly understood for Asian regions. For the first time, in this study, we addressed some of these issues by making continuous in situ measurements of mass concentrations of BC and mixing ratios of carbon monoxide (CO) and carbon dioxide (CO2) at urban and remote sites in East Asia.

The urban site was situated in the main city of Guangzhou, Pear River Delta (PRD) region of South China. PRD is among the main industrial regions of China, where dominant BC emissions are from transport, industries, and domestic sectors. To study the relationships of BC with CO and CO2 in light of local meteorology and emissions, in situ measurements of BC, CO, and CO2 were made in July 2006. The observed average ± standard deviation (SD) concentrations of BC, CO, and CO2 were 4.7±2.3μgC m-3, 798±459 ppbv and 400±13ppmv, respectively, during entire campaign. The trends of these species were mainly controlled by synoptic-scale changes in meteorology during the campaign. Based on back trajectories, data are analyzed separately for two different air mass types representing northerly and southerly flows. Northerly air masses, constituting about 25% of the campaign, were mostly originated within the PRD and represent the observations of regional levels. On the other hand, southerly air masses measured during most of the campaign were continuously diluted by clean marine air. The diurnal patterns of BC, CO, and CO2 exhibited peak concentrations during the morning and evening hours coinciding with rush-hour traffic. The ratios of OC/BC were lower during morning peak hours indicating dominance of fresh emissions. The diurnal variations of BC of southerly air masses closely followed the traffic pattern of heavy-duty vehicles (HDV). The observed slopes of ΔEC/ΔCO, ΔEC/ΔCO2, and ΔCO/ΔCO2 during northerly air masses were 0.0045μgC m-3/ppbv, 0.13μgC m-3/ppmv, and 49.4 ppbv/ppmv, respectively, agreeing reasonably well with their respective emission ratios derived from regional emission inventories.

To study the transport pattern, role of wet removal and the transport of BC relative to CO, in situ measurements of BC and CO were made at Hedo from March 2008 to May 2009. Hedo is remote observation site situated at the Okinawa Island of Japan, in Pacific Ocean. BC was measured using Continuous Soot Monitoring System (COSMOS), an improved version of filter-based instrument. CO was measured using non-dispersive infrared absorption (NDIR) based instrument. Hedo received East Asian outflow in winter and spring during which average concentrations of BC and CO were increases to 0.41μgC m-3, and 183 ppbv from the level of 0.19μgC m-3, and 101 ppbv, respectively, observed during summer and fall when mostly marine outflows were prevailed. Based on back trajectory analysis, the air masses have been classified as North China (NC), South China (SC), Korea (KR), Japan (JP), marine (MA), and free troposphere (FT). Characterizations of air masses arriving from each categories were made for BC and CO considering their residence time of at least 24 hours (RT > 24 hours). Based on the criteria of RT (>24 hours), about 44-52% of air masses were received from Chinese region during spring and winter, while about 75% MA from Pacific Ocean in summer. The observed average BC concentrations in the air masses from source regions were ranges from 0.40 to 0.79, 0.27 to 1.12, 0.22 to 0.45, and 0.16 to 0.29, in NC, SC, KR, and JP, respectively, in all five seasons, expressed inμgC m-3. The levels of wet removal of BC were estimated by deriving wet deposition efficiency (WDE). WDE is the percentage removal of BC, estimated using BC simulated by Community Multiscale Air Quality (CMAQ) system. A good dependency of the slopes of ΔBC/ΔCO against WDE was observed. The observed slopes of ΔBC/ΔCO in the air mass categories of NC, SC, KR, JP, MA, and FT with RT > 24 hours and WDE < 20%, were 0.0065, 0.0080, 0.0062, 0.0050, 0.0016, and 0.0030, respectively, expressed inμgC m-3/ppbv. Region-specific (of NC, SC, KR, and JP) ΔBC/ΔCO slopes are compared with the emissions ratios derived from recent inventory of Zhang et al. [2009]. We found there could be an over estimation of BC by a factor of two in East Asia regions.

ブラックカーボン(black carbon, BC)は炭素性の燃料の不完全燃焼により発生するエアロゾルである。BCは太陽光を強く吸収し大気を加熱するとともに、人間の健康に悪影響があることが知られている。東アジアはBCの大きな発生源であり、その中でも特に中国は最も大きな発生源と推定されている。しかしながら中国でのBC発生量の推定には大きな不確定性がある。この不確定性を改善するためには、正確なBCの濃度とその変動の理解が重要である。この論文では、中国南部の大きなBC発生領域にある広州においてBCと有機炭素(OC)そのトレーサーである一酸化炭素(CO)および二酸化炭素(CO2)の系統的な観測を2006年の夏に行い、その濃度の時間変動を詳細に解析した。

平均BC濃度は4.7μgC m-3であり、早朝に濃度が増大することを見出した。またBCと同時に測定されたCO、CO2との相関から、BCの発生量の日変化を解析した。その結果、早朝から昼間に多く走行するディーゼル車が重要な発生源の一つであり、BCの日変化の主要な原因の一つであることを見出した。また、日中、境界層の混合によりBC濃度は希釈され低下する。北風条件下ではこの様子が特に顕著である。またこの条件下でのBC濃度はこの地域の濃度を代表すると考えられ、これまでの観測との比較も、この点を支持している。

中国の大発生源の下流にあたる沖縄の辺戸観測所で2008年3月から2009年の9月にBCとCOの同時測定が行われた。観測された空気塊のトラジェクトリー解析から、観測されたBCの大部分は中国における排出の影響を強く受けていたことを見出した。またBC/CO比は、空気塊が経験した降水量と共に減少したことから、BCの輸送中での降水除去の重要性を見出した。輸送パターンと降水の変化がBCの季節変化の主な原因であるということを示した系統的な解析は、本研究が初めてであり、科学的価値が高い。特に降水量という、これまでにないパラメーターを導入したことは特筆に値する。

このように、東アジアBCの発生・輸送・除去を理解するうえで、本研究は大きな貢献をし、BC研究の方法論を確立した。本論文の学術的な価値は、極めて高い。

よって本論文は博士(工学)の学位請求論文として合格と認められる