This thesis reports results obtained by the author in studies of ambient atmospheric elemental carbon (EC) in Beijing over a period of three years from winter 2005 to summer 2008. Throughout the text considerable attention is devoted to temporal variation focusing in EC and its dominant sources in Beijing.

By use of a semi-continuous thermal-optical analyzer with high-time resolution of one-hour, levels of EC concentrations in air were determined at Peking University in Beijing (PKU), China. Simultaneously, gaseous components, CO and CO2, meteorological data, and traffic properties were measured. The measurement of those species was also conducted at the Yufa site, about 53 km south of the PKU site, in the summer of 2006 as a part of the Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing 2006). Comparing with the obtained data at the Yufa site, the observed data was found to represent the values in the region within 50 km from the PKU site.

The annual mean of EC concentration was 6.9 μgC m-3, which was about 1.5-4 times higher than the level in other Asian urban areas, such as Seoul and Tokyo. The concentrations of the observed species decreased with increasing near-surface wind speed (WS). The concentrations reached low and stable levels at a WS of about 4 m s-1. The concentrations at stronger winds were considered to the background levels, and those in southerly air were much higher than northerly air. Under low-wind speed conditions (WS < 2.0 m s-1) of each species are interpreted to be more strongly controlled by regional-scale air mass affected by emissions from Beijing.

The slopes of the CO-CO2, EC-CO2, and EC-CO correlations in the weak-wind regime (WS < 2.O m s-1) are used to estimate major EC and CO sources in Beijing. The median EC, △EC/△CO2 (except for summer), and △EC/△CO (except for winter) increased in the late evening and remained high until early morning. These diurnal variations indicate regular sources of EC throughout the year. According to the traffic volume data measured on the North-fourth ring road, the total vehicles were high during daytime, while the numbers of heavy-duty diesel trucks (HDDTs) and their fraction to total vehicles increased at 2200 LT and remained high during nighttime. The nighttime increase in the traffic of HDDTs is due to the traffic regulations, which allow trucks to enter the city only during nighttime (2200-0600 LT). These results indicate dominant contribution of exhaust from diesel vehicles to the nighttime enhancement of EC.

In winter, the CO showed higher concentrations compared with those in other sea. Especially, the nighttime CO and △CO/△CO2 ratio were largely higher than those in the other seasons. CO emissions associated with coal or biofuel combustion for domestic heating are not the main cause of the high CO in winter, considering the lack of expected increases in EC in winter above the levels in the other seasons. Instead, the increase in the CO emissions from the exhaust of gasoline vehicles at low temperature is likely the dominant cause. The time required for catalysts to be heated to temperatures high enough for efficient CO removal is elongated at low temperatures, leading to higher CO emissions. The similarity in △EC/△CO2 between winter and the other seasons indicates no significant additional EC emissions in winter. These results suggest that the vehicular sector is one of the most dominant sources to impact on the diurnal and seasonal variation of CO and EC in Beijing.

EC, CO, and CO2 showed no significant decrease on the weekend. In addition, the diurnal variations of EC, CO, CO2, and △EC/△CO were similar between weekdays and weekends, confirming the absence of the differences in the diurnal variation of emission activity. This finding shows no significant weekend effect in Beijing.

A series of aggressive measures was launched by the Chinese government to reduce pollutant emissions from Beijing and surrounding areas during the Olympic Games. Observations showed significant decreases in concentrations of EC and CO during the Olympic season 2008, relative to the summer seasons in 2006 and 2007. The median concentration of EC was 2.22 μgC m-3 in summer 2008; about 2.5 and 3.9 μgC m-3 (53 and 63%) lower than for summer in 2007 and 2006, respectively. The median concentration of CO in summer 2008 was 660 ppbv, which was reduced about 16 and 33% compared with those in summer 2006 and 2007, respectively. Under weak-wind speed (WS < 1.0 m s-1) conditions, large decrease for EC concentrations during the Olympics would have to be related to the significant reductions of EC emissions in the Beijing urban area. Especially, large reduction of the nighttime EC concentrations and the change in the diurnal variation of △EC/△CO ratios without the nighttime enhancement in summer 2008 strongly reflected the decrease in the traffic of nighttime heavy-duty diesel trucks by traffic restrictions. These indicate that control measures for Olympics targeted at the vehicle would sector were effective in reducing EC emissions in Beijing resulting in a significant source. This result suggests that the dominant EC source in Beijing is the transportation sector, especially diesel-fueled vehicles.

The observed slopes of the CO-CO2-EC correlations for one year, four seasons in 2005-2006, were used to evaluate the emission ratios of these species derived from the emission inventory by Streets et al. [2003] in the Beijing area. The comparison suggests that Streets et al. [2003] largely overestimate the contributions of the emissions from domestic sectors with high CO/CO2 and EC/CO2 emission ratios and/or underestimate those from non-domestic sectors, especially vehicle emissions, in the Beijing study area. These results indicate that the observed slopes are useful parameters in assessing the reliability of emission inventories.

元素状炭素(elemental carbon, EC)は炭素燃料の不完全燃焼により発生するエアロゾルである。EC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。中国はECの大きな発生源であり、そこから輸送されるECはその下流域の環境にも大きな影響を及ぼす。しかしながら中国におけるEC発生量の推定には大きな不確定性があり、より正確なECの発生源の理解が重要な課題となっている。この論文では、中国北部の大きなEC発生領域の中心部にある北京においてECと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2006-2008年に初めて行い、その濃度の時間変動を詳細に解析した。

平均EC濃度は6.9 μgC m-3と他の東アジアの主要都市に比べ1.4-4倍高いこと、ECは夜間に濃度が増大すること、その濃度の日変化には大きな季節変化は無いことなどを見出した。またECと同時に測定されたCO、CO2との相関から、ECの発生量の日変化・季節変化を解析した。その結果、夜間に多く走行するディーゼル車が重要な発生源であり、EC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、測定された北京の交通量変化からもこのことを支持する結果を得た。またEC濃度は週末と平日とに有意な差が無く、これも交通量の測定値と良く対応している。

2008年夏の北京オリンピックの期間中にエアロゾルの濃度を低減する努力(ディーゼル車などの交通量の規制を含む)が、中国政府により行われた。ECの濃度が約50-60%減少したことを初めて示した。特に夜間のECの濃度減少が顕著であることから、ディーゼル車が夜間のECの高濃度の主要原因であることを明確に結論した。

このように、北京におけるECの挙動を理解するうえで、本研究は大きな貢献をした。北京以外の中国の都市におけるECの挙動を調べる方法論を、この研究を通して確立した。このように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