東京都市大気エアロゾルの生成機構と環境影響

平成22年度 成果一覧

近藤 豊(東京大学大学院理学系研究科 教授)
R. L. Verma(東京大学大学院理学系研究科 特任研究員)
大島 長 (気象庁気象研究所 環境・応用気象研究部)
松井 仁志(東京大学大学院理学系研究科 特任助教)
北 和之 (茨城大学理学部理学科 教授)
L. K. Sahu(Physical Research Laboratory Leader)
加藤俊吾 (首都大学東京都市環境学部分子応用化学コース 教授)
梶井 克純 (首都大学東京都市環境学部分子応用化学コース 准教授)
高見 昭憲(国立環境研究所地域環境研究センター広域大気環境研究室 室長)
宮川 拓真 (東京大学先端科学技術研究センター 特任研究員)

要旨

Continuous in situ measurements of the mass concentration of black carbon (BC) aerosols and mixing ratio of carbon monoxide (CO) were made at Cape Hedo on Okinawa Island, Japan, a remote site located in the East China Sea, from March 2008 to May 2009. For the first time, we show temporal variations of BC and CO at Hedo in Asian outflows throughout the year. Annual average concentrations of BC and CO were 0.29 μg m?3 and 150 ppbv, respectively. The origins of the observed air masses were determined by using 5-day back trajectories, suggesting that about 51% of the air masses arriving at Hedo were from the Chinese region during spring and winter, while about 78% of air masses were of maritime origin during summer. Because of the more frequent transport of Chinese air to Hedo in spring and winter, the average and background concentrations of BC and CO in these seasons were higher by about a factor of 2 than those in summer and fall. Air masses from north China made the largest contributions to elevating the BC levels at Hedo because of the high BC emission rate and frequency of transport. The observed ΔBC/ΔCO ratio systematically decreased with the decrease in model-calculated transport efficiency (). On the basis of this result, we derive region-specific ΔBC/ΔCO ratios by selecting data with > 80%. The annually averaged ΔBC/ΔCO ratios for air originated from north and south China were 7.0 ± 3.3 and 7.5 ± 4.6 ng m?3 ppbv?1, respectively, about half the annual BC/CO emission ratio derived from the emission inventory of Zhang et al. (2009). We evaluate the CO emission inventory of Zhang et al. (2009) for China by comparing observed (ground-based and aircraft) and model-calculated CO values. The comparison indicates that the CO emissions from China were underestimated by about a factor of 2. We have estimated the emission rate of BC from China, which is the largest source of BC in East Asia. This has been conducted by comparing BC values observed in the Asian outflows and those predicted by 3-D CTMs that used the emissions inventory of Zhang et al. [2009]. For this purpose, we used BC mass concentrations measured by COSMOS at Cape Hedo from February 2008 to May 2009. Hedo is located sufficiently distant from China to represent a relatively uniform field of BC over the East China Sea while still capturing signals of BC emissions from China. The seasonal median BC at Hedo was highest at 0.25- 0.31 μg m-3 in winter and spring when high-BC plumes from China were often transported to the site. The CMAQ-Modified simulation was used to calculate the mass concentrations of BC using annual mean emission inventories of Zhang et al. [2009] for the base year of 2006. The model well reproduced the temporal variations of surface BC mass concentrations observed at Hedo during the whole observation period and the median BC vertical profile obtained by aircraft over the East China Sea in spring 2009. In order to assess the model variability in transport and removal processes of BC, we inter-compared CMAQ-Modified and EMTACS simulations, in which formulations of atmospheric dispersion and aerosol processes are different. The difference in the predicted BC mass concentrations between the models was small around Cape Hedo with r2 = 0.6-0.8, indicating the smaller uncertainty in the model predictions of BC at the location of Hedo. We selected BC data strongly impacted by emissions from China (fChina > 0.8) and least impacted by wet deposition during transport (ftransport > 0.8). After these selections, r2 for the BC-CO correlation increased to 0.81, with a substantial amount of data (N >1000) remaining for statistical analysis. The Model (med)/Obs. (med) ratios were derived by taking into account the seasonal variations of BC emissions. The Model (med)/Obs. (med) ratio for the entire period was 0.99±0.11. The overall uncertainty of the ratio was estimated to be 54%.   This means we estimate the total BC emission flux over China is very close to the estimate of Zhang et al. [2009] with the uncertainty of about 54%. Although the absolute values of the two estimates turned out to be very similar, partly by coincidence, the uncertainty of our estimate is much smaller than the 208% uncertainty given by Zhang et al. [2009].

キーワード

black carbon, carbon monoxide, East Asia, emission ratio, transport.

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