Environmental Science
First Runner-up: PAHs in Tokyo aerosols
“Compound Class Specific 14C Analysis of Polycyclic Aromatic Hydrocarbons Associated with PM10 and PM1.1 Aerosols from Residential Areas of Suburban Tokyo” by Hidetoshi Kumata, Tokyo University of Pharmacy and Life Science; Masao Uchida, Japan Agency for Marine-Earth Science and Technology (JAMSTEC); Eisuke Sakuma, Tatsuya Uchida, Kitao Fujiwara, and Mikio Tsuzuki, Tokyo University of Pharmacy and Life Science; and Minoru Yoneda and Yasuyuki Shibata, National Institute for Environmental Studies (Japan), 2006, 40 (11), 3474–3480.
Combustion-derived PAHs are important aerosol pollutants that have been linked to cancer in humans, but it is unclear exactly where these PAHs originate in cities, where their concentrations can be high. Are they mainly formed during fossil-fuel burning, or does biomass burning contribute as well? To answer those questions, Hidetoshi Kumata of the Tokyo University of Pharmacy and Life Science turned to a technique for analyzing 14C that was originally developed for the analysis of marine sediments by Masao Uchida of JAMSTEC.
Kumata and Uchida first had to adapt Uchida’s method to analyze atmospheric particles. 14C has a half-life of more than 5000 years, which makes it an ideal tracer for distinguishing between combustion products from fossil fuels (14C-free) and those from modern biomass (contemporary 14C).
“In 2002, no paper on 14C analysis of atmospheric PAHs existed, so we thought we could be the very first to approach this question,” Kumata recalls. However, the work proved much more tedious and difficult than the researchers had expected. After 3 years, they finally got some results. The team collected aerosol samples at a site in suburban Tokyo with no direct source from industrial input. The goal was to learn more about how PAH sources vary depending on the season and particle size: fine particles of less than 1.1 micrometers (µm) in diameter are of particular concern because they can penetrate deeply into human lungs.
The first difficulty the team faced was getting enough sample for the 14C analysis: 20–40 micrograms of carbon per analysis. “We could not take any samples during the rainy seasons between May and July and in the autumn, so it finally took us 2 years to accumulate enough carbon,” recalls Kumata. Another technical problem proved to be the graphitization of the carbon and subsequent radiocarbon analysis of the small samples in the accelerator mass spectrometer facility at the National Institute for Environmental Studies in Tsukuba, Uchida says.
When Kumata and Uchida finally saw their results on the seasonal and particle-size variation in PAH sources, in the summer of 2005, at first they did not believe them. “The 14C signal was too strong, so we first thought we could not explain our results,” Kumata says. However, comparison with other data made the researchers confident that their data were correct. They concluded that biomass burning contributes much more to PAHs in Tokyo aerosols than previously assumed and is responsible for a large part of the PAH elevation seen during the winter months.
“While biomass-based fuels are very useful for reducing CO2 emissions, we need to realize that biomass burning may be bad for the environment for other reasons,” Kumata says. “If we want to develop the use of biomass-based fuels, we also need to develop the technology to use them very efficiently, so that emissions are reduced,” he adds.
Uchida stresses that field-based source discrimination studies such as theirs are essential for the future regulation of atmospheric PAH pollution. “The Tokyo government is eager to reduce emissions, and we need to detect whether new regulations are actually effective,” Kumata says.
The future goal of the researchers is to get more data on the effects on biomass burning on a larger geographical scale. “Because other countries in East Asia are more dependent on biomass-based fuels than Japan and continue to develop fast, I expect that biomass burning will increase its importance as a source of energy and pollution in East Asia countries,” Kumata predicts.
