China keeps pollution in bounds

China may be the world’s biggest emitter of polycyclic aromatic hydrocarbons (PAHs), but the chemicals remain largely within the country’s boundaries, according to researchers from Peking University.

PAHs are mainly produced from fossil fuel and biomass burning and many of the compounds are carcinogenic. ’As a rapidly  industrialising country relying primarily on coal, China emits huge amounts of PAHs,’ Tao Shu, chief researcher at Peking University’s College of Urban and Environmental Sciences, told delegates at  the Chinese Chemical Society’s annual meeting in July. ’However, the amount of outflow [to other countries] is not as big as previously estimated,’ he added. 

According to a recent study by researchers from Peking University, China has the greatest PAH emissions in the world. In 2004, the country’s total emission of 16 of the most important PAHs, as ranked by the US Environmental Protection Agency, was approximately 133000 tonnes, accounting for 22 per cent of global PAH emissions. [1]

However, most of the PAHs either deposit in China or degrade during long-range transport and only a small percentage reach neighbouring countries, says Tao [2]. His team has developed a model to predict energy consumption - based on socio-economic conditions and fuel type - which it has validated using energy data from 135 Chinese counties. The researchers have also designed a model to calculate the probability of PAHs reaching various locations, factoring in wind strength and direction, and photochemical PAH degradation.

Using the models, Tao’s team estimates that over 90 per cent of PAHs emitted in China in 2004 remained within continental China. About 8092 tonnes of PAHs were transported out of mainland China - accounting for 7.1 per cent of the total emissions, estimates the team. And of these emissions, 70 per cent reached no further than the offshore environment, while others were transported to Russia and Mongolia, North Korea, South Korea, Japan, and some central and Southeast Asian countries. 

In an earlier study, US and Japanese researchers took samples from the downwind Japanese island of Okinawa to estimate levels of PAHs coming from China.[3] Their PAH estimates came out slightly higher. ’It’s amazing that the estimates are as close as they are given the drastically different approaches used and that each was estimating different things,’ says Staci Simonich of Oregon State University, US, the corresponding author of the Okinawa study. 

Zhang Gan, a senior researcher at the Chinese Academy of Sciences’ Guangzhou Institute of Geochemistry, says Tao’s study is the first to integrate China’s varying regional emissions of PAHs into meteorological models. ’It offers not only a clear scenario of our own emissions, but also a basis for different countries to negotiate on environmental issues.’ 

Hepeng Jia

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