High levels of particulate pollution in Chinese megacities
The most extensive analysis to date of organic pollutants in the air of Chinese cities confirms that air pollution levels are much higher than in the developed world.
The most comprehensive study yet of organic pollutants in Chinese urban air was posted today to ES&T’s Research ASAP website (DOI: 10.1021/es060291x). Conducted by a team of researchers from China and Japan, the study is the first to compare levels of organic aerosol particles in China’s newly developing midwestern cities with levels in older megacities such as Beijing, Hong Kong, and Shanghai. The research confirms that most of the 14 cities studied have much higher levels of air pollution than cities in developed countries.
Particulate matter (PM) made up of microscopic airborne particles less than 10 micrometers (µm) in diameter (PM10) has been recognized worldwide as a potential cause of respiratory and cardiovascular health problems. PM10 aerosol particles can include a complex mix of inorganic and organic compounds, dust, and soil, and they are a major cause of reduced visibility in urban areas. Fine particles less than 2.5 µm in diameter (PM2.5) pose an even greater threat to human health because they can penetrate deeply into lung tissue. The World Health Organization (WHO) estimates that outdoor particulate pollution accounts for 800,000 premature deaths in the world annually, including 500,000 in Asia alone.
China’s State Environmental Protection Administration (SEPA) regularly monitors urban air quality in the country’s major cities. Among the measured compounds, SO2 and PM10 are the most prominent pollutants. In 2005, SEPA reported that of 522 cities, 4.2% reached grade I (which equates to the lowest levels of pollution) of SEPA’s National Ambient Air Quality Standards (NAAQS), and 56.1% reached grade II. The remaining 39.7% had grade III (inferior) air.
The new research is important because “studies on organic aerosols in China on a molecular level are very scarce,” says one of the study’s coauthors, Gehui Wang, who is from Nanjing University (China) and is currently a postdoctoral fellow at Hokkaido University (Japan). “Organic pollutants are not studied as often as inorganic contaminants,” notes Monica Mazurek, an expert on aerosol chemistry at Rutgers University. Because PM aerosols can contain hundreds of organic compounds—of which many are still unidentified—analyzing urban air pollutant particles for their presence is a very complicated and expensive task.
The researchers designed the study to sample PM2.5 aerosols in 14 cities across China during 2 summer days and 2 winter days in 2003. The sampling conducted during the winter was done simultaneously in all 14 cities. These four days of sampling were enough to provide “a clear overall profile,” Wang explains. “Our data are consistent with earlier reports [in individual cities], suggesting that our results are representative,” he adds.
The levels of most of the 114 organic compounds identified in the new study were higher during the winter. The study’s authors attribute this mainly to the burning of coal for domestic heating. By comparing chemical fingerprints, the researchers could determine whether the pollutants originate from anthropogenic sources, such as coal or vehicular exhaust, or natural sources, such as plants or microbes. According to SEPA, China burned 2.14 billion tons of coal in 2005, accounting for nearly 70% of the energy the country consumed that year.
Coal burning may explain why the predominant PAH detected was benzo[b]fluoranthene, according to the study’s authors. “This is totally different from cases in other countries, especially in developed regions, where PAHs are mainly derived from petroleum combustion (e.g., vehicular exhausts) and benzo[ghi]perylene is the most abundant PAH in the air,” Wang explains. However, Kimitaka Kawamura of Hokkaido University (Japan), the study’s corresponding author, says, “This may be changed soon because the numbers of motor vehicles are increasing quickly.” In 2005, the number of registered motor vehicles in China jumped to more than 31 million, an increase of 30% from 2004.
The study collected the first nationwide data on phthalates—compounds added to plastics that can evaporate into the atmosphere, are suspected to be carcinogenic, and have endocrine-disrupting abilities—in Chinese aerosols, Wang says. Although very high levels of phthalates were found in all of the studied cities in both seasons, the levels were highest during the summer. The higher summertime concentrations can probably be explained by the elevated ambient temperatures, which facilitate evaporation, he says.
Guoshun Zhuang, a professor in atmospheric chemistry at Fudan University (China), says that the high levels of phthalates are “not surprising since phthalates are widely used as plasticizers in China.” The country does not currently regulate the emission of phthalates.
PM aerosols are also associated with environmental problems both within and beyond China: “High levels of carbonaceous aerosols have been linked to the increase of summer floods in the south and drought in the north [of China] in the past 20 years,” Wang says. He explains that anthropogenic aerosols, such as black carbon, can absorb and reflect solar radiation and affect the formation of rain by acting as cloud condensation nuclei, thus changing local atmospheric circulation.
Zhuang says that the study is important because Chinese aerosols have been shown to travel across the North Pacific Ocean and North America. Both pollutants and nutrients, such as divalent iron, are transported to the ocean; this could in turn affect the global environment, he points out.
