WASHINGTON, Nov. 24 -- Severe air pollution in Beijing and other Chinese cities might be directly related to gaseous pollutants rather than particles emitted from urban transportation and regional industry, researchers from China and the United States said Monday.

Photochemical oxidation of gaseous pollutants, including volatile organic compounds (VOCs), nitrogen oxides (NOx) and sulfur dioxide (SO2), is primarily responsible for the formation of a large amount of fine particulate matter (PM), called secondary particles, during China's severe haze pollution events, the researchers said.

The contribution from primary emissions and regional transport of PM, known as primary particles, is small, they reported in the U.S. journal Proceedings of the National Academy of Sciences.

The study, supported by the National Natural Science Foundation of China and China Ministry of Science and Technology, was co-led by Prof. Renyi Zhang at Texas A&M University and Prof. Hu Min from Peking University.

In order to better understand the mechanisms of PM formation, the team used a suite of state-of-the-art instruments to collect extensive air samples in Beijing.

"We find in our study that the conditions in Beijing are prone to PM formation, because of highly abundant condensable gases," Zhang said.

"Emissions of VOCs, NOx from urban transportation and SO2 from regional industry trigger aerosol nucleation and continuous growth over multiple days, leading to particle mass exceeding several hundred micrograms per cubic meter."

The efficient aerosol formation process in Beijing is distinct from that occurring in many other urban centers, the team noted, saying it can explain why the haze in China's capital forms so quickly and severely.

On the other hand, the particle chemical compositions measured in Beijing are quite similar to those commonly measured in other global regions, consistent with the chemical constituents dominated by secondary particles, the team said.

The study also found that meteorology appears to play a key role in the amount and severity of pollution.

"We found that Beijing's PM episodes occur on a periodic cycle, which is largely driven by wind variations," Zhang said.

"When the wind is shifted from the south, that's when trouble starts because most of the factories and power plants are located in the southern region. Under reactively stagnant conditions, the gaseous pollutants from city traffic and surrounding industry react in the air, locally producing a large amount of PM."

The haze problem appears to be more intense in fall and winter months because it rains more in summer and that tends to clean the air by washing out the particles, Zhang noted.

The team also believed that Beijing's pollution is likely representative of other urban areas in China.

"Our results imply that reductions in emissions of aerosol precursor gases, i.e., VOCs and NOx from transportation and SO2 from regional industrial (such as power plants and manufacturing) sources, are critical for remediation of the severe urban and regional haze pollution in China," the team wrote.

"On the contrary, we suggest that an effort to solely control emissions of primary particles is unlikely to be effective."