"The composition of aerosol samples taken for the past few decades shows that in the recent years, particulate nitrate has been increasing, which may come from vehicle exhaust," Yan said.

Wang Yuesi, a researcher at the Institute of Atmospheric Physics under the Chinese Academy of Sciences, recently told Beijing News that vehicle exhaust and the burning of coal are the main contributors to the capital's smog.

Wang Yaqiang, deputy director of the atmospheric composition institute at the Chinese Academy of Meteorological Sciences, said in recent interviews that the only way to reduce the number of smoggy days is to control emissions.

Yan agreed, adding that the smog will significantly decrease once pollutants are reduced dramatically.

"It was proved during the Beijing Olympic Games in 2008," he said.

During the 2008 Games, Beijing authorities kept millions of cars off the roads and halted some construction and factory production near the capital.

Last year, there were 5.2 million vehicles in the capital of nearly 21 million, according to the Beijing environment report in 2012 released in late May by the Beijing environmental protection bureau.

The worsening pollution since winter has triggered public concern over the city's air quality and its health impact. Authorities have responded by putting a greater effort to tackle the problem.

In June, the State Council adopted 10 new measures to curb air pollution, including speeding up the installation of pollution control equipment on coal-fired refineries and restraining the growth of industries that use a high amount of energy. It is also aiming to reduce emissions per unit of gross domestic product by at least 30 percent in heavy polluting industries by 2017.