Beijing residents have bought more than 20,000 masks from an online retailer due to ongoing foggy weather and decreasing air quality.

Beijing shrouded in heavy fog on Monday morning

According to statistics from Taobao Mall, China's largest online B2C marketplace, more than 30,000 masks were sold on Sunday, more than 20,000 of which were bought by customers from Beijing.

Cotton masks priced between 5 to 15 yuan (about 0.8 to 2.4 U.S. dollars) have sold in large number, although more expensive masks with additional features have sold even more.

"My online store has sold more than 5,200 masks in a single month. A customer from Beijing bought more than 100 masks on Monday. About 80 percent of the goods will be sent to Beijing," an online vendor said.

The Beijing Meteorological Center on Sunday afternoon issued a yellow alert, the second-lowest level in the country's weather warning system, to warn about fog and smog in the capital, saying visibility would drop to less than 500 meters in most parts of the city.

A source with the Beijing Capital International Airport said the airport was forced to cancel 56 departing flights and delayed another 27 for at least one hour on Monday.

The air will improve after the fog clears on Wednesday, an official from the Beijing Municipal Environmental Protection Bureau said.