The sandy and dusty weather that has affected most parts of China for a week was caused by a larger amount of sand and dust due to less snow cover, which was then driven back across the country by strong winds, experts from the National Satellite Meteorological Center said on Thursday.

Since Sunday, the sandstorm, which originated in Mongolia, has hit many parts of the country, spreading from Northeast China's Heilongjiang province to as far south as the Yangtze River Basin, causing heavy airborne pollution and low visibility.

The level of PM10 — hazardous particles less than 10 microns in diameter — reached a concentration of more than 2,000 mcg/cubic m in parts of northern China, the National Meteorological Center said.

The PM10 level usually remains below 150 mcg/cubic m when the air quality is good.

The NMC forecast that the sandstorm, despite being weaker than previous weeks, will continue hitting northwestern and northern China for the next week.

Under usual conditions, sand and dust passes southern areas before entering directly into the sea without return. However, it blew back north this year driven by east winds starting on Tuesday night, the NMC said.

"In the past, the reflux usually occurred within the range of northeastern China or began to return after leaving China. However, this year it returned after reaching the south of the Yangtze River," said Wang Xin, chief expert of the National Satellite Meteorological Center's Remote Sensing Application Service Center.

The sand's return journey has swept northward from Jiangsu, Anhui and Henan provinces to Liaoning and Jilin provinces. It affected China for a longer period than the strong sandstorm in March, according to the NMC.

The last sandstorm, which was the strongest so far this year, was in late March, the NMC said.

The amount of sand during March and April monitored by satellites was larger than normal years as snow in the Inner Mongolia autonomous region in March covered 239,000 square kilometers, significantly smaller than the 665,000 square kilometers last year, according to the NSMC.

So far, eight rounds of dusty weather have occurred this year, with the first one on Jan 12 arriving 32 days earlier than an average year, Wang said.

Last spring, northern China had eight rounds of dusty weather. From 2000 to 2021, there was an annual average of 10.7 rounds of dusty weather, according to the 2022 China Climate Bulletin.