Lanzhou is shrouded in smog on Nov. 10.

The municipal government of Lanzhou, the capital city of Gansu province, has issued a letter of apology to residents after restricting the use of private vehicles.

Explaining that car emissions are the biggest factor behind air pollution, the municipal government announced that it will restrict the use of private vehicles through a system of even- and odd-numbered license plates as of Nov. 20. The government also promised that these restrictions will be lifted as soon as the orange alert for air pollution is removed. However, according to meteorologial departments, air quality will likely require some time yet before it improves.

From Oct. 31 to Nov. 19, the city of Lanzhou witnessed the worst and longest-lasting bout of air pollution since 2013. As a result, the municipal government launched a number of measures, which include halting the use of half of all government vehicles. However, the air quality has not improved. In fact, a strong sandstorm that hit the city on the afternoon of Nov. 18 has made the situation even worse.

The city will provide citizens with free bus rides as long as the restriction is in place to minimize its impact.