How Can We Combat Pollution In the Most Populous Areas?

Showcased in China and the article below, pollution in high density population centers is a problem that isn't going away. What can we do as a world to prevent and correct this? Or can we?

http://www.cnn.com/2015/09/04/asia/china-beijing-blue-sky-disappears-after-military-parade/index.html

Less than 24 hours after the end of China's massive military parade, Beijing is back to its usual smoggy self.

Residents woke up Friday morning to find the crystal blue skies that graced the city nearly two weeks suddenly gone -- in their place, the familiar sight and smell of dour gray pollution clouds.

Starting late August, Beijing enjoyed a rare string of continuously clear days as authorities took drastic action to ensure an azure backdrop for the largest parade it's ever held -- a showcase marking the 70th anniversary of Japan's defeat in World War II.

Hundreds of factories were shut during this time, while half of Beijing's five million registered cars were banned from the streets.

China's toxic smog problem

China's toxic smog problem 02:14
It worked. On the morning of the parade, the air quality index (AQI) -- an international standard for measuring the severity of air pollution -- dipped to a pristine 17 out of 500, signifying very healthy air.

Excited Beijingers coined the unusually blue skies "parade blue."

But now the cars are back and the city is back to "Beijing gray."

Friday's AQI shot up past 160 in parts of the city, rated "unhealthy".

According to a guide by the U.S. Environmental Protection Agency, at this level of pollution "Everyone may begin to experience some adverse health effects, and members of the sensitive groups may experience more serious effects."

Beijing's smog has been particularly horrendous this year. Clear days like this one are photo-celebration worthy. Check out the contrast between the good days and the bad.
10 photos: Surprise, Beijing has clean air day
'Parade blue' censored?
China parades huge military

China parades huge military 01:30
Beijing has become infamous for a pollution problem that's increased with severity along with China's economic growth. It's become the norm for the capital city's skies to be completely shrouded by a filthy film of gray smog.

As a result, blue skies are so precious that whenever Beijing enjoys a beautiful day, residents whip out their cameras and excitedly post photos of the sky to social media.

READ: What Beijing looks like on a gloriously clear day

But the government is sensitive even to veiled criticism, and in recent days the phrase "parade blue" seems have vanished from China's Internet -- just like the blue sky in reality.

Only three posts popped up when CNN searched the phrase on Weibo Friday, China's popular micro-blogging site.

"The parade blue disappeared at one blow. It feels so miraculous -- like magic. I have been used to beautiful blue skies, now I have this sudden feeling of uneasiness," lamented one Weibo user.

Other famous skies
It's not the first time a blue sky has gotten its own name in China: Before "parade blue" there was "APEC blue."

Beijing's beautiful, smog-free skyline

Beijing's beautiful, smog-free skyline 01:14
That referred to the clear air during the two-week gathering of world leaders at Beijing's APEC summit last November, a result of a similar set of extreme measures -- shutting factories and ordering mandatory holidays.

When the leaders left town, though, the hazy, smoky air returned immediately. As a rueful joke, Chinese Internet users began using "APEC blue" as a phrase to describe something beautiful that disappears quickly.

READ: The stories that 'broke' China's Internet in 2014

For example, a quip about short-lived relationships: "He's not really into you. It's APEC blue!"

Now, like the end of a fizzled romance, the gray clouds are back -- and with a sigh (and a cough), Beijingers wait for their next chance to name a blue sky.