This could definitely be a good thing, and I definitely think that more countries, especially the United States, need to crack down on their environmental issues throughout the country. Instead many of them are just making the whole situation worse, like the GOP candidates. It could definitely be really good for the world.
Though Beijing's case is a special one, all cities should adopt a pollution warning system like the Chinese capital because cities are more prone to having pollution issues than many other types of geographical regions. Though a city might not have severe pollution problems, implementing a pollution warning system will allow cities around the world to take preventive measures so that pollution may never get to the point of severity as it does in Beijing, Hong Kong, and other cities.
If there is sound public policy, most countries will take steps to avoid the levels of pollution Beijing faces. The air quality in Beijing and surrounding areas is extraordinarily low. This is not an issue often faced in other countries. Should conditions worsen, this issue should be revisited. But for now, it would be better to create policies to prevent the air quality issue from occuring in the first place.
Why spend money building a warning system? Wouldn't the money be better spent fixing the problem? I don't see how a warning system changes anything. Is the warning system making Beijing a cleaner place? Let's focus on solutions and not band aids. There's no point to installing a warning system.