They have crackdowned before, many times actually, but without real continuous enforcement of rules, factory owners are caught in a horrible prisoners dilemma given how hyper competitive production is (if your competitor cheats and you don't, you'll be out of business quickly). In this case, political reform is directly related to the environment.
Here are the rules for Diesel fuel in China.[1] The official standard for sulfur has been been slowly lowered from 0.2% to 10 ppm, but enforcement is weak. There's a weaker standard for "nonroad Diesel" for tractors and such. It's observed that at night, when the blue trucks roll into Shenzhen, the pollution level goes way up, because they're using "nonroad Diesel".
The US is now <15ppm sulfur for all Diesel uses except ships. It took 25 years to get there. The EPA is (was?) trying to get ships down to 1000ppm within 200 miles of the US.
It's not that hard to enforce this, because there aren't that many refineries. China has only 75 refineries. The US has about 125. Not that many places need to be inspected.
Already done (knowledge only due to personal interest), but it only applies within 200 miles of shore. It's also the IMO (International Maritime Organization, a UN agency) rather than the EPA. Currently the limit on open waters is at 35,000 ppm (3.5%), which will tighten to 5000 ppm in January of 2020.
Sulfur aerosols are just as bad as NOx and create just as much smog. They also turn into acid rain. They have a noted anti-geenhouse effect, but again... acid rain.
It should also be noted that there is basically zero technical or economic justification for sulfur pollution. It requires no significant inputs: a catalytic step that results in hydrogen sulfide, then a regenerating amine reaction (effectively solvent distillation), and finally the hydrogen sulfide is burnt (as in energy-producing) to produce elemental sulfur, which is sold. This is how the majority of sulfur is made.
So in that sense nothing has really changed at all.
As a counter example, we can say China can't possibly enforce things like seat belt or drunk driving without political reform but those are largely non-problems now because they were easy politically costless solutions.
The problem is here is prioritization/will rather than mechanism/implementation since China likely believes the environment is a problem with a known solution and wants to milk dirty production for a new more years.
Gorbachev wanted to follow the simultaneous political and economic reform route and that definitely didn't turn out well for anyone.