Air quality in Kagoshima and Japan in general seems to have improved drastically over the past 10 years, what gives?

Greetings,

I just read that there's volcanic ash in Kagoshima regularly and went to check out the air quality data. It shows that it improved drastically for Kagoshima, but also for other cities in Japan, judging from a quick check.

Is it a result of policy, chance with weather and/or are other factors at play? Any insight will be appreciated. Thanks in advance!