This number is actually down sharply because this same institute had previously said China was responsible for about 70% of the pollution in South Korea. This makes me wonder if the ROK government is trying to down play China pollution in order to justify more stringent domestic anti-pollution measures?:

Around 32 percent of ultrafine dust in South Korea can be attributed to China, a piece of joint research by South Korea, China and Japan showed Wednesday, as fine dust pollution continues to be a regional environmental headache.
An average of 51.2 percent of ultrafine dust in South Korea comes from domestic factors and 32.1 percent and 2 percent, respectively, stem from China and Japan, according to a summary of the joint study released by South Korea’s National Institute of Environmental Research.
The remaining 14.7 percent were due to other factors.
The study, the first of its kind, was conducted in major cities of the three countries, including South Korea’s Seoul, Daejeon and Busan.
Yonhap
You can read more at the link, but what do people think the Korean reaction would be if Japan was dumping as much air pollution over the Korean peninsula that China is?