What was done
The authors compared the mean station temperatures of three groupings of cities -- one comprised of four large urban stations (mean 1995 population of 4,830,000), one comprised of six smaller urban stations (mean 1995 population of 548,000), and one comprised of six "rural" stations (mean 1995 population of 214,000) -- over the period 1968-1999.

What was learned
The authors say the results of their study "showed that temperatures of large urban stations exhibit higher urban bias than those of smaller urban stations and that the magnitude of urban bias has increased since the late 1980s [our italics]."  Specifically, they note that "estimates of the annual mean magnitude of urban bias range from 0.35°C for smaller urban stations to 0.50°C for large urban stations."  And, of course, they indicate that "none of the rural stations used for this study can represent a true non-urbanized environment."

What it means
Again, in the words of the authors, "the results showed that the urban growth biases are very serious in South Korea and must be taken into account when assessing the reliability of temperature trends."