Google shares on Google+ that they added custom columns on the keyword and ad level. Because of this update, users can now place the custom columns on the keyword and/or ad level.
“You can now get deeper insight into the business metrics that matter most to you at the keyword and ad level with custom columns. For example, see how engaging your ads are on mobile by creating a custom column for mobile clickthrough rate (CTR). Or see which keywords are converting best on mobile by creating a custom column for mobile conversion rate.”