Google AdSense: Notes from the field
Cannot open ads in new tab/window
Generally, it is our policy to not allow Google ads to open in a new window. However, in a few markets, including China, we've found that opening ads in a new window better aligns with expected internet behavior and provides a more consistent, intuitive user experience.
Keeping in mind the importance of the user experience, we've enabled Google ads to open in new windows in locations where this is expected browsing behavior. This functionality is determined by the IP of the user, and is not an option you can select your AdSense account.
We also ask that you don't modify the AdSense ad code or alter the result of clicking on an ad on your website, as such behavior violates our program policies.