Perhaps you should try some spelling variations:
"learning-based no2 imputation techniques" » "learning-based no2 implantation technique", "learning-based no2 augmentation techniques", "learning-based no2 segmentation techniques", "learning-based co imputation techniques", "learning-based n imputation techniques"
"learning-based novel reduction techniques" » "learning-based novel reduction technique", "learning-based model reduction techniques", "learning-based poverty reduction techniques", "learning-based closed reduction techniques", "learning-based anodes reduction techniques"

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (("learning-based no2 imputation techniques") or ("learning-based novel reduction techniques"))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("learning-based no2 imputation techniques") OR ("learning-based novel reduction techniques"))*.
• Removing quotes may allow a broader search: (( learning-based no2 imputation techniques ) or ( learning-based novel reduction techniques ))*.