Perhaps you should try some spelling variations:
"pollutants dispersion reduction" » "pollutant dispersion reduction", "pollution dispersion reduction", "pollutants detection reduction", "pollutants dispersion prediction", "pollutants dispersion predictions", "pollutants dispersion equation"
"pollutants dispersion relation" » "pollutant dispersion relation", "pollution dispersion relation", "pollutants detection relation", "pollutants dispersion relations"
"pollution dimensions reduction" » "position dimensions reduction", "solutions dimensions reduction", "evaluation dimensions reduction", "pollution dimensional reduction", "pollution dimension reductions", "pollution dimensional reductions"
"pollutant dimension reduction" » "pollutant dispersion reduction", "pollutant diffusion reduction", "pollutant emission reduction", "pollutant dimensionality reduction"

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• If you are trying to use Boolean operators, they must be ALL CAPS: (((("pollutant dimension reduction") OR ("pollution dimensions reduction"))) OR ((("pollutants dispersion reduction") OR ("pollutants dispersion relation")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"pollutant dimension reduction\") OR (\"pollution dimensions reduction\"))) or (((\"pollutants dispersion reduction\") or (\"pollutants dispersion relation\"))))".
• Removing quotes may allow a broader search: (((( pollutant dimension reduction ) OR ( pollution dimensions reduction ))) or ((( pollutants dispersion reduction ) or ( pollutants dispersion relation )))).
• Adding a wildcard symbol may retrieve word variants: (((("pollutant dimension reduction") OR ("pollution dimensions reduction"))) or ((("pollutants dispersion reduction") or ("pollutants dispersion relation"))))*.