step,rows_before,rows_after,rows_removed,pct_removed,notes
Replace 999->NaN,842160,842160,0,0.0,"1,477 values replaced across 18 numeric columns"
Range validation,842160,842160,0,0.0,0 out-of-range values nulled
Forward fill (limit=3),842160,842160,0,0.0,"124 NaN values filled, 0 NaN remain (gaps > 3 hrs)"
Drop all-null rows,842160,842160,0,0.0,Rows where every pollutant column was NaN (extended outages)
Deduplication,842160,842160,0,0.0,"Subset: ['city', 'datetime']"