Use a pivot in a UNION

I am trying to combine ALL the records from BOTH OF these tables. Each TABLE has SOME id's that the other does not have so I don't want to lose any in the process. My problem is the TotalUsages and TotalSites data is all falling under the TotalUsages column. I need to pivot the TotalSites column so it becomes it's own column. SELECT a1.id, a1.city, a1.Year, a1.totalusages FROM table1 a1 UNION ALL SELECT a2.id, a2.city, a2.Year, a2.totalsites FROM table2 a2