PREFIX schema: <http://qweb.cs.aau.dk/airbase/schema/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX air: <http://qweb.cs.aau.dk/airbase/>
PREFIX prov: <http://www.w3.org/ns/prov#>
SELECT (max(?o3) as ?maxo3) WHERE {
  GRAPH <http://qweb.cs.aau.dk/airbase/provenance/> { 
      {
       ?g air:isMetadataGraph ?y .
      } UNION {
       ?g a prov:Entity . 
       ?g prov:wasGeneratedBy ?createTriple .
       ?createTriple prov:used ?tuple .
       ?tuple air:qualityApproved ?x .    
      }
  }
  GRAPH ?g {
     ?obs schema:O3 ?o3 .
  }
}