<Tr> <Td> 5 </Td> <Td> Los Angeles - Long Beach - Riverside, CA </Td> </Tr> <Tr> <Td> 6 </Td> <Td> Modesto, CA </Td> </Tr> <Tr> <Td> 7 </Td> <Td> Visalia - Porterville, CA </Td> </Tr> <Tr> <Td> 8 </Td> <Td> Pittsburgh - New Castle, PA </Td> </Tr>

The most significant source of sulfur dioxide emissions into the atmosphere in the united states is