The issue focused primarily on controlling greenhouse gases—carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), sulfur dioxide (SO 2 ), ozone (O 3 ), nitrogen oxide (NO x ), ammonia (NH 4 ), phosphorus (PO 4 ), phosphate (PO), chlorine (Cl), ammonia vapor (NH 4 PO 4 ), ammonia vapors (NH 4 VO 4 ), ammonia vapors (NH 4 VO ), ammonia vapors (NH 4 VO ), ammonia vapors (NH 4 VO