Well, from a quick bit of searching it looks like it's all NOxes. There are supposed to be known ways of mitigating NOx formation [1][2] but there enough dependencies that I'm not going to do any more digging.

1. https://www.linkedin.com/pulse/nox-reduction-technologies-ga...

2. PDF: https://www.ifc.org/content/dam/ifc/doc/1990/handbook-nitrog...