¹⁾ note that the emission varies widely depending on application temperatures and air/fuel ratios. In general - higher combustion temperature and  higher air/fuel ratios increases NO_x emissions.
²⁾ zero emission for hydrogen is a theoretically value. In practice hydrogen burned in air produces more NO_x than natural gas due to the high flame speed
3) catalytic systems common on most modern vehicles reduces NO_x
NO_x emissions contributes to eutrophication, acidification and the formation of ground-level ozone.