It seems like Nitrogen Oxide output increases as the fuel combustion temperature increases. So when you're at a cruising speed that burns fuel more efficiently, it alters the balance of waste products. Somehow.
I'll defer to anyone who claims any real knowledge about this at all, I'm just doing some motivated googling.
Roughly, a higher combustion temperature increases combustion efficiency, but also increases NOx produced. Running lean increases the air:fuel ratio, which combusts more of the fuel at higher temperatures. On the flip side, EGR [1] is one of the primary techniques to reduce NOx at the expense of efficiency. Incidentally, it's actually somewhat common for diesel truck owners to do an EGR delete to increase fuel economy.
The "somehow" is pretty straightforward chemistry. As temperature increases, oxygen in the chamber begins to react with nitrogen, producing oxides of nitrogen (NOx). Both are present in the atmosphere, but do not react at normal atmospheric temperatures.
It seems like Nitrogen Oxide output increases as the fuel combustion temperature increases. So when you're at a cruising speed that burns fuel more efficiently, it alters the balance of waste products. Somehow.
I'll defer to anyone who claims any real knowledge about this at all, I'm just doing some motivated googling.
http://capita.wustl.edu/CAPITA/CapitaReports/EmisTrends/soxn...
http://www3.epa.gov/climatechange/ghgemissions/gases/n2o.htm...