Ammonia is a natural product ,and vast amounts are produced by bacterial processes,......however the greens pretend to their millenial audience that planet destroying ammonia is being caused by motor vehicles.....by the way,heat urea,and you have ammonia and CO2.
Umm. It's the oxides of nitrogen that cause the problem not the ammonia. The catalyst in a diesel exhaust system causes the ammonia (generated by heating urea) and nitrogen oxides to react together to form nitrogen gas and water.
NOx from agriculture tends to be very spread out. NOx from vehicles tends to be very localized, especially in towns and cities. With time, distance and humidity, it ends up as nitric acid and nitrates, which are good for plants except where they are part of acid rain. In the shorter term, with sunlight, it causes the creation of ozone from oxygen in the air and of smog. Neither of which are good for anything near to ground level.
With particle filters and catalysts, Diesel engines are as clean as petrol engines. and the emmisions standards for new vehicles are essentially identical independent of fuel. This is a Good Thing.
Curious fact:- the same conditions that exist in Diesel engines (high pressure and temperature with excess oxygen) exist in power station gas turbines. oxides of nitrogen were a real problem with early designs. The workaround is to make the combustion occur in several stages rather than all at once in the combustion chambers. This way the peak temperatures are kept closer to the average that the turbine needs instead of having very hot gas that is then mixed with cooler gas.