The [Protocol on Nitrous Oxides] (1988) stipulates a freezing of emissions from their 1987 levels, but nitrous oxide emission have actually increased over the 1980s. Regulations relating to automobile and fuel use are becoming more stringent, particularly in the US, where cleaner fuels are required in certain areas. In the US, Japan and Europe, all new cars must be fitted with catalytic converters, but it is unlikely that fuel and engine design changes alone can suffice and that the only effective answer is to increase motor vehicle usage.
Atmospheric nitrogen may be deposited in dry or wet form. Dry deposition involves the settling of particulates over time with gravity. Wet desposition occurs when particulates and aerosols are removed from the atmosphere by a precipitation event. Wet depositions account for the majority of nitrogen removed from the atmosphere.
Nitrogen-based trace gases emitted during fossil fuel combustion (notably from automobiles) are major contributors to atmospheric pollution. Nitric oxide is an important precursor of ground-level ozone, the component of photochemical smog that is most dangerous to human health and crop productivity. It can also be transformed into nitric acid and, together with sulphuric acid resulting from sulphur emissions, washed out of the atmosphere as acid rain. Acidification of forests, soils and surface waters is increasingly the result of nitrogen emissions in industrialized countries, as sulphur emissions are brought under control.
Once emitted into the atmosphere, nitrogen may be deposited locally or may travel great distances before deposition. Many industrial and urban centers of the central USA emit nitrogen that is not only deposited locally downwind, but also as far away as the east coast of the USA. More than 3.2 million tons of atmospheric nitrogen is deposited on the United States' watersheds each year. In addition, a sizable amount of atmospheric nitrogen is deposited in the Atlantic ocean. It is suggested that that 18% to 27% of the total NOx emitted over the eastern USA is advected over the Atlantic Ocean and deposited.