Emission of oxides of nitrogen (NOx) and ammonia from the earth’s surface

The natural emissions of oxides of nitrogen (odd nitrogen) and ammonia from the earth's surface in the northern hemisphere are estimated to have upper limits of 3 × 10¹³ g(N) year⁻¹ and 13 × 10¹³ g(N) year⁻¹ respectively based on considerations of boundary layer resistance. This estimate indicates that anthropogenic emissions are a major component of the nitrate cycle in the northern hemisphere.