NOx emissions from lakes in the Northern Hemisphere

Nitrogen oxides (NO_x=NO+NO₂) are a key player in the nitrogen cycle affecting health and climate, yet NO_x emissions from lakes have received little attention compared with greenhouse gases. Many lakes are affected by local human activities such as shipping, but the existing bottom-up anthropogenic emission inventories contain large uncertainty in NO_x emissions from lakes due to unrobust proxy data and many untracked vessels in public datasets. Moreover, natural NO_x emissions from lakes away from human activities were traditionally thought to be negligible. However, recent work has discovered strong natural NO_x emissions from 135 lakes on the Tibetan Plateau in summer 2019, which are comparable to anthropogenic emissions in several megacities such as Beijing and New York. Yet, whether such large natural emissions of NO_x from lakes are a global phenomenon remains unknown.

Here, we quantify summertime (June-August) NO_x emissions from 300 large lakes (> 200 km²) in the Northern Hemisphere (NH) during 2018-2024, based on satellite NO₂ VCDs data and a physics-based emission inversion algorithm PHLET at a resolution of 0.05° x 0.05°. To ensure the quality of lake NO₂ VCDs, we further exclude satellite lake pixels with unphysical, negative retrieved water vapor concentrations or affected by sun glint. Then we use PHLET to estimate gridded NO_x emissions from NO₂ VCDs, which describes the relationship between NO_x emissions and NO₂ VCDs and accounts for the nonlinear chemistry and horizontal transport. In this report, we will present the quantity and spatial distribution of lake NO_x emissions.