Distributions of N2O concentration and sea-to-air flux in the western Tropical North Pacific: Influences of eddies and typhoons

Abstract

Marine systems are active regions for producing and emitting nitrous oxide (N₂O), a potent greenhouse gas. In October 2018, samples were collected in the western tropical North Pacific (WTNP) to study the distributions, emissions, and influencing factors of N₂O. The N₂O concentrations in surface seawater showed little variation, ranging from 6.2 to 7.9 nmol L⁻¹ (corresponding to N₂O saturation range of 104–125%), with an average of 6.7 ± 0.6 nmol L⁻¹. The vertical N₂O distribution is a mirror image of dissolved oxygen (DO), increasing with depth from the surface to a maximum in the vicinity of the DO minimum. The sea-to-air fluxes of N₂O ranged from 0.1 to 7.5 μmol m⁻² d⁻¹, with an average of 1.7 ± 2.2 μmol m⁻² d⁻¹, indicating that the WTNP was a net source of N₂O to the atmosphere. Nitrification is the main process for N₂O production. The presence of the Mindanao Eddy noticeably changes the vertical profiles of N₂O in the water column, allowing the N₂O-rich deep water to reach the subsurface layers, but has little effect on the surface N₂O concentration. After Typhoon “Yutu” passed through, N₂O concentrations in surface and subsurface water increased dramatically. The surface concentration increased by about 20%, and the sea-to-air flux of N₂O increased by about 56%. Despite the short duration of the typhoon, its effect on N₂O distribution and sea-to-air flux was more pronounced than that of eddies.