Seasonal and spatial variations of N2O distribution and emission in the East China Sea and South Yellow Sea

Abstract

Continental shelves and estuaries are important regions for the production and emission of N2O, one of the most potent greenhouse gasses and the key ozone depletion substances. Five cruises were conducted in 2011 to investigate the spatial and seasonal variations of N2O distribution and emission in the East China Sea (ECS) and South Yellow Sea (SYS). Generally, the mean N2O concentration in the SYS was higher than that in the ECS, driven by the temperature gradient owing to the latitude and currents. Influenced by riverine input, N2O concentration gradually declines from the coast to the open sea. As bottom N2O concentrations were usually higher than those at the surface, and because of the high sediment–water flux (5.5 × 107 mol yr−1), we suggest that sediments, especially around the Yangtze River Estuary, were an important benthic N2O source. Maximum N2O occurred in spring due to the relatively low temperature and strong vertical mixing. Despite the high temperature in summer, the N2O concentration was still high and the saturation was higher than that in other seasons. N2O concentration and emissions sharply decreased from summer to fall, and both the average concentration and saturation decreased to the lowest. In winter, weakened stratification and lower temperature caused the N2O concentration to increase again. In situ production, mixing of water masses, riverine input, and sediment release were summarized as the main N2O sources in this region. Dominated by surface N2O concentration and wind speed, sea–air fluxes were high in spring and summer, and low in fall and winter. The annual emissions were estimated to be 6.3 × 10−2 Tg yr−1 in the ECS and 1.2 × 10−2 Tg yr−1 in the SYS, accounting for approximately 1.5% and 0.29% of the global oceanic N2O emissions, respectively. Hence, both the ECS and SYS acted as N2O sources to the atmosphere.