Elevated ammonia emission neutralized alleviation of nitrogen deposition to water pollution in China

Over the past few decades, human activities associated with energy and food production (e.g., industrial, traffic, agricultural, and waste disposal sources) have substantially increased emissions of reactive nitrogen (Nr) to the atmosphere which leads to excessive atmospheric nitrogen (N) deposition on land and rivers. Part of the deposited N on land is also transported to rivers (defined as indirect N deposition). China is a global hotspot of N deposition and has implemented strict atmospheric policies in the last decade. Yet, the responses of N deposition (including direct and indirect N deposition) to these policies and inputs into the river are not well known. We couple a global chemistry transport model with the native 0.5°×0.625° horizontal resolution with a water quality model to simulate direct and indirect N deposition from human activities on rivers in 33 Chinese sub-basins in 2011 and 2019. Compared with 2011, the fluxes of direct N deposition on both rivers and land changed from 10.9 to 10.8 Tg N yr^-1 (including deposition of reduced nitrogen (NH_x) changed 4.6 to 5.9 Tg N yr^-1 and deposition of oxidized nitrogen (NO_y) changed 6.3 to 4.9 Tg N yr^-1) among the sub-basins in 2019. The results indicate that the strict air policies since 2013 released by the Chinese government significantly decreased NO_y. However, the amounts of direct and indirect N deposition on water on average only declined by 2% across all sub-basins, mostly due to the increases in dry ammonia deposition. Agriculture was estimated as the largest source, contributing 30% and 37% to the total N deposition in 2011 and 2019. Thus, agricultural ammonia emission control is essential to reduce N deposition-induced water pollution. This integrated air-water model can assess the impacts of N deposition on water quality, providing insights to develop the pollution control policy for both air and water in sub-basins.