Globally gridded dataset of nitrate-nitrite in surface water for the years 1992-2010, monthly observations. Data is available at the 0.5x0.5 degree gridcell level. Units are milligram per liter (mg/l). Data is generated using a machine learning model, as described in the report Quality Unknown: The Invisible Water Crisis (https://www.worldbank.org/en/news/feature/2019/08/20/quality-unknown). See report Appendix for more details.