Fig. 4: Number of studies and changes in focus across different research regions.

This composite visualization presents the global distribution of research volume by country and the temporal shift in pollutant focus across continents, excluding Antarctica. The central map employs circles whose size and color intensity categorize the volume of literature studying each country, identified through geoparsing technology. Surrounding the map are six subfigures corresponding to North America, Europe, Asia, South America, Africa, and Oceania, akin to Fig. 3a. These graphs track the changes in the number of studies over time, alongside the evolution of research attention to various pollutants within each continent across four distinct periods (1990–1997, 1998–2005, 2006–2013, and 2014–2021).