The 1979 Convention on Long-range Transboundary Air Pollution was the first multilateral treaty for dealing with air pollutants.
Links
[1] https://airclim.org/sites/default/files/documents/clrtap.pdf
[2] https://airclim.org/author/christer-%C3%A5gren
[3] https://airclim.org/publication-kind/briefing-and-factsheet