Fig. 1: Site locations and relationships between emissions of SO₂, NO_x and NH₃ and concentrations of SO₄²⁻, NO₃^T and NH₄^T.

a, Site map. Black and red crosses represent measurement sites established before and after 2015, respectively, in the five regions indicated by specific colours. Corresponding site numbers are listed in the legends. The base map was obtained from Natural Earth. The five regions are defined according to the Regional Planning Organizations (Methods). The numbers of samples for these regions for each year are listed in Supplementary Table 2. b–d, Annual SO₂ (b), NO_x (c) and NH₃ (d) emissions (\({e}_{{\rm{S}}{{\rm{O}}}_{2}}\), \({e}_{{\rm{N}}{{\rm{O}}}_{{{x}}}}\) and \({e}_{{\rm{N}}{{\rm{H}}}_{3}}\)) in the five regions. e–g, Annual mean concentrations of SO₄²⁻ (e), NO₃^T (f) and NH₄^T (g). h–j, Orthogonal distance regressions of annual mean \({e}_{{\rm{S}}{{\rm{O}}}_{2}}\) and \({c}_{{\rm{S}}{{\rm{O}}}_{4}^{2-}}\) (h), \({e}_{{\rm{N}}{{\rm{O}}}_{{{x}}}}\) and \({c}_{{\rm{N}}{{\rm{O}}}_{3}^{{\rm{T}}}}\) (i) and \({e}_{{\rm{N}}{{\rm{H}}}_{3}}\) and \({c}_{{\rm{N}}{{\rm{H}}}_{4}^{{\rm{T}}}}\) (j), with each dot indicating one year from 2011 to 2020. The vertical bars show the 25th and 75th percentiles of annual mean concentrations observed at locations within a region.