Fig. 5.

TC and SO₄ 98th quantile trends. Observed 98th quantile TC (Top) and SO₄ (Bottom) trends (calculated using QR methods) from IMPROVE sites for 1988–2016 are shown by black dots with corresponding values in µg·m⁻³·y⁻¹. Krige-interpolated values (calculated from observed data) are shown by the color ramp. In A, the solid black lines with arrows (indicating direction) show the boundary where 90% of the Krige-interpolated TC trends within are positive or negative. Of the 149 sites, 73 show statistical significance. In B, all regions are within the 90% negative slope estimation. Of the 156 sites, 145 show statistical significance.