Secondary Climate Trends
Surface solar radiation (SSR) observed at Stockholm (blue dashed line, y-axis inverted) does not show much similarity to SSR calculated theoretically (dotted purple line, y-axis inverted) from concentrations of pollutants such as SO2, NOx, and black carbon. NOx data from RETRO (Schultz and Rast, 2008) interpolated before 1960 from US data (U. S. Environmental Protection Agency, 2000) are thought to be constant or slightly decreasing since 2000 (Cofala et al., 2009).