We have been alerted by Jos de Laat to a new article
van Beelen, Aldert J. and Aarnout J. van Delden. Institute for Marine and Atmospheric Research Utrecht, Utrecht University, The Netherlands; 2012: Cleaner air brings better views, more sunshine and warmer summer days in the Netherlands. Weather. January 2012. Vol 67 No 1
Excerpts from the paper read [highlight added]
Here we analyse the trends in the frequency of days with high visibility at Schiphol (the main airport in the Netherlands, at 52°18!N and 4°46!E) and at De Bilt (the site of the Royal Netherlands Meteorological Institute, the KNMI, at 52°6!N and 5°11!E) (Figure 2). These stations are roughly 45km apart: Schiphol is about 20km, and De Bilt about 60km, from the sea. Reliable measurements of daily maximum visibility at both stations are available since 1955.
Figure 8 demonstrates that the clearing of the atmosphere is occurring in summer only during daytime. Visibility has changed relatively most strongly in the morning. Visibility has hardly changed during the night, probably because of the competing effect of increasing relative humidity (which, again, is much more important at night than during daytime).
All these changes during daytime are leading to an increase in surface solar radiation. This is confirmed by the measurements of global short-wave radiation at De Bilt, which show that this has steadily increased in summer but changed little in winter. It is likely that this effect is responsible for a significant part of the daytime upward temperature trend in summer, which is reflected also in an accelerated increase of the yearly average temperature after 1985. Nevertheless, we should not jump to conclusions too easily. Apparent agreement between trends does not imply causality. Possible causal links can only be identified by a model study in combination with an analysis of observations.
The conclusion reads
A major clearing of the air has occurred in the Netherlands in the past few decades. These changes are so large that they have become very obvious when looking at the data of individual stations. Strong indications can be found linking human emissions of aerosols to the visibility changes. Coincident with the visibility changes, large trends in cloud cover, sunshine duration and temperature are found, in particular during daytime in summer, showing that these tiny particles might have a significant influence on regional climate.
In addition to documenting the benefit of cleaning up regionally emitted particulate and gas emissions, this analysis suggests that a significant fraction of daytime warming that has been attributed to “global warming” may actually be due to the reduction of aerosols overhead.