Attribution of Wintertime Anticyclonic Stagnation Contributing to Air Pollution in Western Europe

Introduction. In December 2016, western European weather was dominated by persistent anticyclonic conditions (Fig. 14.1a) leading to dry and calm weather. At this time of year, solar radiation does not bring enough energy to develop a well-mixed boundary layer under such calm conditions. The resulting lack of dispersion led to several air pollution episodes in western Europe, as pollutants such as particulate matter (PM10) accumulated. PM10 concentrations reached very high values over the main European air pollution hotspots: the Po-Valley, eastern Europe, and the northern France, Benelux, southern United Kingdom region, which is the primary focus of the present study (marked as a rectangle in Fig. 14.1b). December 2016 was also among the least windy winter months of the last three decades (Fig. 14.1c), as measured from observed wind speeds over 53 measurement stations in the high-emission area. The mean observed concentrations of PM10 in that area were 25.4 μg m−3 that is 55% above expected average of 16.4 μg m−3 accounting for the decadal decreasing trend (Fig. 14.1d). Traffic bans took place in several places such as in Paris (six ban days). The question as to whether climate change had a role in such an event was raised by the media. Detecting a climate change signal in air pollution is difficult as other factors than climatic ones such as emission reduction policies that took place over the last two decades are important. However, several modeling studies showed potential effects of future climate change on particulate matter levels from individual (Carvalho et al. 2010; Colette et al. 2013; Hedegaard et al. 2008, 2013; Manders et al. 2012; Markakis et al. 2014) or ensemble simulations (Lacressonnière et al. 2016, 2017; Lemaire 2016). These studies characterized mean pollutant concentration changes, but potential changes in air pollution episode frequency did not receive much attention. Air pollution results from emissions and several potential meteorological factors: lack of dispersion, lack of precipitation scavenging, and chemical reactions. We focus on one of the main weather factors, the lack of dispersion by horizontal wind. We hypothesize that December monthly winds (as shown among points in Fig. 14.1d) are strong drivers of air pollution as they are more anticorrelated with PM10 concentrations of Fig. 14.1d (r = −0.75) than monthly precipitations (r = −0.51) over the area and over the 2001–16 period. Shallow planetary boundary layers are also important, but long-term measurements and model assessments are lacking for attribution studies. Near-surface wind speeds are determined by the large-scale circulation, which forces the wind in the free atmosphere, and by surface roughness and stability, which modulate near-surface momentum fluxes. We examine whether human influence could have affected stagnation episodes at monthly time scales. This issue is of particular relevance as previous studies showed that more stagnant conditions are expected (Horton et al. 2014), as well as a decrease in mean wind speeds and wind power production (Tobin et al. 2016) in future decades in some areas of Europe, however not over the area under consideration. AFFILIATIONS: VautaRd, tobin, and Yiou—LSCE/IPSL CEA/ CNRS/UVSQ, Université de Paris-Saclay, Paris, France; Colette and Meleux—Institut National de l’Environnement Industriel et des Risques, Verneuil en Halatte, France; Van MeijgaaRd and Van oldenboRgh—Royal Netherlands Meteorological Institute (KNMI), De Bilt, Netherlands; otto—Environmental Change Institute and Oxford e-Research Center, University of Oxford, Oxford, United Kingdom