Supplementary material to "What caused a record high PM10 episode in northern Europe in October 2020?"

Abstract. In early October 2020, northern Europe experienced an episode with poor airquality due to high concentrations of particulate matter (PM). At severalsites in Norway, recorded weekly values exceeded historical maximumPM10 concentrations from the past 4 to 10 years. Daily mean PM10values at Norwegian sites were up to 97 µg m−3 and had amedian value of 59 µg m−3. We analysed this severe pollution episode caused by long-range atmospheric transport based on surface andremote sensing observations and transport model simulations to understandits causes. Samples from three sites in mainland Norway and the Arcticremote station Zeppelin (Svalbard) showed strong contributions from mineraldust to PM10 (23 %–36 % as a minimum and 31 %–45 % as a maximum) and biomass burning (8 %–16 % to 19 %–21 %). Atmospheric transport simulations indicate that Central Asia was the main source region for mineral dustobserved in this episode. The biomass burning fraction can be attributed toforest fires in Ukraine and southern Russia, but we cannot exclude othersources contributing, like fires elsewhere, because the model underestimates observed concentrations. The combined use of remote sensing, surfacemeasurements, and transport modelling proved effective in describing theepisode and distinguishing its causes.