Articles | Volume 17, issue 19
Research article
12 Oct 2017
Research article |  | 12 Oct 2017

Typical meteorological conditions associated with extreme nitrogen dioxide (NO2) pollution events over Scandinavia

Manu Anna Thomas and Abhay Devasthale

Abstract. Characterizing typical meteorological conditions associated with extreme pollution events helps to better understand the role of local meteorology in governing the transport and distribution of pollutants in the atmosphere. The knowledge of their co-variability could further help to evaluate and constrain chemistry transport models. Hence, in this study, we investigate the statistical linkages between extreme nitrogen dioxide (NO2) pollution events and meteorology over Scandinavia using observational and reanalysis data. It is observed that the south-westerly winds dominated during extreme events, accounting for 50–65 % of the total events depending on the season, while the second largest annual occurrence was from south-easterly winds, accounting for 17 % of total events. The specific humidity anomalies showed an influx of warmer and moisture-laden air masses over Scandinavia in the free troposphere. Two distinct modes in the persistency of circulation patterns are observed. The first mode lasts for 1–2 days, dominated by south-easterly winds that prevailed during 78 % of total extreme events in that mode, while the second mode lasted for 3–5 days, dominated by south-westerly winds that prevailed during 86 % of the events. The combined analysis of circulation patterns, their persistency, and associated changes in humidity and clouds suggests that NO2 extreme events over Scandinavia occur mainly due to long-range transport from the southern latitudes.

Short summary
Episodes of extreme pollution events of nitrogen dioxide (NO2) can seriously hamper air quality. But under which meteorological conditions do such extreme pollution events occur over Scandinavia? Using observational and reanalysis data it is shown that south-westerly winds (sustained for at least a few days) dominate during extreme events and cause an increase in humidity and clouds. South-easterly winds have the second largest contribution and the pollution transport is rapid when they prevail.
Final-revised paper