Influence of atmospheric circulation patterns on urban air quality during the winter

Relationships between urban nitrogen dioxide (NO2) and atmospheric circulation at two spatial scales were studied for Southern Sweden. Lamb Weather Types (LWT) describe the circulation (scale: thousands of kilometers) including non–directional (cyclonic and anticyclonic) and directional types depending on the wind direction. LWTs with low wind speeds (anticyclonic, NW and N) were associated with strongly elevated [NO2], between 46–52% of the daily averages of NO2 exceeded the 60 μg m−3 air quality standard (AQS) when occurring during these LWTs. The lowest fractions of exceedances of NO2 AQS were generally observed for LWTs E, S, SW and W. A larger scale circulation (several thousands of kilometers) was represented by the North Atlantic Oscillation (NAO) affecting meteorology over middle and high latitudes in the Northern Hemisphere. While a negative NAO index (NAOI) favors stagnant high pressure weather over Northern Europe, a positive NAOI is often associated with windy conditions. High [NO2] was found to be frequent under negative NAOI. Additionally, larger fractions of exceedances of NO2 AQS were observed for the majority of LWTs occurring at negative NAOI. It’s concluded that both LWTs and NAOI had partly independent effects on the urban air quality in a North European city. These circulation indices can be useful tools for air pollution risk assessment and forecasting.