Nocturnal atmospheric conditions and their impact on air pollutant concentrations in the city of Stuttgart

Abstract

Abstract Meteorological and air pollutant measurements were conducted in the area of Stuttgart during winter and summer seasons. Stuttgart is situated in moderate mountainous terrain in southwestern Germany. We focus on the connection between atmospheric conditions and air pollutants in the urban nocturnal boundary layer. This is done by relating the bulk Richardson number (Rib), turbulence intensity, cloudiness, and winds, as well as NOx and O3 data. Turbulence intensity is inversely related to Rib, with the lower values occurring at Rib >0.33. The coefficient of determination for the exponential regression is only moderate, which partly can be attributed to sporadic turbulence in the transition from dynamically unstable to stable flows. Dynamically unstable flows (Rib 1.25) are found preferably under clear skies in summer with the build‐up of strong surface inversions, so that buoyant suppression is strong and shear generation of turbulence is weak. The nocturnal NOx concentrations are positively correlated with Rib. The correlation is weak, which is mainly related to the large variability of air pollutant concentrations in a range around Rib = 0.33. In this range, many low‐level jets are present that can cause sporadic turbulent coupling between the atmosphere and the surface. Reduced mixing under dynamically stable flows causes NOx values about 3 times higher than under dynamically unstable flows. The overall lowest NOx concentrations occur during winter when low clouds and strong winds are present.

Keywords

• [UC]2
• complex terrain
• low‐level jet
• NOx
• O3
• surface inversion