Spatial variability of particle number concentrations and NOx in the Karlsruhe (Germany) area obtained with the mobile laboratory ‘AERO-TRAM’

For the first time in Germany, we obtained high-resolution spatial distributions of particle numbers and nitrogen oxides in an urban agglomeration using a tram system. In comparison to particle numbers the NOx concentration decreased much faster with a significantly steeper gradient when going from the inner city to the surrounding area. In case of NOx the decrease was 70% while for particle number concentration it was only 50%. We found an area in the rural surrounding with a second increase of particle numbers without simultaneous enhanced NOx levels. The source of the high particle numbers could be ascribed to industry emissions about 5–10 km away. The mean spatial distribution of particle number concentration depended on wind direction, wind velocity and boundary layer stability. The dependency was particularly strong in the rural area affected by industrial emissions, where individual wind directions led to concentration differences of up to 25%. The particulate concentration was 40% higher during low wind velocities (1–5 m s−1) than during high wind velocities (>5 m s−1). We observed similar findings for the impact of boundary layer stability on particle numbers concentration. Particle pollution was 40% higher for stable stratification compared to neutral or unstable cases.