Aerosol ageing in an urban plume – implications for climate and health

Abstract

Abstract. The climate and health effects downwind of an urban area resulting from gaseous and particulate emissions within the city are as yet inadequately quantified. The aim of this work was to estimate these effects for Malmö city in Southern Sweden (population 280 000). The chemical and physical particle properties were simulated with a model for Aerosol Dynamics, gas phase CHEMistry and radiative transfer calculations (ADCHEM) following the trajectory movement from upwind Malmö, through the urban background environment and finally tens and hundreds of kilometers downwind Malmö. The model results were validated with measurements of the particle number size distribution and chemical composition. The total particle number concentration 50 km (~3 h) downwind in the center of the Malmö plume is about 3800 cm−3 and the Malmö contribution is roughly 35%. Condensation of nitric acid, ammonium and to a smaller extent oxidized organic compounds formed from the emissions in Malmö increases the secondary aerosol formation with a maximum of 0.6–0.7 μg/m3 6 to 18 h downwind of Malmö. The secondary mass contribution dominates over the primary soot contribution from Malmö already 2 to 3 h after the emissions and gives an enhanced total top of the atmosphere direct or indirect aerosol shortwave radiative forcing in the center of the urban plume ranging from −0.3 to −2.3 W m−2 depending on the distance from Malmö, and the cloud properties. It also gives an increased respiratory tract deposited mass dose, which increases with the distance downwind Malmö.