Abstract
Abstract. The climate 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 of Malmö, through the urban background environment and finally tens and hundreds of kilometers downwind of Malmö. The model results were evaluated using 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 3700 cm−3 of which the Malmö contribution is roughly 30%. 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 0.7–0.8 μg m−3 6 to 18 h downwind of Malmö. The secondary mass contribution dominates over the primary soot contribution from Malmö already 3 to 4 h downwind of the emission sources and contributes to an enhanced total surface direct or indirect aerosol shortwave radiative forcing in the center of the urban plume ranging from −0.3 to −3.3 W m−2 depending on the distance from Malmö, and the specific cloud properties.
Highlights
In recent years several studies have shown that anthropogenic emissions of trace gases and aerosol particles from urban areas are important for particle properties relevant for climate and population health, within the source region itself and several hundred kilometers downwind (e.g. Seinfeld et al, 2004; Gaydos et al, 2007; Hodzic et al, 2009; Tie et al, 2009)
This indicates that the modeled particle number size distributions in the urban plume between Malmoand Vavihill are realistic
A fraction (∼ 30%) of these particles survives in the atmosphere all the way to Vavihill, where they appear as a mode around 18 nm in diameter
Summary
In recent years several studies have shown that anthropogenic emissions of trace gases and aerosol particles from urban areas are important for particle properties relevant for climate and population health, within the source region itself and several hundred kilometers downwind (e.g. Seinfeld et al, 2004; Gaydos et al, 2007; Hodzic et al, 2009; Tie et al, 2009). In recent years several studies have shown that anthropogenic emissions of trace gases and aerosol particles from urban areas are important for particle properties relevant for climate and population health, within the source region itself and several hundred kilometers downwind The distances from the emission sources are much shorter than the spatial scales used in global and regional CTMs (Pierce et al, 2009). It is important to study the chemical and physical transformation of particles from urban background scale (0.1–1 km) to CTM grid scale (10–100 km scale).
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