Seasonal and diurnal cycles of 0.25–2.5 μm aerosol fluxes over urban Stockholm, Sweden

Size resolved aerosol and gas fluxes were measured in Stockholm from 1 April 2008 to 15 April 2009 over both urban and green sectors. CO₂ and H₂O fluxes peaked in daytime for all seasons. CO₂ concentrations peaked in winter. Due to vegetation influence the CO₂ fluxes had different diurnal cycles and magnitude in the two sectors. In the urban sector, CO₂ fluxes indicated a net source. The sector dominated by residential areas and green spaces had its highest aerosol fluxes in winter. In spring, super micrometer concentrations for both sectors were significantly higher, as were the urban sector rush hour fluxes. The submicrometer aerosol fluxes had a similar diurnal pattern with daytime maxima for all seasons. This suggests that only the super micrometer aerosol emissions are dependent on season. During spring there was a clear difference in super micrometer fluxes between wet and dry streets. Our direct flux measurements have improved the understanding of the processes behind these aerosol emissions. They support the hypothesis that the spring peak in aerosol emissions are due to road dust, produced during the winter, but not released in large quantities until the roads dry up during spring, and explain why Stockholm has problems meeting the EU directive for aerosol mass (PM10).