Abstract
Abstract. This paper describes aerosol modelling in Europe with a focus on Switzerland during summer and winter periods. We modelled PM2.5 (particles smaller than 2.5 μm in aerodynamic diameter) for one summer and two winter periods in years 2006 and 2007 using the CAMx air quality model. The meteorological fields were obtained from MM5 simulations. The modelled wind speeds during some low-wind periods, however, had to be calibrated with measurements to use realistic input for the air quality model. The detailed AMS (aerosol mass spectrometer) measurements at specific locations were used to evaluate the model results. In addition to the base case simulations, we carried out sensitivity tests with modified aerosol precursor emissions, air temperature and deposition. Aerosol concentrations in winter 2006 were twice as high as those in winter 2007, however, the chemical compositions were similar. CAMx could reproduce the relative composition of aerosols very well both in the winter and summer periods. Absolute concentrations of aerosol species were underestimated by about 20 %. Both measurements and model results suggest that organic aerosol (30–38 %) and particulate nitrate (30–36 %) are the main aerosol components in winter. In summer, organic aerosol dominates the aerosol composition (55–57 %) and is mainly of secondary origin. The contribution of biogenic volatile organic compound (BVOC) emissions to the formation of secondary organic aerosol (SOA) was predicted to be very large (>95 %) in Switzerland. The main contributors to the modelled SOA concentrations were oxidation products of monoterpenes and sesquiterpenes as well as oligomerization of oxidized compounds. The fraction of primary organic aerosol (POA) derived from measurements was lower than the model predictions indicating the importance of volatility of POA, which has not yet been taken into account in CAMx. Sensitivity tests with reduced NOx and NH3 emissions suggest that aerosol formation is more sensitive to ammonia emissions in winter in a large part of Europe. In Switzerland however, aerosol formation is predicted to be NOx-sensitive. In summer, effects of NOx and NH3 emission reductions on aerosol concentrations are predicted to be lower mostly due to lower ammonium nitrate concentrations. In general, the sensitivity to NH3 emissions is weaker in summer due to higher NH3 emissions.
Highlights
Atmospheric aerosols are known to have adverse health effects
New studies indicate that oxygenated organic aerosol (OOA) is formed from Organic aerosol (OA) and its precursor gases, which becomes increasingly oxidized, less volatile, and more hygroscopic (Jimenez et al, 2009)
Analysis of the submicron aerosol in Switzerland revealed that only a small fraction of OA originates from freshly emitted fossil fuel combustion, and that a high fraction is composed of OOA (Lanz et al, 2010)
Summary
Atmospheric aerosols are known to have adverse health effects. They play an important role in climate change by modifying the radiative balance of the atmosphere (IPCC, 2007). Analysis of the submicron aerosol in Switzerland revealed that only a small fraction of OA originates from freshly emitted fossil fuel combustion, and that a high fraction is composed of OOA (Lanz et al, 2010). This conclusion was based on analyses of the organic aerosol. Separate scavenging models for gases and aerosols were implemented in CAMx to calculate the wet deposition (Environ, 2008). We performed the simulations for the same periods as the detailed field campaigns where measurements were conducted with an aerosol mass spectrometer (AMS) at Zurich in January 2006 (Lanz et al, 2008) and at Payerne in. Partitioning of condensable organic gases to secondary organic aerosols was www.atmos-chem-phys.net/11/7355/2011/
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