Secondary aerosols in Switzerland and northern Italy: Modeling and sensitivity studies for summer 2003

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The formation and transport of secondary aerosols during a 4‐day period in summer 2003 were studied using the 3‐dimensional air quality model CAMx (Comprehensive Air quality Model with eXtensions) over an area covering regions north and south of the Swiss Alps with different air pollution characteristics. The modeled components were particulate sulfate, nitrate, ammonium, primary (POA) and secondary organic aerosols (SOA) and elemental carbon (EC) with a particle diameter smaller than 2.5 μm. Highest concentrations were predicted in northern Italy. The modeled sum of particle mass concentrations was in general lower than PM2.5 measurements most probably due to an underestimation of organic aerosols. Differences between the regions in the north and the south of the Alps are discussed with respect to the aerosol concentrations and to the sensitivity of aerosol formation. Sensitivity tests using reduced NH3 and NOx emissions suggest that in northern Switzerland secondary aerosol formation is unlikely to be limited by NH3 but rather by HNO3. On the other hand, aerosol formation around Milan seems to be similarly dependent on HNO3 and NH3 most of the time. However, there are times when limitation by NH3 is stronger. The contribution of biogenic sources to SOA was predicted to be rather high, about 80% in the north, matching the measurements whereas it was lower in southern Switzerland (40%).