Chapter 2.1 Modeling of secondary aerosols in Switzerland in summer 2003

Abstract

Abstract The formation and transport of secondary aerosols during a 4-day period in summer 2003 were studied using the three-dimensional air quality model CAMx (Comprehensive Air quality Model with eXtensions) and meteorological model MM5 over an area covering Switzerland and part of the surrounding countries. The modeled components are particulate sulfate, nitrate, ammonium, and secondary organic aerosols (SOA) with a particle diameter smaller than 2.5 μm. Higher concentrations are predicted in southern Switzerland and northern Italy. 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 NH 3 and NO x emissions suggest that in northern Switzerland secondary aerosol formation is unlikely to be limited by NH 3 but rather by HNO 3 . On the other hand, aerosol formation around Milan seems to be similarly dependent on HNO 3 and NH 3 most of the time. However, there are times when limitation by NH 3 is stronger. The contribution of biogenic sources to SOA was predicted to be rather high, about 70% in the north, matching the measurements whereas it was lower in southern Switzerland (40%). In northern Italy, anthropogenic sources contribute much more to SOA formation.