Abstract Number: 289 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Tropospheric aerosol was characterized during the ARPA Supersite Intensive Operational Period in the Po valley during fall 2011. The results presented here refer to observations performed in San Pietro Capofiume, a rural site located about 30 kilometers from Bologna (Italy). During the campaign, several techniques were deployed to characterize aerosol composition and trace-gas concentration, including off-line analysis of size segregated aerosol samples and on-line measurements with an Aerodyne high resolution aerosol mass spectrometer (HR-ToF-AMS). Submicron particles were composed mainly by organic aerosol (more than 40%), nitrate (about 30%), and ammonium (about 14%). The organic aerosol was mainly water-soluble and the AMS data show that oxidized organics were predominant. The campaign was characterized by frequent fog events, which lasted from one hour up to several hours. These events will be used to investigate the effect of aerosol wet removal, which is a significant source of uncertainty in global models especially for the organic aerosol component. During fog events the aerosol mass concentration decreased by a factor larger than two, and the accumulation mode particles (dry diameter between 100 nanometer and 1 micron) were those scavenged with the highest efficiency. The comparison of aerosol composition prior to and following fog formation indicates that scavenging removed ammonium and nitrate efficiently and left an interstitial aerosol enriched in organics, although the organics were dominated by water soluble oxidized components. The concentration of water soluble organic aerosol decreased by a factor of 1.6 – 1.8 during fog episodes, corresponding to 40-50% of scavenging efficiency. Therefore, these results point towards an overestimation in organic aerosol wet removal in the global models.