Seasonal variation of aerosol size distributions based on long term measurements at GAW Global Monte Cimone Station (2165 m asl), Italy

Particle number concentration and size distribution are important variables needed to constrain the role of atmospheric aerosol in the Earth radiation budget, both directly and indirectly through CCN activation. They are also linked to regulated variables such as particle mass (PM) and therefore of interest to air quality studies. However, data on their long-term variability are scarce, in particular at high altitudes, where atmosphere is representative of baseline conditions. In this paper, we investigate the diurnal and seasonal variability of the aerosol total number concentration and size distribution at the Monte Cimone GAW global station (CMN, 2165 m a.s.l.). The measurement site, placed in Northern Appenines, with 360° free horizon, can be considered representative of the background conditions of the Southern Europe/Mediterranean basin and it is particularly suitable to characterize the background conditions, but also the influence of the polluted Po Valley carried up with valley breezes during summertime. We analyze the 10-500 nm aerosol size distribution performed at CMN over more than 7 years (November 2005 - May 2013), by using a Differential Mobility Particle Sizer (DMPS), set up and improved in the framework of EUSAAR and ACTRIS EU-projects, following the standard recommendations of both projects. Current knowledge of aerosol number size distributions has generally allowed to divide the number size distribution into a Nucleation mode (10-25 nm) representative of recent new particle formation, an Aitken mode (40-90 nm), and an accumulation mode (100-500 nm) representing aged particles. The modal aerosol size distributions have been described using lognormal fitting, that identifies 2 or 3 modes. Moreover, the aerosol size distributions have been classified as a function of the air mass type. The seasonal aerosol number distribution showed a behavior typical for high altitude conditions, characterized by highest values in summer (2430 ± 1198 cm-3) and lowest in winter (1050 ± 737 cm-3). Typical autumn-winter conditions are represented in Figure 1, showing the case study of November 3rd 2010. During these seasons CMN observations are well representative of the free troposphere conditions, with very low particles concentration (<100 cm-3), mainly found in the Aitken mode. On the other hand, during summertime, CMN can be also affected by polluted air masses transported from the regional PBL. The spring-summer conditions also favor the formation of new particles, as shown in Figure 2, (August 30th 2010), when relatively clean conditions were observed in the morning (~300 cm-3), while since 14:00 the accumulation (aged) particles increase indicated pollution transport from PBL and a subsequent strong new particle formation event since 15:00. The averaged particle size distribution shows very clearly the presence of three aerosol modes: nucleation, Aitken and accumulation. A statistical analysis of new particle formation events occurring at the site will be shown, describing the atmospheric conditions favoring such type of events. This work was supported by MIUR through Ev-K2-CNR/SHARE, CNR-DTA/NEXTDATA projects and by EUSAAR, ACTRIS EU-projects.