Two years of harmonized aerosol number size distribution data from 24 European field monitoring sites have been analysed. The results give a comprehensive overview of the European near surface aerosol particle number concentrations and number size distributions between 30 and 500 nm of dry particle diameter. Spatial and temporal distribution of aerosols in the particle sizes most important for climate applications are presented. We also analyse the annual, weekly and diurnal cycles of the aerosol number concentrations, provide log-normal fitting parameters for median number size distributions, and give guidance notes for data users. Emphasis is placed on the usability of results within the aerosol modelling community. We also show that the aerosol number concentrations of Aitken and accumulation mode particles (with 100 nm dry diameter as a cut-off between modes) are related, although there is significant variation in the ratios of the modal number concentrations. Different aerosol and station types are distinguished from this data and this methodology has potential for further categorization of stations aerosol number size distribution types. The European submicron aerosol was divided into characteristic types: Central European aerosol, characterized by single mode median size distributions, unimodal number concentration histograms and low variability in CCN-sized aerosol number concentrations; Nordic aerosol with low number concentrations, although showing pronounced seasonal variation of especially Aitken mode particles; Mountain sites (altitude over 1000 m a.s.l.) with a strong seasonal cycle in aerosol number concentrations, high variability, and very low median number concentrations. Southern and Western European regions had fewer stations, which decreases the regional coverage of these results. Aerosol number concentrations over the Britain and Ireland had very high variance and there are indications of mixed air masses from several source regions; the Mediterranean aerosol exhibit high seasonality, and a strong accumulation mode in the summer. The greatest concentrations were observed at the Ispra station in Northern Italy with high accumulation mode number concentrations in the winter. The aerosol number concentrations at the Arctic station Zeppelin in Ny-\AA lesund in Svalbard have also a strong seasonal cycle, with greater concentrations of accumulation mode particles in winter, and dominating summer Aitken mode indicating more recently formed particles. Observed particles did not show any statistically significant regional work-week or weekday related variation in number concentrations studied. Analysis products are made for open-access to the research community, available in a freely accessible internet site. The results give to the modelling community a reliable, easy-to-use and freely available comparison dataset of aerosol size distributions.

Ground Surface Temperature (GST) is defined as the near-surface temperature of the ground (bedrock or surficial deposits), measured in the uppermost centimeters of the ground. GST is not a direct proof of permafrost existence but a proxy for estimating potential permafrost presence or absence in the subsurface and can be used for calibrating and validating numerical models. GST has to be distinguished from the Bottom Temperature of Snow cover (BTS), which is the temperature measured at the snow/ground interface in late winter. The APD does not collect raw GST data but the Mean Annual Ground Surface Temperature (MAGST) of a certain depth.

GM are punctual evidence of potential presence or absence of permafrost derived from geophysical methods. Electrical Resistivity/Impedance Tomography (ERT/EIT), Ground Penetrating Radar (GPR) or Seismic Refraction (SR) are suitable methods for the indirect detection of permafrost.

Temperature measured in boreholes (BH) is the most reliable evidence of permafrost presence or absence. BH data provides min/max/mean subsurface temperature at differing depth, maximum active layer thickness (ALT) and (if available) the depth of zero annual amplitude (ZAA). The APD does not collects raw borehole data but yearly synthesis data.

Other indirect permafrost evidence (OIE) such as patterned ground (features produced by the repeated annual freezing and thawing of the active layer in permafrost soils) or thermokarst depressions.

The presence of massive ice-lenses in periglacial areas can be a direct proof of permafrost occurrence. Such evidence from trenches or construction sites in high-mountain areas can provide useful information. Massive ice can also be observed in rockfall scars and can represents evidence of permafrost.

The Alpine Permafrost Data (APD) is an on-line service for collecting and sharing permafrost data, or permafrost evidence, in the European Alps. The permafrost evidence is a direct or indirect proof of the presence or absence of permafrost in a specific location obtained by field measures or observations. The Alpine Permafrsot Database (APD) collects Rock Glacier Inventories and the following Permafrost evidence: - Boreholes Temperature (BH) - Ground Surface Temperature (GST) - Surface Movement in periglacial areas (SM) - Results of Geophysical Prospecting - Perennial-ice in rock-fall scars, trenches or construction sites - Other indirect permafrost evidence

It is important to understand the relative contribution of primary and secondary particles to regional and global aerosol so that models can attribute aerosol radiative forcing to different sources. In large-scale models, there is considerable uncertainty associated with treatments of particle formation (nucleation) in the boundary layer (BL) and in the size distribution of emitted primary particles, leading to uncertainties in predicted cloud condensation nuclei (CCN) concentrations. Here we quantify how primary particle emissions and secondary particle formation influence size-resolved particle number concentrations in the BL using a global aerosol microphysics model and aircraft and ground site observations made during the May 2008 campaign of the European Integrated Project on Aerosol Cloud Climate Air Quality Interactions (EUCAARI). We tested four different parameterisations for BL nucleation and two assumptions for the emission size distribution of anthropogenic and wildfire carbonaceous particles. When we emit carbonaceous particles at small sizes (as recommended by the Aerosol Intercomparison project, AEROCOM), the spatial distributions of campaign-mean number concentrations of particles with diameter >50 nm (N50) and >100 nm (N100) were well captured by the model (R2>0.8) and the normalised mean bias (NMB) was also small (-18% for N50 and -1% for N100). Emission of carbonaceous particles at larger sizes, which we consider to be more realistic for low spatial resolution global models, results in equally good correlation but larger bias (R2>0.8, NMB = -52% and -29%), which could be partly but not entirely compensated by BL nucleation. Within the uncertainty of the observations and accounting for the uncertainty in the size of emitted primary particles, BL nucleation makes a statistically significant contribution to CCN-sized particles at less than a quarter of the ground sites. Our results show that a major source of uncertainty in CCN-sized particles in polluted European air is the emitted size of primary carbonaceous particles. New information is required not just from direct observations, but also to determine the "effective emission size" and composition of primary particles appropriate for different resolution models.

In order to evaluate the possible effects of heatwave phenomena on background O3 concentrations, the average summer O3 concentrations at the high mountain station of Mt. Cimone (MTC—2165 m a.s.l.) have been analyzed. In particular, at this baseline station unusually high O3 concentrations were recorded during August 2003, when an intense heatwave (the “August heatwave”) affected Europe. During this heatwave, the highest O3 concentrations were recorded at MTC in connection with air masses coming from continental Europe and the Po basin boundary layer as shown by three-dimensional air mass back-trajectory and mixing height analyzes. However, high O3 concentrations were also recorded in air masses coming from the middle troposphere (above 3000 m a.s.l.), thus suggesting the presence of O3-rich atmospheric layers over Europe. This could be due to the large extension of the mixing layer which favoured the transport of high concentrations of O3 and its precursors to altitudes that would usually be in the free troposphere. Other than from traffic and industrial activities, a contribution to the high O3 concentrations recorded at MTC during the August heatwave could derive from fires in the North of Italy, as suggested by a well-documented episode and supported by in situ CO2 measurements used as non-conventional tracer for fire emissions.

Halocarbons are powerful greenhouse gases capable of significantly influencing the radiative forcing of the Earth’s atmosphere. Halocarbons are monitored in several stations which are globally distributed in order to assess long term atmospheric trends and to identify source regions. However, to achieve these aims the definition of background mixing ratios, i.e. the mixing ratio in a given air mass when the recent contribution of local sources is absent, is necessary. This task can be accomplished using different methods. This paper presents a statistical methodology that has been devised specifically for a mountain site located in Continental Europe (Monte Cimone, Italy), characterised by the vicinity of strong sources. The method involves the decomposition of the observed data distribution into a Gaussian distribution, representative of background values, and a Gamma distribution, ascribable to contribution from stronger sources. The method has been applied to a time series from a European marine remote station (Mace Head, Ireland) as well as to time series from Monte Cimone. A comparison of the methodology described in this paper with a well-established meteorological filtering procedure at Mace Head has shown an excellent agreement. A comparison of the baselines at Mace Head, Mt. Cimone and the Swiss alpine station of the Jungfraujoch highlighted the occurrence of a specific background concentration. Although this paper presents the application of the method to three hydrofluorocarbons, the proposed methodology can be extended to any long lived atmospheric component for which a long term time series is available and at any location even if affected by strong source regions.