Abstract
Abstract. Cluster~analysis of particle number size distributions from~background sites across Europe~is presented. This generated a total of nine clusters of particle size distributions which could be further combined into two main groups, namely: a south-to-north category (four clusters) and a west-to-east category (five clusters). The first group was identified as most frequently being detected inside and around northern Germany and neighbouring countries, showing clear evidence of local afternoon nucleation and growth events that could be linked to movement of air masses from south to north arriving ultimately at the Arctic contributing to Arctic haze.~The second group of particle size spectra proved to have narrower size distributions and collectively showed a dependence of modal diameter upon the longitude of the site (west to east) at which they were most frequently detected.~These clusters indicated regional nucleation (at the coastal sites) growing to larger modes further inland. The apparent growth rate of the modal diameter was around 0.6–0.9 nm h−1. Four specific air mass back-trajectories were successively taken as case studies to examine in real time the evolution of aerosol size distributions across Europe. ~While aerosol growth processes can be observed as aerosol traverses Europe, the processes are often obscured by the addition of aerosol by emissions en route. This study revealed that some of the 24 stations exhibit more complex behaviour than others, especially when impacted by local sources or a variety of different air masses. Overall, the aerosol size distribution clustering analysis greatly simplifies the complex data set and allows a description of aerosol aging processes, which reflects the longer-term average development of particle number size distributions as air masses advect across Europe.
Highlights
Airborne particle size distributions vary in space and time and can be interpreted in terms of known sources, meteorological processes and aerosol dynamical processes affecting such particles
By clustering all of the spectra collected across the 24 EUSAAR sites, an understanding of how the spectrum types are shared between these sites can be observed
The clusters detected most frequently within the central European region – in and around Germany where the spatial density of emissions is anticipated to be high – had less seasonal dependence and much broader nucleation and accumulation modes, reflecting the increased likelihood of more diverse emission sources from anthropogenic emissions mixing into an air mass
Summary
Airborne particle size distributions vary in space and time and can be interpreted in terms of known sources, meteorological processes and aerosol dynamical processes affecting such particles. Sources contributing to particle concentrations in the atmosphere are both primary and secondary. Particles in rural areas are from a wider range of sources, both primary and secondary. New secondary particles formed by nucleation are generated from gas-to-particle conversion. Such particles have been observed at many surface locations around the world and within the free and upper troposphere (Kulmala et al, 2004; Venzac et al, 2008; Boulon et al, 2010, 2011). Reddington et al (2011) recently stressed the importance of understanding the relative contribution of primary and secondary particles in regional and global aerosol so that models can attribute aerosol radiative forcing to different sources
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