Abstract
We examine the meteorological conditions favourable for new particle formation as a contribution to clarifying the responsible processes. Synoptic weather maps and satellite images over Southern Finland for 2003–2005 were examined, focusing mainly on air mass types, atmospheric frontal passages, and cloudiness. Arctic air masses are most favourable for new aerosol particle formation in the boreal forest. New particle formation tends to occur on days after passage of a cold front and on days without frontal passages. Cloudiness, often associated with frontal passages, decreases the amount of solar radiation, reducing the growth of new particles. When cloud cover exceeds 3–4 octas, particle formation proceeds at a slower rate or does not occur at all. During 2003–2005, the conditions that favour particle formation at Hyytiälä (Arctic air mass, post-cold-frontal passage or no frontal passage and cloudiness less than 3–4 octas) occur on 198 d. On 105 (57%) of those days, new particle formation occurred, indicating that these meteorological conditions alone can favour, but are not sufficient for, new particle formation and growth. In contrast, 53 d (28%) were classified as undefined days; 30 d (15%) were non-event days, where no evidence of increasing particle concentration and growth has been noticed.
Highlights
Atmospheric nucleation events receive increasing attention as a potentially important source of aerosol particles affecting climate and human health (e.g. Charlson et al, 1987; Donaldson et al, 1998)
Aerosol particle size distributions are measured with a Differential Mobility Particle Sizer (DMPS) system (Aalto et al, 2001), which consists of two Differential Mobility Analyzers (DMAs) producing overlapping size distributions and two Condensation Particle Counters (CPCs) to detect the selected particles
No significant yearly differences in the frequency of the different types of air masses and atmospheric frontal passages were observed at Hyytialafor 2003–2005
Summary
Atmospheric nucleation events receive increasing attention as a potentially important source of aerosol particles affecting climate and human health (e.g. Charlson et al, 1987; Donaldson et al, 1998). Since Aitken (1891, 1897) first reported evidence for new particle formation in the atmosphere, new particle formation has been observed across the world in different environments (Kulmala et al, 2004a, and references cited therein) Despite these abundant observations of new particle formation and growth, the mechanism remains largely unknown. Existing theories for this mechanism include binary water– sulphuric acid nucleation theory (Kulmala and Laaksonen, 1990) and its improvement for tropospheric and stratospheric conditions (Vehkamaki et al, 2002), ternary water–sulphuric acid– ammonia nucleation theory (Napari et al, 2002; Merikanto et al, 2007), cluster activation theory Within this multitude of theories for new particle formation, several important factors affect nucleation, such as the chemical composition of the atmosphere, water content, and the amount
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