Spatial and temporal scales of new particle formation events in eastern North America

New particle formation (NPF) events have been observed in numerous locations. However, questions remain as to the scale of the events and their importance to regional and global particle number concentrations, size distributions and climate forcing. This study presents measured particle size distributions (PSD) at multiple sites across eastern North America and evaluates the degree of coherence on large (hundreds of kilometer) scales and the site-to-site variability across scales of tens of kilometers. Long-term data from sites separated by 1500 km demonstrate frequent and synchronous NPF, that over 80% of event days at both sites are followed by another event day and that event sequences are best described by a Markov Chain of order 1. Estimates of the mean spatial scale of NPF from a site in southern Indiana range from at least 120–850 km depending on the season and the precise assumptions applied. Despite the evidence for regional coherence in NPF, detailed measurements along an 80 km transect in southern Indiana also indicate some important sub-regional variability. While PSD from individual days typically indicate NPF at all three sites or at none of the sites, PSD measured in two urban environments show greater coherence than those from a centrally located site in a forest, and both the number of ultrafine particles and their growth rates are typically (but not uniformly) higher at the forested site. Some of the site-to-site variability appears to be causally linked to planetary boundary layer dynamics and variations in land cover.