Ultrafine (< 100 nm) aerosol particles, emitted during combustion and generated from atmospheric chemical reactions, are important to both human health and climate.  Thanks to more widespread use of ultrafine particle counters and Scanning Mobility Particle Sizing, ambient measurements of these particles at high (~5 min) time resolution are becoming increasingly common.  At the University of Iowa, we have developed tools to organize and analyze size distribution data from multiple locations, together with collocated meteorological, gas-phase, and aerosol-phase data.  Both tools and analysis results will be discussed, using datasets from the USEPA Particulate Matter Supersites Program, Bondville Illinois, the MILAGRO field campaign, the Southern California PM Center, and other studies.  

In terms of results, we will present summary statistics regarding ultrafine particles at these locations, the measured occurrence of new particle formation at these locations, and the impact of vehicular emission on each of the measured sites.  For example, the Pittsburgh site had an average number concentration (3-500 nm) of 22,000 cm^-3 with an average number mode of 40 nm.  Regional new particle formation events are most common in Pittsburgh (~3 of 10 days averaged over 1 year).  Sampling duration was not long enough in Bondville (2 months) or Mexico City (1 month) to establish an overall frequency, but particle formation events appear to be somewhat less frequent (than Pittsburgh) in both locations.  In Mexico City, the role of the boundary layer and residual layer in new particle formation appears to be very important.  The Mexico City basin has been shown to have strong thermally and topographically driven circulations.  The connection of this unique meteorology to new particle formation is potentially important and may help to rule out some mechanisms for new particle formation.  On several of the study days in Mexico City, rapid decreases in PM concentrations in the early afternoon are simultaneous with rapid increase in boundary layer height, high friction velocities, and the appearance of a newly formed and growing sub-50 nm mode in the number size distribution. 

The following data analysis problems will be discussed: separating local-source combustion ultrafines from new particle formation, integrating data and deterministic models of vehicle and atmospheric ultrafine particles, and using CO₂ and particle size distribution time series. 

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