Aerosol nucleation and growth in a moderately polluted United States continental environment: Statistical analysis of aerosol number-size distributions

Atmospheric aerosols influence global climate directly and indirectly by acting as cloud condensation nuclei (CCN). Two key variables that are needed to constrain the role of aerosols in the Earth's radiation budget are aerosol sizes and number concentrations. In this presentation, we will present statistical analysis of aerosol number concentrations in the diameter range from 3 nm to 600 nm measured with two scanning mobility particle sizers (SMPS) performed over a 4 year period (January 2006 - December 2009) in a moderately polluted continental environment in United States (Kent, Ohio). New particle formation (NPF) was studied using the observed aerosol number-size distributions. The particle growth and nucleation model (PARGAN) was used to calculate aerosol nucleation and growth rates from the measured aerosol number-size distributions. The total aerosol number, surface, volume concentrations, and condensational sink were determined to be 6150 ± 3900 cm-3, 206 ± 126 μm2 cm-3, 6.5 ± 4.7 μm3 cm-3, and 9.6×10-3 ± 5.8×10-3 s-1, respectively. The PARGAN inversion model calculated growth rates are reasonably correlated with those derived from the measured geometric mean diameters (R = 0.6). Growth rates were in the range from 2 to 20 nm h-1 similar to those observed at other locations worldwide and were highest in the summer (with a mean value of 6.3 ± 2.4 nm h-1) and lowest in winter (with a mean value of 2.0 ±1.0 nm h-1). Aerosol nucleation rates were observed highest in spring and fall, in the range from 0.1 to 100 cm-3 s-1. The averaged particle number concentrations in the size range from 3-20 nm (nuclei mode), 20-100 nm (Aitken) and 100-600 nm (accumulation) were 1300 ± 1080 cm-3, 3360 ± 2670 cm-3 and 1640 ± 1060 cm-3, respectively. Nuclei and Aitken mode particle number concentrations were highest during the spring and fall and lowest during the summer and winter; this seasonal variation was consistent with the observed frequency of NPF events. The NPF frequency was higher in spring (April, 48%) and fall, (October, 32%) than in summer (June, 17%) and winter (December, 11%). Accumulation mode particle number concentrations were greatest in late summer, which was likely linked to the highest growth rates observed in summer. We will discuss the observed NPF frequency in relation with meteorological parameters (temperature, relative humidity, wind speed, and wind direction), water vapor concentrations, solar flux, and air mass history.