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Item Open Access Aerosol particle size distributions in the lower Fraser Valley: evidence for particle nucleation and growth.(Copernicus Publications, 2004) Mozurkewich, M.; Chan, T.W.; Aklilu, Y.; Verheggen, B.Particle size distributions from 9 to 640nm diameter were measured at Eagle Ridge in the lower Fraser Valley from 13 August to 1 September 2001 as part of the Pacific 2001 Air Quality Study. The site was on top of a ridge, about 300m above the valley floor, in a predominantly agricultural area about 70km ESE of Vancouver. To further characterize the particles, their hygroscopic properties (affinity for water) were measured. The maximum of the number distributions was generally between 40 and 100nm diameter, but the number distribution was sometimes dominated by ultrafine particles with diameters below 40nm. These ultrafine particles, which appeared to some extent on all days, were frequently associated with elevated levels of CO and NOx, as expected for fresh vehicular emissions. The appearance of these fresh emissions was most pronounced when the growing mixed layer reached the altitude of the site. In contrast, pronounced nucleation events occurred on the five cleanest days; these resulted in particle number concentrations as high as 5x104 particles cm-3 and growth rates of 5 to 10nmhr-1. Nucleation appears to have been triggered when the UV flux reached about 25Wm-2. The growth of these newly formed particles was probably driven by the photochemical oxidation of biogenic organic compounds. Dramatic growth events were also observed on the afternoons of the more polluted days; these produced an extremely narrow mode σ<0.3) at a diameter of about 40nm. Rainy days showed low number concentrations with the size distributions shifted to small sizes. On one of these days there was evidence of nucleation not far from the site; this may have been occurring in the vicinity of the clouds.Item Open Access Determination of the nucleation rate from observation of a SO2 induced atmospheric nucleation event(AGU, 2002) Verheggen, B.; Mozurkewich, M.A method to determine the particle nucleation rate directly from atmospheric measurements is presented. During the Southern Ontario Oxidant Study (SONTOS) field campaign in rural Ontario, Canada, particle size distributions and concentrations of a range of photochemical species were measured. On 25 August 1993, the size distribution showed a pronounced peak in the concentration of nucleation mode particles. This correlated with, but lagged behind, a peak in the SO2 concentration. The data imply that nucleation occurred aloft as an SO2 plume was entrained into the growing boundary layer. The particle growth rates were determined from the evolution of the measured particle size distributions, while accounting for coagulation and dilution. In principle, measurements of precursor species are not needed. However, in this case study, the ground-based measurements did not reflect the aerosol concentrations in the plume aloft; as a result, extrapolation of the growth rate was necessary. This was accomplished by using a one-dimensional model to calculate the gas phase sulfuric acid concentration. The particle growth rate due to condensation of H2SO4 was calculated and used to extrapolate the observed growth backward to obtain the time of formation. From the particle number in a certain size interval, suitably corrected for losses by coagulation and dilution, and the time interval in which they formed, the nucleation rate can be determined. We obtained nucleation rates of 5–40 cm−3 s−1 for sulfuric acid mixing ratios of 3–10 pptv. These nucleation rates are higher than predicted by classical binary nucleation theory for H2SO4 and H2O.Item Open Access An inverse modeling procedure to determine particle growth and nucleation rates from measured aerosol size distributions(Copernicus Publications, 2006) Verheggen, B.; Mozurkewich, M.Classical nucleation theory is unable to explain the ubiquity of nucleation events observed in the atmosphere. This shows a need for an empirical determination of the nucleation rate. Here we present a novel inverse modeling procedure to determine particle nucleation and growth rates based on consecutive measurements of the aerosol size distribution. The particle growth rate is determined by regression analysis of the measured change in the aerosol size distribution over time, taking into account the effects of processes such as coagulation, deposition and/or dilution. This allows the growth rate to be determined with a higher time-resolution than can be deduced from inspecting contour plots ("banana-plots''). Knowing the growth rate as a function of time enables the evaluation of the time of nucleation of measured particles of a certain size. The nucleation rate is then obtained by integrating the particle losses from time of measurement to time of nucleation. The regression analysis can also be used to determine or verify the optimum value of other parameters of interest, such as the wall loss or coagulation rate constants. As an example, the method is applied to smog chamber measurements. This program offers a powerful interpretive tool to study empirical aerosol population dynamics in general, and nucleation and growth in particular.Item Open Access Observation of a SO2 induced nucleation event in the atmosphere(Elsevier, 1998) Verheggen, B.; Mozurkewich, M.