Estimating nanoparticle growth rates from size-dependent charged fractions: Analysis of new particle formation events in Mexico City

A method to estimate nanoparticle diameter growth rates (GR) during new particle formation (NPF) events from the measured dependence of charged fraction, f, on size, D-p, is introduced. The method is especially useful for observations during intense particle production rates, when the mode in the distribution of newly formed particles does not grow monotonically with time. This method assumes that the observed profile of f versus Dp during the nucleation and growth period is controlled by condensational growth, ion-particle combination/recombination, scavenging by preexisting particles, and coagulation among growing nanoparticles. Values of growth rates obtained by this method (GRf) agree well with independently obtained particle growth rates due to gas-to-particle conversion processes (GR(PSD)) during regional NPF events. The method was then applied to characterize the NPF events observed at Tecamac, Mexico. These growth rates were found to range from 15 - 40 nm/h, which is significantly higher than values reported for other urban areas. The production rates for 1 nm particles calculated from the estimated growth rates and measured Fuchs surface area (J(1nm) = 1900 - 3000 particles/cm(3) s) are comparable to those recently observed in New Delhi. Because critical nuclei are likely close to 1 nm in size, J(1nm) should provide a reasonable estimate for nucleation rates.