Abstract
New particle formation (NPF) is a common occurrence in urban environments where it is promoted by a ready supply of gaseous precursors such as sulfuric acid, ammonium and volatile organic compounds originating mainly from motor vehicle emissions, In this paper, we investigate the influence of wind speed on the occurrence of NPF events in a subtropical urban environment. The analysis was based on a large data set obtained with a neutral cluster and air ion spectrometer (NAIS) over 485 days of observations, out of which NPF events were observed on 213 days. Particles formation was most likely to occur during the morning hours and we observed NPF events between 8 am and noon on 123 of these days. In order to assess its influence, we estimated the mean wind speed between 8 and 9 am on these 123 days and on the days with no NPF. We found a statistically significant difference in average wind speeds during days with NPF (1.31 m s(-1)) and days with no NPF (1.84 m s(-1)) (p < 0.05). Several studies have shown that increasing wind speed enhances NPF in polluted megacities by removing pre-existing particles. Our observations indicate that this effect is less important in smaller cities, so that higher wind speeds can mitigate NPF by the removal of condensable gases. The wind speed does not affect the rate of NPF in clean environments.