The continuous growth of newly formed particles in urban environments

Abstract

New particle formation events (NPF) are observed to take place in all environments, including megacities where the concentrations of pre-existing particles acting as sink are exceptionally high. NPF produces high number concentration of small particles, which can act as seed particles suitable for the accumulation of particle mass after their subsequent growth. However, the reason of the frequently observed NPF in urban environments is still unclear, and the contribution of NPF and subsequent growth to air pollution is still controversial. To improve the understanding of the link between air pollution and NPF, comprehensive observations, including gaseous precursors, size distributions and chemical compositions of atmospheric aerosols, as well as meteorological conditions, were conducted in the west campus of Beijing University of Chemical and Technology (BUCT, 39o 56’31” N, 116o17’50” E), near the West Third Ring Road of Beijing, China. We also performed simultaneous measurements of aerosol composition and particle number size distributions at ground level and at 260 m based on the 325 m Beijing meteorological tower. We divided NPF events into two types based on whether the newly formed particles grow continuously or not. By comparing the meteorological conditions, gaseous precursors, and chemical composition between these two types, we investigated the conditions favour the continuous growth of new formed particles. Our results also showed that the continuous particle growth could contribute to the formation of haze. Due to the stronger emission of gaseous precursors near ground coupled with the less effective boundary layer mixing, particles originating from NPF continue to grow at ground level while their mean diameter remains relatively stable at a higher altitude, resulting in the more severe haze pollution at ground level than at high altitude.