Closing the ultrafine particle number concentration budget at road-to-ambient scale: Implications for particle dynamics

摘要

Freshly emitted vehicle exhaust particles are diluted quickly as they mix into ambient air, but the contribution of evaporation, coagulation, and/or nucleation of new particles to the number concentration has been the subject of some debate. We analyzed one-second time resolution size distribution data from an early morning field campaign, data collected at a time at which dilution has a smaller (but still dominant; approximate to 70-80%) impact on particle concentrations. Because the plume is diluted over an hour, and a distance of 1500m, we can constrain the processes with higher accuracy. We find that concentrations in the smaller size bins (5.6-23.7nm) peak further downwind than the reference particles (42.1-562nm), and decay significantly faster than larger particles particularly in the area 100-400m downwind. Comparisons of the cumulative contributions of van der Waals enhanced coagulation, dry deposition, and dilution and the observed decay curves, imply that for up to the first 50-100m there is nucleation and/or growth of particles smaller than 5.6nm. In contrast, in the approximate to 100-400m region, some of the smaller particles evaporate. In the further downwind areas (>400m) the particles all appear to decay at rates consistent with the sum of dilution, coagulation, and deposition. We also find that a dry deposition parameterization at the low end of those available in the literature is most consistent with the observational data.(c) 2016 American Association for Aerosol Research