An Approach Developed to Investigate New Particle Formation in Vertical Direction on Basis of High Time-resolution Measurements at the Ground Level

Abstract:

A combination of high time-resolution measurements of particles and an eddy covariance system has been used to study the transport of particles in vertical direction. Building on this idea, we developed a new approach to investigate new particle formation in vertical direction using high time-resolution measurements at the ground level alone. The coefficient of variation (CV), i.e., the ratios of standard deviation to mean for number concentration and geometric mean diameter of <100 nm particles (N₁₀₀, GMD₁₀₀) in every 30s, is introduced as an indicator to identify horizontal particle transport from vertical particle transport. We first examine the typical ranges of CVs for horizontal and vertical transport of particles, respectively, using the measurement conducted in the summer of 2007 at a semi-urban site in Toronto, Canada. We found that the CVs associated with horizontal transport of particles were 2-10 times smaller than those associated with vertical transport of particles. Thus, we assumed that 1) the higher CVs reflect higher contribution from vertical transport; 2) the lower CVs reflect higher contribution from horizontal transport. We then calculate the N₁₀₀, GMD₅₅ (GMD of <55 nm particles) and GMD₁₀₀ corresponding to 5% maximum and 5% minimum CVs for N₁₀₀ in five NPF events, respectively. Our results inferred that the growth rates of new particles at ground level were 0-30% larger than that at height. The formation rates of new particles at ground level were 0-30% larger than those that at height, except one event in which the formation rate at ground level was 20% lower than that aloft. However, the size of the cases used in this study was small and the findings need more data to further confirm.