Determining PM2.5 dry deposition velocity on plant leaves: An indirect experimental method

Abstract

Trees and green spaces have been proved to play an important role in purifying urban atmospheric particulate matter and result to the overall improvement of air quality. However, a primary setback was the need for an accurate and easier method to measure the deposition velocity (Vd) of PM2.5 retained on plant leaves. This paper established an indirect method of measurement for determining Vd: the indirect method was derived from a closed system (smog chamber), and an exponential attenuation model was used to measure the decline in PM2.5 concentration under conditions caused by hanging leaves in the chamber, and an empty chamber without leaves, respectively. The value of Vd was thus calculated by fitting data to prevent inaccurate results caused by a single measurement error. The experimental conditions such as initial concentration, test duration, and time interval (Δt) were then determined from empirical observations. The equation showed that Vd could be determined only by measuring the attenuation rate constant, k, and the leaf area, LA. Although the particle size, tested tree species and some other experimental conditions may affect the determination results of Vd, the values of Vd tested by indirect method were still within scientifically acceptable range. Overall, along with the traditional wind tunnel method, the indirect measurement is considered as the complement for a less expensive and easier way to determine Vd, and offers a novel idea that Vd might be gained by attenuation equation, and can augment the choices for researchers to quantify the ability of retaining particles.