A simplified approach to calculate particle growth rate due to self-coagulation, scavenging and condensation using SMPS measurements during a particle growth event in New Delhi

Abstract

The scanning mobility particle sizer (SMPS) technique is a widely employed technique to measure the particle number size distribution, and thus calculate the particle total growth rate. However, growth due to individual atmospheric processes needs to be known precisely and accurately to better model the secondary aerosol distribution. In this study, we use simplified analytical formulas to calculate the growth rates due to self-coagulation, coagulation scavenging and condensation processes of particle size distribution (9–425 nm) measured using SMPS. Firstly, total growth rate is determined from the regression fit of SMPS data plotted between the geometric mean diameter (GMD) of particle size (nm) versus time (hour) measured during a particle growth event. The SMPS measurements were conducted during November-December 2011 in New Delhi. The particle growth event days and non-event days were classified according to the protocol discussed elsewhere. Assuming that the particle number size distribution of a growing population can be described by a unimodal distribution and particles are neutral in the population, we calculated the growth rate due to selfcoagulation (GRscoag), which is proportional to the total number of particles in the mode and mode peak diameter. Similarly, assuming that particles with mode peak size 25 nm and above act as a coagulation sink and grow due to scavenging of newly formed nucleation range particles (< 12 nm), we calculated the coagulation scavenging growth rate (GRscav) as a time derivative of the mode peak diameter, which is equivalent to the product of particle diameter and its coagulation sink. The condensation growth rate (GRcond) is calculated based on the assumption that total growth rate is the summation of the growth resulting of three physical processes: self-coagulation, coagulation scavenging and condensation. During the study period, three event days were recorded at the measurement site. To explain the growth rate calculation approach, which is presented here in detail, we have taken SMPS data of one event day (November 4, 2011) as an example (two other event days are also briefly discussed). On November 4, the total average growth rate was found to be 15.4 ± 11 nm/h, while the average GRscoag, GRscav and GRcond were calculated to be 3.8 ± 0.4 (with min and max values of 2.9–5.1 nm/h), 8.0 ± 6 (0.6–19.3 nm/h) and 3.6 nm/h, respectively. These growth rates are comparable to those reported for other urban sites around the world using different methods. This approach is simple, and growth by individual processes can be calculated without knowing several other parameters, which include vapor concentration of atmospheric constituents, heterogeneous processes, and complex modeling procedures.