Modeling and Characterization of a Particle-into-Liquid Sampler (PILS)

Abstract

A modified particle-into-liquid sampler (PILS), based on the original design of Weber et al. (2001), is presented. The principal modification in this design is that collected liquid sample is delivered to vials held on a rotating carousel as opposed to an on-line analytical detector. A model is developed to predict aerosol mass concentrations measured by a PILS based on operating parameters and characteristics of the sampled aerosol. A backward model predicts the concentrations of the sampled aerosol based on operating parameters and concentrations measured by the PILS. Both models, which consider plumbing transmission efficiencies, droplet growth, mixing effects, and volatilization losses, predict mass concentrations that are consistent with laboratory tests for step changes in concentration. The average collection efficiency for species (Na^+, K^+, SO_4^(2−), Cl^−, NO_3^−) from a variety of aerosols compared to simultaneous measurements with a differential mobility analyzer (DMA) exceeded 96% except for NH_4^+ (88%); NH_4^+ is theoretically shown to be the most vulnerable to volatilization, followed by Cl^- and then NO_3^− , with greater losses caused by increasing droplet pH and temperature. The characterization tests highlight the importance of reducing NH_4^+ volatilization by keeping a stable tip temperature of 100°C at the point where steam and ambient air mix in the condensation chamber. Maintaining a stable tip temperature also avoids fluctuations in supersaturations that lead to increased deposition losses of larger droplets. Sample data from the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign are presented.