Limitations of First-Order Deposition Models for Indoor Aerosol Concentration Decay

LI LI, Himanshi Saini, Jeff Tithof, Christopher J. Hogan, University of Minnesota

     Abstract Number: 51
     Working Group: Aerosol Physics

Abstract
Traditionally, first-order kinetic models are widely used to interpret indoor aerosol concentration decay and estimate particle deposition rate or loss coefficients. Although these models lead to a straightforward and mathematically simple approach, they rely on assumptions of a well-mixed system and a constant deposition rate, which often fail to represent realistic indoor environments and aerosol chamber experiments. In many real-world measurements, concentration time-evolution curves deviate significantly from the exponential trend predicted by first-order decay models. This is primarily due to complex turbulent flow structures and convective bulk motion driven by the room geometry and ventilation settings. In this study, we introduce a more general approach to model the time-dependent deposition rate by statistically analyzing the cumulative distribution function of particle residence times within an enclosed space. Through this approach we show that the first order decay model is only accurate when the cumulative distribution function is exponentially distributed and initial position-independent, which is not always satisfied in many environments. The approach is applied to a two-dimensional test environment where the turbulent the flow field is resolved by Direct Numerical Simulation, and particle motion is tracked using a Lagrangian approach, covering particle diameters from 10nm to 10µm. We systematically evaluate how convective bulk velocity, Reynolds number, turbulent vortex structures (including their number and configuration), and the extent of the convection influence deposition behavior.