A simple model for the outcomes of collisions between exhaled aerosol droplets and airborne particulate matter: Towards an understanding of the influence of air pollution on airborne viral transmission

TL;DR

This paper introduces a new model for droplet-particle collisions that predicts outcomes and applies it to understand how air pollution may enhance airborne viral transmission, including COVID-19, by affecting aerosol pathogen content.

Contribution

The paper develops a novel droplet-particle collision model adapted from droplet-droplet models, validated against experiments, and applies it to assess pathogen transfer in polluted air.

Findings

Aerosol-particle collisions can increase pathogen load in smaller aerosols.

Pathogens can transfer to particulate matter surfaces, facilitating deep respiratory entry.

High ambient particulate matter levels may elevate airborne infection risks.

Abstract

A model that predicts the outcome of collisions between droplets and particles in terms of the distribution of the droplet volume post-collision is lacking, in contrast to the case for droplet-droplet interactions. Taking existing models that successfully predict the outcomes (coalescence, stretching or reflexive separation) and post-separation characteristics (sizes, numbers and velocities of the resulting droplets) of droplet-droplet collisions and adapting them to take into account an inextensible, non-deformable particle with varying wettability characteristics, a new model is presented for droplet-particle collisions. The predictions of the new model agree well with experimental observations of droplet-particle collisions in low-viscosity regimes. The model is then applied to the case of collisions between respiratory aerosols generated by breath, speech, cough and sneeze and ambient airborne particulate material (PM) in order to assess the potential contribution of these interactions to the enhanced transmission of pathogens contained in the aerosol, including COVID-19. The results show that under realistic conditions it is possible for aerosol-PM collisions to enrich the pathogen content of smaller (and so more persistent) aerosol fractions, and to transfer pathogens to the surface of PM particles that can travel deep into the respiratory tract. In the context of better knowledge of the size and velocity distributions of respiratory aerosols, this model may be used to predict the extent to which high ambient PM levels may contribute to airborne infection by pathogens such as COVID-19.