A review of hygroscopic growth factors of submicron aerosols from different sources and its implication for calculation of lung deposition efficiency of ambient aerosols

Hygroscopic properties are an important parameter in determining the atmospheric behaviour of aerosols and their optical properties, influencing the direct and indirect effect of aerosols upon climate. As a result, particle hygroscopicity has received much attention with a rapid increase of publications in recent years. Likewise, hygroscopicity is an important characteristic influencing the deposition efficiency of particles in the human respiratory tract by affecting the particle size. The object of this study is to review the existing knowledge on the hygroscopic growth factor (G<inf>f</inf>) of atmospheric submicron particles and its influence on the lung deposition calculation. The study briefly reviews first the G<inf>f</inf> values of particles generated from various sources, including nucleation, traffic emissions and biomass burning, discussing the spatial and temporal variations. It then summarises G<inf>f</inf> values of submicron particles and number fraction of each hygroscopic group measured in different ambient environments. These include marine, roadside, urban background and rural environments. The study concludes by estimating the lung deposition efficiency of ambient particles using a modified version of the International Commission on Radiological Protection (ICRP) model for hygroscopic particles. Furthermore, the effect of hygroscopicity on lung deposition efficiency of ambient particles has been estimated. The ICRP model seems to predict well the deposition efficiency (DE) values for small ambient particles in the extra-thoracic and tracheo-bronchial region, but not the alveolar region, where they are overestimated. However, for larger particles (D<inf>p</inf> > 200 nm), the ICRP model underestimates the DE values, with the extra-thoracic region the most affected of the three. Hygroscopic atmospheric particles with a small diameter (D<inf>p</inf> < 200 nm) showed a lower total lung deposition than hydrophobic particles of the same initial size due to their hygroscopic properties. On the other hand, larger hygroscopic particles (D<inf>p</inf> > 200 nm) showed much higher lung deposition than hydrophobic particles.