Evaluating Environmental Film Maturation through a Deliquescence–Efflorescence Model

Abstract

As atmospheric particulate and (semi)volatile molecules gradually deposit on outdoor surfaces, they create heterogeneous coatings known as environmental films. The unique chemical environments within these films will impact local environmental chemistry. We report the effects of water vapor and deliquescence/efflorescence cycling on particles incorporated into these films within three model systems created in our laboratory: pure salt, pure organic, and mixed salt-organic films, ranging from 1 to 5 μm thickness. To do this, we monitor morphology changes to inorganic and organic particles that comprise each film type before and after relative humidity (%RH) cycling. We track the model film behaviors by quartz crystal microbalance and optical microscopy, including detailed image analysis to track particle sizes, shapes, and number density. Our results show maximum particle diameter and shape distributions (≥100 nmhundreds of μm and circular to elongated), as well as the number of film particles per unit area change in different ways depending on the composition of the model film. Specifically, the pure salt films show fewer, larger particles after %RH cycling. Pure organic films show a small decrease in particle sizes but no significant morphology changes. In mixed films of inorganic and organic species, the %RH exposure leads to matured films with higher numbers of smaller, more compact particles. Based on these observations, we suggest the effects of %RH on increasing mobility (mixing, lateral reach, and aggregation) of deliquesced or liquid-phase particles within the film. These maturation, or ripening, effects alter the capacity for environmental films to affect local environmental chemistry.

What Are the Differences between Two Environmental Films Sampled 1 km Apart?

“…8 Quantifying water affinities for different film chemistries has important implications for stormwater quality and tracking fate and transport of persistent organic pollutant materials entrained within the films. 2,16 In this study, we examine films collected at two sites near Iowa City, Iowa. The two sampling sites are separated by ∼1 km.…”

“…For example, Baergen et al have shown a surprising trend of higher water uptake of films collected in the summer in comparison to their winter counterparts and rationalized this suggesting an unidentified species controlling water uptake . Quantifying water affinities for different film chemistries has important implications for stormwater quality and tracking fate and transport of persistent organic pollutant materials entrained within the films. , …”