16Indoor air quality has become a growing concern as people are spending more time 17indoors, combined with the construction of highly sealed buildings that promote 18 thermal efficiency. Particulate matter (PM) is a common indoor air pollutant, with 19 exposure to high concentrations associated with several detrimental health outcomes. 20Active botanical biofilters or functional green walls are becoming increasingly 21 efficient and have the potential to mitigate high suspended PM concentrations. These 22 systems, however, require further development before they become competitive with 23 industry standard in-room air filters. Whilst the plant growth substrate in active 24 biofilters can act as a filter medium, it was previously not known whether the plant 25 component of these systems played a function in PM filtration. This study thus 26 examines the influence of the botanical component on active green wall PM single 27 pass removal efficiency (SPRE), with a focus on evaluating the air filtration features 28 of different plant species in green wall modules. All tested botanical biofilters 29 outperformed biofilters that consisted only of substrate. Green walls using different 30 plant species had different single pass removal efficiencies, with fern species 31 recording the highest removal efficiencies across all measured particle sizes 32 (Nephrolepis exaltata bostoniensis SPRE for PM 0.3-0.5 and PM 5-10 = 45.78% and 33 92.46% respectively). Higher removal efficiencies were associated with increased 34 pressure drop across the biofilter. An assessment of plant morphological data 35 suggested that the root structure of the plants strongly influenced removal efficiency. 36 These findings demonstrate the potential to enhance active botanical biofiltration 37 technology with appropriate plant species selection. 38 39 Keywords: air quality; PM; phytoremediation; active green wall; sustainable 40 buildings; living wall 41 42 Highlights 43 44 • Active botanical biofilters can reduce ambient atmospheric particulate matter 45 concentrations. 46 • Particulate matter removal efficiency is influenced by active green wall plant 47 species. 48• Removal efficiency is correlated with pressure drop across the green wall 49 module. 50

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Indoor air quality has become a growing concern due to the increasing proportion of time people spend indoors, combined with reduced building ventilation rates resulting from an increasing awareness of building energy use. It has been well established that potted-plants can help to phytoremediate a diverse range of indoor air pollutants. In particular, a substantial body of literature has demonstrated the ability of the potted-plant system to remove volatile organic compounds (VOCs) from indoor air. These findings have largely originated from laboratory scale chamber experiments, with several studies drawing different conclusions regarding the primary VOC removal mechanism, and removal efficiencies. Advancements in indoor air phytoremediation technology, notably active botanical biofilters, can more effectively reduce the concentrations of multiple indoor air pollutants through the action of active airflow through a plant growing medium, along with vertically aligned plants which achieve a high leaf area density per unit of floor space. Despite variable system designs, systems available have clear potential to assist or replace existing mechanical ventilation systems for indoor air pollutant removal. Further research is needed to develop, test and confirm their effectiveness and safety before they can be functionally integrated in the broader built environment. The current article reviews the current state of active air phytoremediation technology, discusses the available botanical biofiltration systems, and identifies areas in need of development.

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Functional green walls are gaining attention due to their air cleaning abilities, however the air cleaning capacity of these systems may be improved through substrate modification. This experiment investigated the capacity of several green wall media to filter a range of air pollutants. Media, consisting of differently sized coconut husk-based substrates, and with different ratios of activated carbon were evaluated through the use of scaled down model 'cassettes'. Tests were conducted assessing each substrate's ability to filter particulate matter, benzene, ethyl acetate and ambient total VOCs. While the particle size of coconut husk did not influence removal efficiency, the addition of activated carbon to coconut husk media improved the removal efficiency for all gaseous pollutants. Activated carbon as a medium component, however, inhibited the removal efficiency of particulate matter. Once the substrate concentration of activated carbon approached ∼50%, its gas remediation capacity became asymptotic, suggesting that a 50:50 composite medium provided the best VOC removal. In full-scale botanical biofilter modules, activated carbon-based substrates increased benzene removal, yet decreased particulate matter removal despite the addition of plants. The findings suggest that medium design should be target pollutant dependent, while further work is needed to establish plant viability in activated carbon-based media.