The Chamber Wall Index for Gas-Wall Interactions in Atmospheric Environmental Enclosures.

Secondary organic aerosol (SOA) particles, which are formed and aged in Earth's oxidizing atmosphere, influence climate and human health. Quantifying properties of SOA particles and oxidized organic compounds (OVOCs) requires controlled experiments in enclosures, but enclosures have walls that can alter the chemistry. Comparing wall effects for widely used large environmental chambers (ECs) and portable oxidative flow reactors (OFRs) is difficult. In this work, the Chamber Wall Index (CWI) is developed as the minimum ratio of the initial wall uptake time constant divided by the enclosure residence time. This index demonstrates that walls alter the chemistry less in OFRs than in ECs, due primarily to shorter residence times. Much shorter residence times may not be feasible because oxidation chemistry and microphysics need time to produce atmospherically relevant SOA and OVOCs. While all current OFRs have wall effects, it may be possible to develop a "wall-less" OFR.