Secondary organic aerosol formation indoors: experimental and modeling investigations of impacts of surface reactions and equilibrium partitioning processes

Chunyi Wang

doi:10.17918/wxt3-r395

People in developed countries spend about 90% of their time indoors, so controlling in-door air quality (IAQ) is of primary importance for not harming public health. Airborne particu-late matter (PM) is one of the most problematic pollutants indoors, since exposure to particles with aerodynamic diameters smaller than 2.5 [mu]m (i.e, PM2.5) is associated with respiratory dis-eases, as well as morbidity and mortality outcomes. Organic aerosol components, so called organic aerosol (OA), generally comprise the ma-jor portion of indoor PM, owing to its large indoor emission. One important component of OA indoors is secondary organic aerosol (SOA), which are condensed phase particles composed of semi- and low-volatility compounds. Most research has focused on SOA generated by terpene ozonolysis occurring in the gas phase. This work, however, explores a lesser researched for-mation mechanism, which is the possibility of airborne SOA generated by ozone surface reac-tions with sorbed squalene (C30H50), which is a nonvolatile constituent of skin oil. As such, thirteen steady state chamber experiments were performed to measure the SOA formation en-tirely initiated by ozone reactions with squalene sorbed to glass at two RH conditions of 21% and 51%, in the absence of seed particles. SOA was initiated from these surface reactions, and all experiments but one exhibited nucleation and mass formation. Mass formation increased with ozone concentration at RH = 51% while nucleation was more obvious at RH = 21%. Additionally, most indoor OA, either emitted or generated (i.e., not only SOA), is at composed of semivolatile compounds (SVOCs) in a state of dynamic equilibrium between gas and particle phases. Filters might have a reduced efficiency on removing these kinds of particles since they coexist in gas and condensed aerosol phases. The preferential filtration of particle phase material of the OA system could disrupt the equilibrium, and the removed aerosols might be enhanced by desorption from surfaces and repartitioning from gas phase. To explore this phenomenon, three types of particles, including non-volatile ammonium sulfate ((NH4)2SO4) aerosol, incense aerosol (which might be partly semi-volatile), and SOA derived from ozone + d-limonene reactions (the majority of which are SVOCs), were characterized and compared in terms of their effective removal by a portable air cleaner. For this comparison, the metric of the Clean Air Delivery Rate, CADR (m3/h), was used, which is the volumetric flow of pollutant-free air produced by an air cleaner. Results demonstrated that the lowest effective CADR was for SOA, followed by the incense, and then the ammonium sulfate particles, indicating a repar-titioning processes reduced the filter efficiency. Then a model based on the principles of desorp-tion and repartition process was developed, to quantify the reduced CADR as a function of par-ticle concentration and distribution, in terms of parameter ATSP, which is the ratio of particle surface area to mass. Finally, the influence of the above two parameters on amount of CADR reduction was discussed. Using some details gleaned from the above two experimental studies, a thermodynamic equilibrium model was developed using the volatile basis set (VBS), to predict indoor organic aerosol concentrations and behavior. The model outcomes are the total organic mass indoors (gas + condensed phase), and the fraction of it that partitions to the aerosol phase, including that existing as SOA formed by ozone + d-limonene reactions. With this model, the total OA concentration was simulated at key locations in an indoor environment, such as in the occupied space and different positions in a building mechanical system. The impacts of different condi-tions were compared, including commercial against residential buildings, surface against gas reactions, and winter against summer, within a Monte Carlo framework. Indoor OA concentra-tion indoors were higher when reactions were involved, and gas phase reactions had much more influence on SOA than surface reactions. Finally, the result dataset was used to evaluate the influence of key factors on the indoor OA concentrations, using multiple linear regression sen-sitivity methods. The most important factor that enhanced indoor particles was d-limonene emission rate with average SRC of 0.73, while the negative related factors were filtration effi-ciency with SRC of -0.33 for commercial and surface deposition rate with SRC of -0.22 for resi-dential buildings. Beyond the three SOA studies discussed above, humidifiers used indoors might be strong PM emitters. So, as a supplementary piece, this work also investigated the influence of three humidifier types (ultrasonic, evaporative, and steam humidifiers), and water type used (tap water, de-ionized (DI) water or distilled water), on indoor aerosol number/mass concentra-tions by performing 16 experiments. Particle size distribution during emission periods and size-resolved emission rates were explored to compare the emission ability of humidifiers. Two lung deposition models were also applied to simulate the deposition percentage of particles breathed in on three lung regions (HA, TB, and AL), and total percentage on varying age groups. Results showed that two year-old group was most vulnerable, with number deposition fractions of 0.36, compared with 0.25 for adults. Furthermore, roughly 70% of the total emitted particles pene-trates into the AL region of the lung.

Secondary organic aerosol formation indoors: experimental and modeling investigations of impacts of surface reactions and equilibrium partitioning processes

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Secondary organic aerosol formation indoors: experimental and modeling investigations of impacts of surface reactions and equilibrium partitioning processes

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