The project aims to answer the following question: is heterogeneous photosensitized chemistry important indoors?
Most humans spend a large fraction of their lives indoors, breathing air that is typically not rapidly exchanged with the outdoor environment, and so is potentially quite different in some aspects of its chemistry. One major difference lies in the initiation of chemical reactions in the indoor atmosphere: the very different lighting conditions compared to solar illumination, combined with the typically very low ozone concentrations preclude the occurrence of the “normal” photochemically-initiated generation reactions to form highly-reactive free radical species such as OH. Indeed, there is a considerable knowledge gap in our understanding of how indoor atmospheric oxidation is initiated.
Combining this lack of significant UV radiation of wavelength less than 320 nm indoors and the very high surface-to-volume ratio of the indoor environment strongly suggests that photosensitized surface chemistry could well be important. During such reactions, a molecule (i.e., the photosensitizer) absorbs light and therefore gains energy, then transfers this energy to other molecules, allowing a series of chemical reactions to take place that would otherwise be impossible. This sequence leads to the possibility of photochemistry occurring at longer than “normal” wavelengths – that is, at wavelengths available indoors. It is highly likely that such processes take place indoors, involving ubiquitous compounds adsorbed to surfaces there.
We will directly explore whether and which compounds present on indoor surfaces can contribute to gas phase oxidant and particle concentrations via heterogeneous photosensitized chemistry. We aim to provide some fundamental understanding of this chemistry indoors, laying the groundwork for future work to quantify the impact of such chemistry.
The work we propose will explore how photosensitized processes indoors may affect the oxidation capacity in confined spaces. The proposed work is organized around three main questions, i.e.,
- Photosensitized cycling of reactive nitrogen oxides on indoor surfaces. This action is devoted to the understanding and quantification of processes involving nitrogen oxides and various ubiquitous indoor surfaces (paint, grime, combustion “grime”). It will explore how NO2 or nitrates can be photochemically converted indoors into HONO, a precursor for gaseous OH radicals, as well as give rise to various noxious compounds.
- Photosensitized production of gas phase radicals and reactive organics by illuminated indoor surfaces. Here, we will explore, but also quantify, processes involving organic compounds adsorbed on various ubiquitous indoor surfaces. It will explore how photosensitized chemistry can produce HOx, ROS and ultrafine particles indoors.
- Processes occurring in engineered systems deployed to treat indoor air. We will use real (or realistic) HVAC filter and wall materials, coupled with the same illumination sources as used in commercial HVAC units to explore heterogeneous photochemistry occurring in the air-conditioning systems of modern buildings.
In summary, we will address the following fundamental research questions: Can photosensitized chemical reactions take place on indoor surfaces, in an analogous fashion to what is known from outdoor atmospheric chemistry? Do such reactions give rise to gas phase oxidants which could be important to indoor air? Does this chemistry happen to an extent that it becomes an important unidentified source of indoor atmospheric oxidants?