Research
Here I present a short overview of some of the problems we are currently studying:
Indoor air cleaning with photocatalytic oxidation technologiesOzone-driven chemistry in indoor environments
Indoor fate and transport of secondhand tobacco smoke
Indoor air cleaning with photocatalytic oxidation technologies
In many types of buildings, the indoor generated air pollutants of concern are primarily volatile organic compounds (VOCs) and various types of particles. Implementation of air cleaning technologies for both VOCs and particles may improve indoor air quality, or enable indoor air quality levels to be maintained with reduced outdoor air supply and concomitant energy savings. Air cleaning, unlike ventilation, also can reduce indoor exposures to outdoor pollutants.
Practical air cleaning technologies for aerosol particles are widely available, typically consisting of fibrous filters installed in incoming outdoor air and recirculated air streams.
Air cleaning technologies for VOCs are less advanced, and little information is available on their effectiveness and practicality for use in heating, ventilating and air conditioning (HVAC) systems of buildings. One promising technology is ultraviolet photocatalytic air cleaning (UVPCO), which has been under development for a number of years. UVPCO devices utilize a honeycomb or similar high-surface monolith reactor coated with titanium dioxide (TiO2). The monolith is irradiated with ultraviolet light of either 254 nm (UVC) or 365 nm (UVA). Air flows through the monolith, where the VOCs adsorb reversibly on the photocatalyst and react. The semiconductor acts as a sensitizer for light-induced redox processes through an electron-hole separation upon absorption of a UV photon. Excited-state electrons and holes can recombine, remain trapped in metastable surface states or react with electron donors and acceptors adsorbed on the catalyst surface. The latter process initiates a series of oxidation steps conducing to the production of intermediate organic species and final inorganic products such as CO2 and water. The complete oxidation to inorganic products, often referred to as mineralization, is the ultimate goal of the air cleaning process.
Figure 1: Schematic representation of the photocatalytic process: upon absorption of a UV photon by a TiO2 particle, an electron is excited to the conduction band. Atmospheric oxygen sorbed to the particle serves as electron acceptor, while a VOC (electron donor, D) is oxidized.
UVPCO has been studied almost exclusively in laboratory settings, and most investigations have employed relatively high concentrations (ppm range) of a single or a few VOCs often in an attempt to better understand photocatalytic reaction mechanisms. We are currently investigating the performance of UVPCO under realistic conditions consisting of multicomponent mixtures of common indoor VOCs at low concentrations (<10 ppb in most cases) and high flow rates (200 - 600 m3/h). Figure 2 illustrates the conversion efficiency observed in a single-pass flow reactor as a function of residence time, for two VOC mixtures simulating office building air (OB) and cleaning product emissions (CP).
Figure 2: conversion efficiencies of VOCs present in a mixture simulating office building air (OB) and cleaning product emissions (CP) as a function of the reactor residence time.
We also investigate the photocatalytic mechanisms that lead to the efficient removal of target VOCs (alcohols, aromatics), in order to minimize the generation of harmful partially oxidized secondary pollutants such as formaldehyde and acetaldehyde (as shown on Figure 3). A potential drawback of this technology that needs to be better understood is the inactivation of the photocatalyst due to accumulation of surface-bound oxidation products. Techniques used for this study involves analysis of surface species by FTIR and XPS, and gas phase species by GC/MS and HPLC.
Figure 3: a) Sorption of a VOC on an irradiated TiO2 film; b) formation of secondary pollutants (2nd) re-emitted into treated air, and surface-bound oxidation species (surf).
Ozone-driven chemistry in indoor environments
Ground-level ozone is a major constituent of urban air pollution. It is of particular concern in megacities with high incidence of photochemical smog, such as Los Angeles or Mexico City, where ozone concentrations regularly exceed the EPA 120 ppb 1-hour standard and the 80 ppb 8-hour standard.
Figure 1: The city of Los Angeles, as (not) seen from atop Mt. Wilson.
Ozone levels in the indoor environment are typically 20-70 % of the corresponding outdoor values. The observed attenuation is attributed to reactive deposition on indoor surfaces and to reactions with indoor volatile organic compounds (VOCs). As compared with many important chemical processes in (outdoor) atmospheric chemistry, indoor chemistry is not directly driven by a photochemical initiation step. Instead, ozone of outdoor origin has a critical role in initiating oxidation processes in buildings. Other indoor sources of ozone also exist, such as copier machines, laser printers and certain types of "air purifiers". In addition to ozone, nitrogen oxides and radicals generated by combustion sources can also induce chemical processes in the indoor environment.
Indoor chemistry presents other distinctive characteristics:
1) very large surface-to-volume (S/V) ratios (as compared to well-known processes in the lower troposphere), and a significant role played by heterogeneous chemistry
2) exposed surfaces and structural building materials may function as reservoirs for pollutant uptake, accumulation and long-term re-emission to indoor air. These surfaces can also play an important role in pollutant control and abatement.
3) specific challenges with respect to chemical characterization and exposure assessment are related to the presence of complex mixtures (e.g., tobacco smoke, household product emissions), and to partitioning dynamics between various environmental compartments.
4) human exposure to pollutants in the indoor environment is magnified by their relatively high concentrations (as compared with outdoor air), and by the fact that urban populations spend most of the time indoors (according to a recent survey of California adults, >90% of the time is spent indoors, mostly at home and at work).
We are currently investigating the gas phase and surface chemistry of indoor ozone, in order to better understand the formation of potentially harmful secondary pollutants and ultrafine organic aerosol. This information can also serve to improve the formulation of household products, and to design building materials that can reduce exposure to ozone and certain oxidation byproducts. Formaldehyde is a byproduct formed in the reaction of ozone with several VOCs and with surface materials. It is also emitted directly by certain building materials and present in background levels in outdoor air. Considering its various sources and concerns about health effects due to acute and chronic exposure, improving source apportionment of formaldehyde is a critical research need.
OZONE CHEMISTRY IN THE GAS PHASE: in controlled chamber studies, we evaluated the relative reactivity of VOC mixtures emitted by cleaning products and air fresheners. Figures 2 and 3 show representative examples of results obtained in our experiments. Terpenes and terpenoids are key fragrance constituents in the formulation of household products. The three terpenes shown in Figure 2 were among the most reactive constituents in the presence of ozone.
Figure 2: Ozone reaction of three reactive terpenes present in emissions from a pine oil-based cleaner. The ozone concentrations indicated are inlet levels. Chamber steady state levels are approximately an order of magnitude lower. Chamber experiments were carried out at air exchanges rates of 3 h-1 or 1 h-1.
In these experiments, we determined the yield of formation of various volatile carbonyls and carboxylic acids as stable oxidation products. The yields are calculated on a molar base with respect to ozone reacted, hence the values higher than 100% indicate that OH radical formed during these reactions also participate in subsequent steps yielding additional oxidation products.
Figure 3: Yields of the main volatile oxidation products.
We also investigated the formation of ultrafine aerosol particles, observing nucleation of a large amount of ultrafine (>100 nm) aerosol particles upon initial mixing of ozone with reacting VOCs.
Figure 4: Processes involved in SOA formation.
OZONE SURFACE CHEMISTRY: In several projects, we are currently studying ozone surface chemistry with semivolatile organic chemicals (SVOCs) sorbed to indoor surfaces and reactions with building materials (such as ventilation filters). Specific details on a study carried out on sorbed nicotine are given below.
Indoor fate and transport of secondhand tobacco smoke
Nicotine, the principal alkaloid in tobacco, is emitted in the sidestream and exhaled mainstream smoke of cigarettes. Owing to its specificity, nicotine is the most commonly used tracer for secondhand tobacco smoke (SHS), and its metabolite cotinine is the standard biomarker for SHS exposure in epidemiological and intervention studies. However, it is also known that the dynamic behavior of nicotine differs from that of SHS particles and nonsorbing gases. Nicotine sorbs rapidly and extensively (>95% sorbed within 2 h) to indoor materials, greatly reducing concentrations immediately following smoking and creating the potential
for exposure after subsequent desorption. Sorption also
limits the spread of nicotine as SHS mixes throughout
residences and offices. With repeated smoking,
nicotine accumulates on materials and rates of mass desorption
increase to yield higher daily “background” concentrations. Nicotine loading has been measured at tens of micrograms per square meter on hard surfaces, and estimated at tens of mg per square meter on carpet in homes with unrestricted smoking.
Substantial levels of airborne and surface nicotine were
measured in the homes of smokers who had previously ceased
smoking indoors.
In addition to long-term, low-level re-emission of nicotine and other semivolatile SHS components from contaminated surfaces, the presence of significant nicotine deposits on indoor materials becomes a potential source of secondary pollutants from its reaction with atmospheric oxidants and other reactive species.
Figure 1: Indirect exposure pathways to aged tobacco smoke constituents.
In controlled chamber experiments, we deposited nicotine on the surface of various model materials (Teflon, cotton, wallboard), and recorded the desorption profile as a function of time in the presence and absence of ozone. Relatively low ozone levels (10-40 ppb) were able to decrease nicotine chamber concentrations with respect to values measured in the absence of ozone by up to two orders of magnitude after a week of continuous exposure. At the same time, we observed the formation and re-emission to the gas phase of various oxidation products. These byproducts were present at levels comparable to residual nicotine. Figure 2 shows some of the identified nicotine oxidation products and their concentration profile. These results suggest that under realistic (oxidative) conditions, the lifetime of SVOCs adsorbed to indoor materials is likely much shorter than what can be estimated on the basis of simple partitioning. In addition, surfaces loaded with SVOCs become a relatively constant source of oxidation products. These processes can be mediated by the nature of the surface and by the presence of co-adsorbed water molecules (at moderate or high relative humidity).
Figure 2: Nicotine oxidation products detected in the gas phase. The figures indicate chamber concentration of nicotine (black), cotinine (green), N-methyl formamide (red) and nicotinaldehyde (blue) as a function of reaction time.