Indoor Pollutant Chemistry
Learning Objectives
Students will be able to:
- Explain how indoor chemistry differs from outdoor atmospheric chemistry
- Describe the role of hydroxyl radicals (OH) and ozone in indoor oxidation
- Analyze surface reactions as a major pathway for indoor pollutant transformation
- Identify sources of reactive oxygen species in indoor environments
- Predict products of indoor oxidation reactions
The Big Question
"What happens chemically when outdoor air pollutants enter an indoor space? Do indoor-specific reactions create new pollutants?"
Indoor vs. Outdoor Chemistry
Indoor environments differ fundamentally from the outdoor atmosphere in ways that alter chemical reaction pathways:
| Parameter | Outdoor | Indoor | Implications |
|---|---|---|---|
| Surface-to-volume ratio | Very low | ~3-6 m-1 | Surface reactions dominate indoors |
| UV light | Full spectrum | Window glass filters UV-B | Reduced photochemistry indoors |
| OH radical concentration | ~106 molecules/cm3 | ~104-105 | Slower gas-phase oxidation |
| Ozone | 30-100 ppb | 5-50 ppb | Still reactive with unsaturated VOCs |
| NOx | Variable (ppb) | Higher near gas stoves | Affects radical chemistry |
The Hydroxyl Radical (OH)
The hydroxyl radical is the primary oxidant in atmospheric chemistry, sometimes called the "atmospheric detergent."
Indoor OH Sources
- Ozone photolysis: O3 + hv → O(1D) + O2; O(1D) + H2O → 2OH
- HONO photolysis: HONO + hv → OH + NO (can occur with visible light)
- Ozonolysis of alkenes: O3 + R=R' → [Criegee intermediate] → OH + products
Indoor OH concentrations are typically 10-100 times lower than outdoors due to reduced UV light, but can be significant in certain conditions (e.g., near windows, with high ozone and terpene levels).
Surface Chemistry
With surface-to-volume ratios 10,000 times higher indoors than outdoors, surface reactions become critically important:
Ozone-Surface Reactions
- Deposition velocity: 0.01-0.1 cm/s
- Reactions with skin oils produce aldehydes
- Carpet and fabric are reactive sinks
- Produces submicron particles
HONO Formation
- 2NO2 + H2O (surface) → HONO + HNO3
- HONO photolyzes to produce OH
- Major indoor OH source
- Enhanced in kitchens with gas stoves
Ozone-Terpene Reactions
One of the most important indoor chemical processes is the reaction between ozone and terpenes (from cleaning products, air fresheners, and natural sources):
Example: Limonene Ozonolysis
C10H16 (limonene) + O3 → various products including:
- Formaldehyde (HCHO) - carcinogen
- Acetone and methyl vinyl ketone
- Hydroxyl radicals (OH)
- Secondary organic aerosol (SOA)
- Ultrafine particles
Rate constant: k = 2.0 x 10-16 cm3/molecule-s at 298 K
Research Highlight: Human Oxidation Field
The "Oxidation Field" Around Humans
Recent research (Wisthaler & Weschler, 2010; Wang et al., 2022) has shown that humans themselves are significant indoor chemical reactors:
- Ozone reacts with squalene and other skin oils
- Produces 4-oxopentanal, 6-methyl-5-hepten-2-one (6-MHO), and other aldehydes
- OH radicals are produced and consumed in an "oxidation field" around each person
- A single person can reduce room ozone by 30-50% and produce measurable secondary pollutants
Reference: Weschler, C.J. (2011). Chemistry in indoor environments: 20 years of research. Indoor Air, 21(3), 205-218.
Activity: Indoor Chemistry Analysis
Scenario Analysis
A classroom has the following conditions: outdoor ozone = 50 ppb, air exchange rate = 1 h-1, indoor surface loss rate for ozone = 2 h-1, and a teacher is using a citrus-scented cleaning product containing 500 ppb limonene.
- Ozone Balance: At steady state, what is the indoor ozone concentration? Use: Cin = Cout * (AER)/(AER + ksurface)
- Reaction Products: If 10% of the limonene-ozone reaction produces formaldehyde, estimate the formaldehyde production rate in ppb/hour.
- Recommendations: What strategies would you recommend to minimize secondary pollutant formation in this classroom?
Key Takeaway
Indoor chemistry is not simply a diluted version of outdoor chemistry. The high surface-to-volume ratio, reduced UV light, and unique pollutant sources create distinct chemical environments. Surface reactions, ozone-terpene chemistry, and even human skin chemistry produce secondary pollutants that may be more harmful than the primary pollutants. Understanding these processes is essential for designing healthy indoor spaces.