Ozone Initiated Secondary Emissions of Aldehydes
from Indoor Surfaces
Paper # 1232
Hong Wang, Meredith Springs, Glenn Morrison
University of Missouri-Rolla, 1870 Miner Circle, Rolla, MO 65409
ABSTRACT
Secondary emissions of oxidized organic compounds, primarily aldehydes, from building
surfaces in the presence of ozone may contribute significantly to indoor air pollution.
Ozone-induced formation of aldehydes was studied to determine how daily use consumer
products contribute to secondary emissions. A field ready method was developed to
quantify secondary emission rates on existing surfaces. Four different products, canola
cooking oil, a spray cooking oil, bath soap, and a liquid detergent, were evaluated after
being coated on a laminated countertop. Upon exposure to 100 ppb O3, nonanal and
hexanal were emitted at rates similar to secondary emission rates from carpet. Total
aldehyde emission rates from canola oil, soap, and spray cooking oil were respectively
180, 110, 125 g m-2 h-1. The liquid detergent released small amounts of hexanal, octanal,
and nonanal. The aldehyde emission pattern from these products roughly correspond to
the anticipated relative aldehyde emission rates due to the proportion of unsaturated fatty
acids in vegetable oils and animal fats. If a substantial area of indoor surfaces are coated
with these products, secondary emission rates of aldehydes (especially nonanal) may
result in concentrations approaching or exceeding odor thresholds.
INTRODUCTION
The emissions of organic compounds in buildings are of great concern due to their
potential to cause sick-building syndrome, cancer, irritation, and unpleasant odors. Past
research has focused on primary emissions of organic compounds; however, researchers
are beginning to also look at secondary emissions. Secondary emissions of volatile
organic compounds (VOCs) are defined as emissions resulting from transformations
taking place indoors, significantly increasing the concentrations of odorous and irritating
compounds such as aldehydes. This paper discusses our research that evaluates the
possibility that “daily-use” consumer products, such as soaps and cooking oils, can react
with ozone on surfaces to produce secondary emissions (aldehydes).
Weschler et al.1 was the first to show that ozone reacted with carpet to form
formaldehyde, acetaldehyde, and aldehydes containing between five and ten carbons.
They proposed that the source of the homologous series of C5-C10 aldehydes was the
reaction between ozone and condensed-phase material associated with carpets. Other
indoor sources, such as paint, were also studied, and results showed organic acids and
carbonyl groups (aldehydes and ketones) were formed as a result of latex paint exposure
to ozone2. Morrison et al.3,4 showed secondary aldehydes, especially nonanal, were the
result of reactive chemistry between ozone and carpet surfaces. They demonstrated
particular aldehydes may emit at rates high enough to exceed the odor threshold for long
periods of time. They found evidence that the carpet was coated with unsaturated fatty
acids, or nonvolatile polymers with some degree of unsaturation.
Morrison et al.4 anticipated these surface precursors compounds would eventually
become depleted as ozone oxidized the limited number of unsaturated bonds in the
coating.To our knowledge, researchers have not evaluated the potential for daily use
consumer products to produce aldehydes via reactions with ozone. Product such as soaps
and oils are composed of unsaturated fatty acids or esters. During normal consumer use
of these products, indoor surfaces may become covered with chemically reactive species
that will emit aldehydes and carboxylic acids in the presence of ozone. For example,
cooking oils evaporate somewhat during cooking and condense on nearby surfaces. Soaps
precipitate in hard water and may remain on surfaces even after rinsing. Consumer
products can generate aldehydes on surfaces that do not otherwise react with ozone (e.g.
laminated kitchen countertops) or they may regenerate the potential for secondary
emissions from depleted surfaces (e.g. carpet).
Since we are most interested in identifying how consumer use of products can enhance
the ability of a surface to chemically produce oxidized volatile products, we recognized
the importance of being able to evaluate secondary emission rates on real, in-use, indoor
surfaces. The field and laboratory emission cell (FLEC) has been used widely for
measuring the primary emission rates of VOCs from existing surfaces. However, it was
unclear if this device was suitable for measuring secondary emission rates which
incorporate deposition and emission in one chamber. To our knowledge, no field ready
method has been developed to measure secondary emission rates from existing indoor
surfaces. Therefore, the objectives of this paper are (1) to develop methods to measure
secondary emission rates of aldehydes in residential field experiments; and (2) to evaluate
the secondary emission rates from consumer products that may coat a kitchen countertop
during normal use.
MATERIALS AND METHODS
Consumer Products
Four commonly used products were tested in this study: cooking canola oil, spray
cooking oil (Pam®), bath soap (Ivory®), and a liquid dish detergent (Palmolive®), These
products contain long, unsaturated, fatty acids from either animals or plant sources. In
Table 1 are shown the anticipated fatty acid content of canola oil and animal fats.
Coating Methods
A laminated countertop was used as the test surface and coated with various consumer
products. First, the countertop was cleaned with methanol and rinsed with distilled water.
Then, the countertop was coated with the products in the same manner as they are used in
homes. The coating methods are as follows.
(1) Canola oil. Before being coated, half of the countertop was covered with aluminum
foil and half remained uncovered. This technique represented an unexposed and exposed
surface, respectively. The countertop was placed near a stove where 5g of canola oil was
cooked with fresh tomatoes and eggs. In the process of cooking, vaporized oil deposited
on the exposed side of the countertop. No visible droplets of oil or obvious splatters were
observed on the panel.
(2) Soap. A bar of bath soap was wetted and rubbed uniformly on the counter top. The
soap lather was rinsed using tap water.
(3) Spray cooking oil. The cooking oil was sprayed on a piece of paper held
perpendicular to the countertop surface (as a consumer might spray a pan before cooking),
allowing excess spray droplets to settle naturally on the countertop surface.
(4) Detergent. The detergent was used to clean the countertop. For one experiment, the
detergent cleaned top was rinsed with tap water later. For the other experiment, the top
was not rinsed with water after cleaning.
Table 1. Chemical composition of oil and soap
Fat type
Canola oil (rapeseed oil)
(cooking oil and spray
cooking oil)
Animal fat or tallow
(soap and detergent)
Fatty acid composition 5
Palmitic
2.3%
Stearic
2.3%
Acachidic
1.1%
Behenic
1.2%
Lignoceric
0.7%
Hexadecenoic
1.6%
Oleic
15.0%
Eicosenoic
6.2%
Erucic
46.5%
Linoleic
13.8%
Docosadienoic
1.0%
Linolenic
8.5%
Lauric
0.2%
Myristic
4.4%
Palmitic
29.8%
Stearic
22.5%
Arachidic
1.0%
Tetradecenoic
0.4%
Hexadecenoic
1.7%
Oleic
39.8%
Octadecadienoic
1.8%
C20-C22 unsaturated 0.4%
Notes
Average fatty acid
percentages for rapeseeds
from various countries.
Average fatty acid
percentages for beef from
various countries.
Secondary Emission Rate Experiments
Secondary emissions of aldehydes formed by reactions between consumer products and
ozone were measured using the apparatus shown in Figure 1. The system consisted of an
ozone generator, humidifier, field emission chamber, sampling system, and an UV
photometric ozone analyzer. The 4.25-L Teflon-coated field emission chamber is open
on the bottom so that it can be positioned on the surface, isolate a defined area of the
surface, collect emissions and deliver reactants (e.g. ozone) to that surface. Compressed
air was introduced into the system and split between three streams. The air stream was
passed through a humidifier (two gas sparging bottles, filled with distilled water, in series)
before being mixed with an additional stream to produce air with a relative humidity of
50±5%. The third air stream passed through an ozone generator and then mixed with the
humidified air. The chamber was continuously ventilated at 2.0±0.01 L min-1 of either
humidified air or humidified air containing ozone. A three-way valve located in front of
the chamber was used to measure ozone concentrations at the chamber inlet. Stainless
steel sampling tubes packed with Tenax-TA tubes were attached to the chamber exhaust
to collect samples. A Tenax-TA tube sampling airflow rate of 0.1 L min-1 was controlled
by the use of a sampling pump and flow splitter. The sampling air flow rate was
continuously monitored by a mass flow meter. Sample volume was 2.0 L. VOC
emissions were measured before and after the system was exposed to ozone. Inlet and
outlet concentration measurements were used to determine the ozone deposition on the
surface.
Prior to the experiment, the laminated countertop was either cleaned using soap/
detergent or exposed to cooking oil (see enumerated list). Then the test chamber was
placed on top of the panel, with a narrow ring of foam ensuring a tight seal between the
chamber and surface. Ozone-free humidified air was introduced into the system for one
hour. Two samples, representing emissions from unexposed products, were collected at
the exhaust point. The system was then exposed to an ozone concentration of
approximately 100 ppb for one hour with subsequent samples collected. A final sample
was obtained at the chamber inlet to represent a system blank.
System Leak Test
Surface emission chambers that are intended to seal onto the surface itself (instead of
completely enclosing a surface sample) may leak around the point of contact between the
chamber and the surface, especially if the surface is not smooth. The apparatus shown in
Figure 2 was designed to test the total leakage rate for the chamber. The leak test
conditions were the same as those for the secondary emission rate measurements, except
that a sulfurhexafluoride monitor (INOVA 1302) was positioned at the sampling stream.
Sulfur hexafluoride was used as tracer gas, and a known mass was injected into the inlet
point A, as shown in Figure 2. Assuming the system acts as a completely-mixed flow
reactor model, the gas exchange rate was determined from the rate of decrease of the
concentration of sulfurhexafluoride during a 0.5 h period. The percent leakage was
determined by comparing the % difference between the estimated and measured gas
exchange rates.
Figure 1. Experimental apparatus for measuring ozone-induced secondary emission rates
from in-use surfaces.
humidifier
4.25L teflon
coated chamber
counter-top
mass-flow
controller
tenax
tube
ozone
generator
UV Absorption
ozone analyzer
mass-flow flow
meter
splitter
sampling
pump
Air
mass-flow
meter
vacuum pump
Figure 2. System leak test. Sulfurhexafluoride was injected into the inlet branch at
position A and the concentration decay was measured at the exhaust to determine the
actual gas exchange rate.
humidifier
A
4.25L teflon
coated chamber
counter-top
mass-flow
controller
ozone
generator
multi-gas monitor
Air
mass-flow
meter
vacuum pump
Target Analyte Measurement
Analytes collected on Tenax-TA sample tubes were analyzed using a thermal
desorber/gas chromatograph/ flame-ionization detector (TD/GC/FID) system with an
autosampler. Tubes were desorbed at 140°C for 10 minutes. The TD cold-trap was then
desorbed at 150°C before the gas was carried into the GC for analysis. The GC inlet
pressure was 14.7 psi with a total gas flow rate of 7.3 mL min-1, operated in split-less and
constant pressure modes. The oven temperature was initially set at 50°C and held there
before increasing at a rate of 30°C min-1 up to a maximum temperature of 250°C. The
FID was set at 250°C with gas flows as follows: hydrogen, 40 mL min-1; compressed air,
450 mL min-1, and nitrogen, 45 mL min-1.
Analytes diluted in methanol were used as standards for system calibration of aldehydes.
By a double dilution method, a methanol solution composed of 3 mg L-1 of each aldehyde
was created. For example, 50±5 µL samples of each pure aldehyde liquid were injected
into a 25 mL volumetric flask containing 25 mL of acetone-free methanol. The sample
was sonicated for ten seconds to ensure complete mixing. A 50±5 µL sample of this
mixture was injected into a second volumetric flask containing 25 mL of acetone-free
methanol and further sonicated. From this final dilution, samples representing
approximately 50 and 100 ng (16±2 and 32±4 µL) of the aldehyde solution were injected
onto the inlet packing of a clean Tenax TA tube and analyzed using the same TD/GC/FID
method described above for the air samples. Using the data obtained from the GC,
standard calibration curves were generated based on plots showing the peak area versus
the mass. Calibrations curves for pentanal, hexanal, heptanal, octanal, nonanal, decanal,
2-nonenal, and 2,4-nonadienal were generated.
RESULTS AND DISCUSSIONS
Methods Development
The method used in this study was developed to test for secondary emissions from in-use
surfaces in residential field experiments. We found that the method consistently provided
reproducible secondary emission rate measurements. For example, the average emission
rates standard deviation for duplicate measurements was about 13% for canola oil, soap,
and Pam®. Leak tests are vital to ensuring reliability of secondary emission rate
measurements. For these experiments, a leakage fraction of <0.1 was considered
acceptable. In the future, positive and negative control experiments will be developed to
improve our confidence in the method results.
Secondary Emissions
Several consumer products tested in this research generated secondary carbonyl
emissions in the presence of ozone. Results of these experiments are reported in terms of
the analyte mass emission rate per exposed surface area (µg m-2 h-1). In Figure 3 are
shown the ozone induced secondary aldehyde emission rates. Uncertainty bars represent
error propogation of the standard error of calibrations and replicate experiments for each
product. Secondary mass emission rates were obtained by subtracting the emission rate
of unexposed products from the emission rate of exposed products.
While most of the target aldehydes were observed to be emitted from each consumer
product, nonanal was the most prominent secondary aldehyde. Nonanal was emitted in
significant amounts from the cooking vapor deposited canola oil, soap, and spray canola
oil. Total secondary emission rates of aldehydes, of the order of 150 µg m-2 h-1, are
comparable to or higher than ozone-induced secondary emission rates from carpet4. The
secondary emission rates from the detergent, with and without a tap water rinse, were
small, but the relative emission rates of nonanal, hexanal and octanal were remarkably
similar to one another. This suggests that a film of detergent remains on the surface even
after rinsing. While detergents should rinse somewhat thoroughly, rinsing soaps with
hard water is likely to leave a substantial film of unsaturated soap precipitate that can act
as a source of oxidized products.
When the countertop was exposed to cooking vapor deposited canola oil, nonanal was the
primary compound produced. A small amount of hexanal was also emitted. Nonanal and
hexanal were also observed for the spray canola oil; however, the spray released more
hexanal than the cooking vapor deposited canola oil. As listed Table 1, canola oil consists
predominantly of 18-carbon, unsaturated, fatty chains such as oleic acid, linoleic acid,
and linolenic acid and eurcic acid, a 22-carbon unsaturated fatty acid. Ozone will attack
the double-bonds in these esters, forming aldehydes and carboxylic acids at the point of
attack. For example, nonanal and 3-nonenal will be formed when ozone attacks the ninth
carbon of the esters of erucic (nonanal), oleic (nonanal), and linoleic acids (3-nonenal).
Similarly, 3, 6-nonadienal is the product of the ozonation of the double-bond located at
the ninth carbon of linolenic acid. Hexanal, 3-hexenal, and propanal are also released
from the reaction of canola oil and ozone. Canola oil may have been transformed by
cooking or by processing so that oleic acid dominates. As a result, aldehydes released
from canola oil were not consistent with reported proportions of oleic and linoleic acid.
However, the spray canola oil appeared more consistent than the vapor deposited from
cooking canola oil.
The emission patterns of secondary aldehydes (i.e. relative emission rates), from soap
were similar to those observed during carpet experiments4. Soaps are made by rendering
animal tallow with a strong alkali to produce saturated fatty acids, such as stearic acid, as
well as unsaturated fatty acids, such as oleic acid. The secondary emissions of aldehydes
from soap were expected to be similar to those from oils given the relative composition
of tallow; however, a different pattern was observed. Pattern variations may have been
caused by a shift in the unsaturated bond of the esters during processing with a strong
base. For example, shifting the double-bond one position away from the acid
functionality would result in octanal upon oxidation. Two shifts results in heptanal and so
forth. The declining relative emission rates from nonanal down to pentanal suggests this
may be the case. However, we did not observe decanal, the secondary product generated
if the double-bond shifted one position towards the carboxylic acid functional group.
Small relative amounts of secondary aldehydes were observed for the detergent. This
suggested the chemical constituents of detergent do not have many unsaturated double
bonds. Small amounts of oleic acid may have been present during the sulfonation process,
resulting in small amounts of sulfonated, unsaturated precursors being produced. Figure 3
shows that hexanal, octanal, and nonanal were consistently emitted at low secondary
emission rate from the detergent.
For the clean panel, a negative emission rate of pentanal was observed. We believe this
was due to a contaminated sampling tube which produced an artificially high baseline.
The emissions of other aldehydes, such as heptanal, nonanal and decanal were probably
caused by a modest primary emission from panel.
Figure 3. Secondary aldehyde emission rates for each consumer product.
Aldehyde Formation Factors
The ozone-induced aldehyde “formation factor” is an indicator of secondary carbonyl
emissions based on the assumption that secondary-product emission rates are
proportional to ozone deposition rates. The factor is defined as the ratio of the ozoneinduced molar emission rate of carbonyl species to the molar loss rate of ozone to that
surface. Shown in Figure 4 are emission formation factors for target aldehydes composed
of five to ten carbons. Typically, the nonanal formation factor for oil and soap is ~0.2,
indicating 20% of the uptake ozone produced nonanal via ozone-product reactions. The
total of all detected aldehydes should theoretically equal 0.5 since the ozone-product
reaction will produce aldehydes 50% of the time and carboxylic acids the other 50% of
the time. The summed formation for all target aldehydes ranged between 0.2 and 0.4, less
than the expected 0.5. This may be due to the fact that we were unable to positively
identify or quantify some aldehydes (such as 3-nonenal), or any short-chain aldehydes.
Negative formation factors and large uncertainties observed for one detergent experiment
was due to very low total ozone uptake for that experiment.
Figure 4: Ozone-induced aldehydes formation factor of each consumer product. The
category “detergent1” represents the countertop that has been cleaned with detergent but
rinsed with tap water. The category “detergent2” represents detergent that was not rinsed
off.
The influence on indoor air quality of secondary aldehyde emissions
from consumer products
Using our observed formation factors, we can estimate the indoor concentrations of target
aldehydes that are emitted as a result of ozone oxidation of unsaturated fatty acids. We
consider nonanal emitted from the laminated countertop (exposed to cooking vapor
deposited canola oil) as an example. The emission rate of an aldehyde can be estimated
by combining the aldehyde formation factor with the anticipated ozone flux to the
countertop surface. The indoor ozone concentration is estimated based on a steady-state
material balance model6 of a well-mixed residence. Given typical indoor conditions: (a)
outdoor ozone concentration, 25ppb7; (b) average deposition velocity, 1.4m h-1 8; (c) air
exchange rate, 0.5h-1 9; (d) estimated area-to-volume ratio, 3m-1,the calculated timeaveraged indoor ozone concentration is 2.7ppb. A carpet-specific deposition velocity of
1m h-1 for ozone4 is combined with indoor ozone concentration to estimate the ozone flux.
The secondary emission rate of the aldehyde is obtained by multiplying ozone flux by our
observed nonanal formation fraction, 0.2. We assume that nonanal was not present in
outdoor air and was produced only by the canola oil-ozone reaction. For nonanal, the
estimated emission rate was 4.5 µg m-2 h-1. Assuming that the countertop exposed area to
kitchen volume ratio was 0.1 m-1, the resulting indoor nonanal concentration was 0.9 µg
m-3. This concentration is lower than the nonanal odor threshold of 13 µg m-3. Since oils
present on the floor, walls, and ceiling from cooking were not take into account, this may
be a substantial underestimate of the impact of these ozone induced emissions. Since
other products such as 3-nonenal and 3,6-nonadienal (not quantified in this research)
have much lower odor thresholds, consumer usage of these products may indeed degrade
indoor air quality due to ozone reactions.
CONCLUSIONS
Our experimental results demonstrate that in-use consumer products react with ozone to
produce aldehydes. The secondary emission rates and formation factors are comparable
to those observed for carpet, which were estimated to significantly degrade indoor air
quality4. While “fresh” carpet may still be the major source of these aldehydes indoors,
time softens the impact as ozone eventually depletes the precursor unsaturations on the
surface. However, daily use of cooking oils, soaps and other products may regenerate
these surfaces, continuously supplying precursor unsaturated fatty acids. Since many
personal hygiene products also contain unsaturated fatty acids (we note that the major
ingredient in a popular facial cleanser in the United States is “C14-C16 olefin sulfonate”),
these reactions are also likely to take place on the skin immediately adjacent to the
breathing zone, further degrading the “personal cloud”. Even natural skin oils themselves
could contribute to secondary emissions of oxidized volatile organic compounds.
It is presently unclear the extent to which consumer products contribute to the secondary
emission of aldehydes in indoor air. One objective of our laboratory research on
consumer product reactions with ozone is to simulate residential field studies. Our next
step is to take a validated field emission chamber to residences to evaluate in-use surfaces
for secondary emission rates. In addition, we hope to develop methods to measure other
products such as carboxylic acids which may have a greater impact on odor and or
irritation potential of indoor air.
ACKNOWLEDGEMENTS
This research is funded by the National Science Foundation through a CAREER award
fellowship. We thank Maneerat Ongwandee for her assistance with the experiments.
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