Understanding the Indoor Concentrations of Outdoor Aerosols in Residences
In 1997, the U.S. Environmental Protection Agency (EPA) issued new National Ambient Air Quality Standards for airborne particles less than 2.5 mm in diameter (PM2.5). These standards are based on evidence of associations between outdoor PM2.5 concentrations and adverse health effects. However, the relationship between indoor and outdoor concentrations of these fine particles is poorly understood and has important implications for public health.
Although the standards involve outdoor concentrations, people spend, on average, about 90 percent of their time indoors—70 percent of that in homes. The extent to which indoor concentrations of airborne particles track outdoor concentrations over time is not well understood. Perhaps more importantly, even less is known about the behavior of important chemical species that make up the particulate matter. It is suspected that some chemical species are more harmful than others. If the type and condition of residences affect indoor concentrations, then relevant regulations need to be reevaluated in light of scientific research focused on indoor exposures.
With support from the U.S. Department of Energy through the National Petroleum Technology Office and the Western States Petroleum Association, a team of Environmental Energy Technologies Division (EETD) scientists, led by Tracy Thatcher and Melissa Lunden, has been investigating indoor particulate matter that originates outdoors. The objective of the study is to characterize the fate and transport of outdoor PM2.5 in the indoor environment. The team conducted a series of intensive field experiments in an unoccupied, single-story residence in Clovis, California, a suburb of Fresno, in the San Joaquin Valley (Figure 1). Figure 2 shows a schematic representation of the numerous physical processes that affect the transport and fate of outdoor aerosols that travel indoors.
Field Study Methods
The field study collected time and chemical data on both indoor and outdoor concentrations of PM2.5, while ventilation, heating and cooling conditions were manipulated in the house. Measurements were made in October and December of 2000 and January of 2001. The house was unoccupied during these measurements to remove any confounding effects of indoor sources.
The research house was outfitted with a number of instruments to characterize particle size and chemistry simultaneously both indoors and outdoors, as well as meteorological variables including temperature and relative humidity. In addition, the house was instrumented to continuously measure ventilation rate. A new instrument, developed in part for this study, was key to characterizing the time-resolved behavior of important chemical species. Developed by Aerosol Dynamics Inc. (Berkeley CA), the instrument is an integrated collection and vaporization cell (ICVC) that enables measurement of concentrations of ammonium sulfate, ammonium nitrate, and carbonaceous aerosols with 10-minute time resolution.
Variability in Indoor Aerosol Concentrations
Figure 3 shows results from the ICVC system, which show the variation in indoor and outdoor aerosol concentrations for a four-day period during the December measurement effort. The figure also shows the ventilation rate, indicated as air changes per hour (ACH). The results show that there is considerable variability in both the indoor and outdoor concentrations of all three species as well as in the magnitude of the difference between the indoor and outdoor concentrations.
In general, during periods of increased ventilation rate, the difference between the indoor and outdoor concentrations decreased. The most striking feature of Figure 3 is that the individual chemical constituents of PM2.5 behave differently after entering into the residence. The difference between indoor and outdoor ammonium nitrate concentrations is much greater than the differences measured for sulfate or carbon. Ammonium nitrate is a chemically active species that exists in equilibrium with gaseous nitric acid and ammonia. Upon entering the residence, the ammonium nitrate dissociated into its gas phase precursors, which were subsequently lost to the house surfaces by diffusion.
The differences in behavior between individual PM2.5 chemical species and the dissociation of the ammonium nitrate aerosol illustrate that an exposure assessment based on total particle mass measured outdoors may not accurately represent actual human exposures to indoor particles of outdoor origin and may obscure the causal relationships involved. Ammonium nitrate is a significant outdoor pollutant in the Western United States. The extent to which it may or may not be a significant source of indoor exposure has important policy implications for control of sources that lead to ammonium nitrate formation. These results emphasize the need for chemical characterization of PM2.5, and further studies of the physical and chemical transformation processes influencing the indoor concentration of particles that originate outdoors.
For more information, contact:
- Melissa Lunden
- (510) 486-4891; fax (510) 486-5928
Nancy Brown served as principal investigator for this research, and Rich Sextro along with Susanne Hering (of Aerosol Dynamics Inc. of Berkeley CA) were co-principal investigators. Other EETD scientists who contributed to the project are Marc Fischer, David Littlejohn, Lara Gundel, Thomas Kirchstetter, and Ray Dod.
This research was funded by the Department of Energy through the National Petroleum Technology Office and the Western States Petroleum Association.