Exploration and Production Technologies
A Predictive Model of Indoor Concentration of Outdoor PM2.5 in Homes


This is a Natural Gas and Oil Technology Partnership project initiated in FY1998. The purpose is to determine impacts of particulate matter (PM) on indoor air quality.

Project Goal
The San Joaquin Valley has some of the country's highest PM2.5 (PM less than 2.5 microns) concentrations. A large percentage is NH3NO3. This study utilizes species- and size-specific residential PM2.5 measurements as input into a model. The validated, semi-mechanistic model is general enough to predict probability distributions for species-specific indoor PM2.5 concentrations based on outdoor PM, gas-phase species and concentrations, weather conditions, building characteristics, and heating, ventilation, and air conditioning. This model is a component in understanding human exposure to airborne particulate matter.

Lawrence Berkeley National Laboratory (LBNL)
Berkeley, CA

Project Results
Analyses show relationships between indoor and outdoor particle chemistries. Chemical and time-resolved data are necessary to understand the transport and fate of indoor particles of outdoor origin. Results suggest that significant differences exist among chemical constituents. Exposure assessments based on total outdoor particle mass measurements may obscure causal relationships for indoor exposure of outdoor origin. For example, results indicate indoor exposure to ammonium nitrate (NH3NO3) is much smaller than outdoor concentrations suggest. These results could impact future PM2.5 standards.

Because ambient-air PM2.5 standards are based on health risk, population exposure to PM2.5 is an important issue. Individuals spend about 90% of their time indoors (70% in homes). To date, standards have focused on outdoor suspended particle mass because there is no scientific evidence to implicate any particular mass component(s). Exposure evaluation is a critical element for apportioning particulate characteristics to health risks.

This new predictive model is a tool for relating indoor PM2.5 concentrations of outdoor origin to measurements at central monitoring stations. It is based on mass balance principles, where the residential building is treated as a single, well-mixed zone. By bringing sound science to regional air quality regulation, such research helps refiners avoid needlessly stringent emissions standards for their operations and products.

San Francisco Bay area refineries are subjected to strict NOx rules because they are believed to be contributors to San Joaquin Valley particle loading. This research provides sound science for possible future regulatory frameworks. Exposures must be quantified to eliminate arbitrary control decisions that could be ineffective. Project findings suggest a methodology to minimize conflict between ozone and PM control strategies.

The health research community is also faced with understanding causes, if any, of adverse health effects resulting from exposure to ambient PM2.5. Current correlations between adverse health effects and PM concentration are based upon short-term increases in outdoor particle concentrations, not on indoor exposure. An essential component to determine PM2.5 exposure is to establish the fate and transport of outdoor particles crossing the building shell and becoming resident indoors. Understanding processes affecting indoor concentrations of outdoor PM is required. Although indoor sources and resuspended particles also may have an influence on human health, it is unlikely that particles of indoor origin would track outdoor particle concentration changes.

Project Summary
During fall 2000 and winter 2001, experiments were conducted at an unoccupied suburban Clovis, CA, research house located in the San Joaquin Valley. This is the first such experiment to characterize time- and chemically resolved PM2.5 levels.

Researchers sought to determine that:

  • Data analysis and model building showed that indoor/outdoor concentration relationships depend on particle chemistry.
  • Chemical and time-resolved data that were necessary to understand transport and fate of indoor particles of outdoor origin.
  • A mechanistic understanding could be achieved for individual chemical particle species transformations of physical loss of PM indoors and for infiltration behavior.
  • Results could be extended to other locations and housing types using the LBNL infiltration model.

Results showed that concentrations of atmospheric aerosols, nitrate sulfate, and carbonaceous matter are variable both within a single day and between days. Aerosol constituents behave differently upon entrance into a building. On average, indoor sulfate aerosol concentrations are about half of those outdoors. However, when the house air exchange rate is elevated, indoor sulfate aerosol levels can approach those outdoors. Measured indoor sulfate aerosol concentrations can be predicted using a mass-balance model and knowledge of penetration loss through the building shell, deposition loss rate within the building, and air exchange rate.

Results indicate that indoor exposure to NH3NO3 in the San Joaquin Valley is small. In contrast to sulfate aerosol, chemically resolved data revealed that indoor NH3NO3 concentrations are much less than predicted. Additional reductions were attributed to indoor transformation of NH3NO3 into NH3 and nitric acid gasses, which are subsequently lost by deposition and surface sorption.

Current Status (August 2005)

The first-generation model is nearing completion. Recent analysis of time-resolved organic carbon data shows that, even at high air exchange rates, indoor concentrations are lower than outdoor values. Differences between predicted and measured concentrations correlate with temperature and sulfate air change rates. This is not true for NH3NO3. Results suggest that gas-to-particle partitioning of organic gases is an important factor controlling indoor aerosol concentrations.

Lunden, M.M., Revzan, K.L., Fischer, M.L., Thatcher, T.L., Littlejohn, D., Hering, S.V., and Brown, N.J., 2003, The Transformation of Outdoor Ammonium Nitrate Aerosol in the Indoor Environment, invited paper, Atmospheric Environment, 37, 5633-5644 (LBNL Report No. 52795).

Lunden, M.M., Thatcher, T.L., Hering, S.V., and Brown, N.J., 2003, The Use of Time- and Chemically Resolved Particulate Data to Characterize the Infiltration of Outdoor PM2.5 into a Residence in the San Joaquin Valley, Environmental Science and Technology, 37, 4724-4732 (LBNL Report No. 52221).

Thatcher, T.L., Lunden, M.M., Revzan, K.L., Sextro, R.G., and Brown, N.J., 2003, A Concentration Rebound Method For Measuring Particle Penetration And Deposition In The Indoor Environment, Aerosol Science and Technology, 37, 847-864 (LBNL Report No. 51631).

Fischer, M.L., Littlejohn, D., Lunden, M.M., and Brown, N.J., 2003, Automated Measurement of Ammonia and Nitric Acid in Indoor and Outdoor Air, Environmental Science and Technology, 37, 2114-2119 (LBNL Report No. 51385).

Project Start: January 15, 1999
Project End: September 30, 2005

DOE Contribution: $2,291,000
Performer Contribution: $290,000 (11.2% of total)
$75,000 from American Petroleum Institute in 2005

Contact Information
NETL - Betty Felber (betty.felber@netl.doe.gov or 918-699-2031)
LBNL - Nancy J. Brown (njbrown@lbl.gov or 510-486-4241)

A schematic of indoor/outdoor particle chemistry.


Time- and chemically resolved indoor/outdoor particle data.

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