A Predictive Model for Determining Indoor Concentrations of Outdoor Volatile Organic Compound Air Toxics

FEW EE-1682

Program
This project was funded through DOE's Natural Gas and Oil Technology Partnership Program. The program establishes alliances that combine the resources and experience of the nation's petroleum industry with the capabilities of the national laboratories to expedite research, development, and demonstration of advanced technologies for improved natural gas and oil recovery.

Project Goal
The goal of this project is to develop a model to estimate concentrations of indoor concentrations of outdoor volatile organic compounds (VOCs) from outdoor measurements. The approach will be to integrate existing models with information derived from experiments to provide essential data required for environmental risk assessment.

Performer
Lawrence Berkeley National Laboratory (LBNL)
Berkeley, CA

Project Results
A model to estimate indoor VOC concentrations from outdoor VOC concentrations has been developed. Outdoor samples of ambient air were measured to provide the data for modeling.

Benefits
This research will provide a model that can provide a quantitative estimate of indoor inhalation exposure to hazardous air pollutants (HAPs). Putting the HAP standard on a risk basis will significantly restrict the number of compounds regulated and will target concentrations of those deemed to pose significant risk. This is especially critical for refiners faced with ever-tightening fuel emissions standards that may not be based on sound science.

Background
Congress has directed the Environmental Protection Agency to establish and promulgate emission standards for HAPs. This is consequential for refineries and may require refineries to change the composition of fuels produced for mobile and stationary combustion sources and to reduce their fugitive emissions.

When suitable tools are available to assess health risks associated with specific HAPs, EPA plans to base HAP standards on achieving risk reduction rather than applying a mandated reduction level for all HAPs. If risk is to be assessed reliably, exposure must be known.

Key to quantifying risk is determining actual exposure of the population to the air toxics. This requires determining the quantitative relationship between air toxic concentrations measured at stationary outdoor monitoring sites and the actual exposures of individuals to them. One of the key exposure pathways is inhalation. Determining indoor concentrations of outdoor HAPs is particularly crucial for quantifying the inhalation exposure pathway because individuals spend, on average, about 90% of the time indoors (70% in homes).

Project Summary
A model is being developed to estimate indoor VOC concentrations from outdoor VOC concentrations determined from outdoor measurements. The approach is to integrate existing models (e.g., LBNL infiltration model and gas-phase reaction models) with information derived from experiments to provide essential data required for risk assessment.

Ambient air concentrations of VOCs vary temporally and often exhibit a diurnal pattern in response to changes in source terms and atmospheric processes. When these compounds enter buildings from outdoors by infiltration, they interact with indoor surfaces. Thus, when ambient concentrations are high, indoor concentrations may be reduced relative to outdoor values due to sorption. Conversely, re-emission from surfaces can result in higher indoor concentrations after outdoor concentrations have decreased. To capture this, design and coding have been completed for a coupled outdoor/indoor air model.

For the outdoor portion of the model, a Lagrangian photochemical box model is used. The chemical mechanism SAPRC-99 has been implemented into the model because it is the best representation of atmospheric VOC chemistry. A set of diurnally varying emissions representative of an urban area is used as model input. Dilution and entrainment of air aloft due to cell height variations induced by a temporally varying mixing height are included in the model to generate representative VOC concentrations. A state-of-the-art radiation model will be included to calculate actinic flux that will drive the photochemistry.

Outdoor air concentrations of VOCs will be computed for several days for different seasons and for different spatial locations. The project model then has the outdoor air flow into an indoor space of specific volume at a fixed infiltration rate, which is an input parameter. Calculations have revealed that the photochemistry ceases indoors because there is insufficient actinic flux to support photochemistry. However, the VOC reactions continue to occur because they are driven by reactions with ozone. Physical loss via sorption and gain via desorption also are represented in the model, and appropriate rate parameters required to evaluate them is to be evaluated experimentally or taken from the literature.

Current Status (October 2005)
The project is complete, but final results have not been submitted.

Project Start: April 12, 2001
Project End: April 11, 2005

Anticipated DOE Contribution: $435,000
Performer Contribution: $120,000 (22% of total)

Contact Information
NETL - Kathy Stirling (kathy.stirling@netl.doe.gov or 918-699-2044)
LBNL - Nancy Brown (njbrown@lbl.gov or 510-486-4241)