Volatile Organic Compound Concentrations and Emission Rates Measured over One Year in a New Manufactured House
A study to measure indoor concentrations and emission rates of volatile organic compounds (VOCs), including formaldehyde, was conducted in a new, unoccupied manufactured house installed at the National Institute of Standards and Technology (NIST) campus. The house was instrumented to continuously monitor indoor temperature and relative humidity, heating and air conditioning system operation, and outdoor weather. It also was equipped with an automated tracer gas injection and detection system to estimate air change rates every 2 h. Another automated system measured indoor concentrations of total VOCs with a flame ionization detector every 30 min. Active samples for the analysis of VOCs and aldehydes were collected indoors and outdoors on 12 occasions from August 2002 through September 2003. Individual VOCs were quantified by thermal desorption to a gas chromatograph with a mass spectrometer detector (GC/MS). Formaldehyde and acetaldehyde were quantified by high performance liquid chromatography (HPLC). Weather conditions changed substantially across the twelve active sampling periods. Outdoor temperatures ranged from 7 °C to 36 °C. House air change rates ranged from 0.26 h-1 to 0.60 h-1. Indoor temperature was relatively constant at 20 °C to 24 °C for all but one sampling event. Indoor relative humidity (RH) ranged from 21 % to 70 %. The predominant and persistent indoor VOCs included aldehydes (e.g., formaldehyde, acetaldehyde, pentanal, hexanal and nonanal) and terpene hydrocarbons (e.g., a-pinene, 3-carene and d-limonene), which are characteristic of wood product emissions. Other compounds of interest included phenol, naphthalene, and other aromatic hydrocarbons. VOC concentrations were generally typical of results reported for other new houses. Measurements of total VOCs were used to evaluate short-term changes in indoor VOC concentrations. Most of the VOCs probably derived from indoor sources. However, the wall cavity was an apparent source of acetaldehyde, toluene and xylenes and the belly space was a source of 2-butanone, lower volatility aldehydes and aromatic hydrocarbons. Indoor minus outdoor VOC concentrations varied with time. Adjusted formaldehyde concentrations exhibited the most temporal variability with concentrations ranging from 25 µg m-3 to 128 µg m-3 and the lowest concentrations occurring in winter months when indoor RH was low. A model describing the emissions of formaldehyde from urea-formaldehyde wood products as a function of temperature, RH and concentration reasonably predicted the temporal variation of formaldehyde emissions in the house. Whole-house emissions of other VOCs generally declined over the first three months and then remained relatively constant over a several month period. However, their emissions were generally lowest during the winter months. Also, an apparent association between TVOC emissions and outdoor temperature was observed on a one-week time scale.