The relationship between airborne acidity and ammonia in indoor environments

Indoor acid aerosol, nitric acid (HNO3), and ammonia (NH3) concentrations were measured in 47 homes in State College, Pennsylvania, during the summer of 1991. From each home, 12-hour indoor, 12- and 24-hour outdoor, and 12-hour air exchange samples were collected continuously over a 5-day period. Additionally, questionnaires were administered daily by field technicians to obtain information on house occupant number, ventilation, gas stove use, pets, and other housing characteristics. In this paper, we discuss the relationship between NH3 and corresponding concentrations of aerosol strong acidity (H+) and HNO3 inside these homes. As part of this analysis, we also examined indoor/outdoor concentration relationships and identified housing factors that may influence indoor levels. In State College, indoor NH3 concentrations were higher than corresponding outdoor levels, with air conditioner use, air exchange rates, and occupant number identified as important determinants of indoor levels. Indoor concentrations of both H+ and HNO3 were substantially lower than outdoor levels, as homes with air exchange rates less than one exchange per hour were found to have essentially no acid indoors. These low H+ and HNO3 levels likely resulted from their reaction with indoor NH3 and with indoor surfaces. Indoor NH3 concentrations were higher than outdoor levels, indicating the presence of indoor NH3 sources; however, correlations between indoor NH3 and both pets and occupants, its primary indoor sources, were weak and negative, respectively. Mass balance models that included an NH3 neutralization term were found to predict indoor H+ concentrations reasonably well, representing a substantial improvement over outdoor concentrations alone. The accumulation of NH3 indoors was found to be the primary determinant of indoor H+ and HNO3 levels.