Assessing the Influence of Humidity and Surface Reservoirs on Indoor Ammonia Dynamics at the CASA Experiment: A Modeling Study

Marc Webb, Glenn Morrison, Stephen Zimmerman, Michael Link, Dustin Poppendieck, Marina Vance, Delphine K. Farmer, BARBARA TURPIN, University of North Carolina at Chapel Hill

     Abstract Number: 590
     Working Group: Indoor Aerosols

Abstract
Although indoor ammonia (NH₃) is an important contributor to the chemistry of indoor environments, its dynamics in residential environments are understudied. Because NH₃ is a water-soluble base, its behavior indoors is expected to be regulated by surface reservoirs, especially when humid. Surface interactions of NH₃ can alter building chemistry and human exposure to other indoor pollutants (e.g., gaseous acids and bases). As part of the Chemical Assessment of Surfaces and Air (CASA) experiment at the Net-Zero Energy Residential Test Facility (NZERTF), NH₃ was injected into a test house at 75% RH and 45% RH, and the concentration was monitored and modeled. NH₃ levels in room air were higher at 45% RH (avg: 239 ppb) than 75% RH (avg: 171 ppb). Maximum concentrations of NH₃ in house air reached 446 ppb at 75% RH and 695 ppb at 45% RH. Roughly ~80% (at 75% RH) and 70% (at 45% RH) of NH₃ was taken up by house reservoirs which indicated an RH effect associated with surface reservoirs. A mass balance model incorporating reversible surface sinks of RH-dependent volumes was developed to reproduce NH₃ measurements and provide fitted parameters for mass transport and surface reservoir volumes. Simulation of NH₃ dynamics required reversible surface sinks of RH-dependent volumes in ventilation ductwork and building materials. The effective “air” volumes of surface reservoirs increased with humidity and were predicted to be ~60,000 m³ at 75% RH and ~40,000 m³ at 45% RH. We estimate the liquid water content corresponding to the effective air volumes and influence on pH of sorbed water associated with NH₃ uptake.