Behavior of nitrous acid (HONO) and isocyanic acid (HNCO) in a test house

Abstract:

Nitrous acid (HONO) and isocyanic acid (HNCO) are two small nitrogen containing acids with combustion sources both indoors and outdoors. In addition, HONO is known to form from heterogeneous reaction of nitrogen oxides, arising from either transport from outdoors or indoor combustion. HONO is an important source of hydroxyl radical indoors. While HNCO is toxic, its source and behavior indoors are poorly studied. In this study, the behavior of HONO and HNCO indoors during different activities was examined for better characterization of their sources, sinks, chemistry in the indoor environment during the House Observation of Microbial and Environmental Chemistry (HOMEChem) campaign. HONO and HNCO were measured in-situ with a time of flight chemical ionization mass spectrometer (CIMS) in a test house. HONO was also measured with Laser-Photofragmentation/Laser-Induced Fluorescence which showed a good agreement with CIMS measurements. Overall, both HONO and HNCO concentrations were higher indoors than outdoors, indicating the presence of indoor sources that might contribute to outdoor pollution through air exchange. We observed effects of cooking, bleach cleaning and air conditioning on indoor levels of both acids. Cooking, especially the use of an oven, enhanced indoor HONO and HNCO significantly with effects lasting for a few days. Bleach cleaning led to a substantial decrease of HONO, yet was an important source of HNCO due to their different chemistry. Air conditioning was the dominant modulator of indoor HONO and HNCO levels during unoccupied periods, related to the chemical’s volatility and water solubility. This study thoroughly explored various common indoor activities on indoor levels of two important small acids.