Continuous measurement of the dynamics of residential indoor and outdoor NO and the contributions to human exposure.

In residential environment, NO is an important air pollutant. Yet, the dynamics of indoor NO and source contributions to human exposure are not well understood. Here, we conducted a continuous NO measurement in and out of eight households in Guangzhou, China. Paired high time-resolution NO data sets indoors (kitchen, living room) and outdoors (balcony) were obtained with NO monitors. We summarized the indoor and outdoor NO levels, identified temporal variation patterns, analyzed indoor-outdoor relationships, and quantified source contributions to indoor NO exposure. Indoor NO were overall higher than outdoor NO, and in most cases, the highest NO levels were observed in the kitchen. NO in the kitchen was characterized by multiple spikes associated with use of gas stoves, while NO in the living room was also elevated but the peaks were generally smaller. The indoor-outdoor correlations were stronger in winter than in summer, and were stronger in nighttime than daytime. The sources contributing to indoor NO were separated with a conceptual model. Overall, the outdoor NO source contributed 73%-76% of the NO in the kitchen, and 76%-85% in the living room. The source pattern was quite different: outdoor NO sources were present indoors all the time; by contrast, indoor NO sources were present sporadically but with a very high contribution. This has important implication to the exposure assessment that indoor NO sources lead to short-term high exposure, and deserves attention regarding acute health effects.