Impact of NO emissions from household heating systems with wall-mounted gas stoves on indoor and ambient air quality in Chinese urban areas.

Nitrogen dioxide (NO) has been discussed as a typical indoor pollutant for decades. As an increasingly popular heating method, household heating system (HHS) with wall-mounted natural gas stoves has led to a continuous increase in the emission of NO. The absence of legal regulations and strict limits for NO emissions from wall-mounted gas stoves has led to a significant exceedance of indoor NO concentrations beyond the permissible value. However, this issue is rarely taken into consideration. In this study, we present the first report on NO emissions from wall-mounted gas stoves for household heating and their impact on indoor and ambient air quality in Chinese urban areas based on in-situ measurements and numerical simulations. On heating days, the observed indoor NO concentration is within 80-200 μg/m, much higher than the outdoor atmospheric concentration. With a low emission grade of the wall-mounted gas stoves, it is estimated that >10 % of residents in a typical residential building area are exposed to a high NO concentration of >200 μg/m, and >50 % of residents are exposed to a concentration of >80 μg/m. In addition, the indoor NO concentration shows an obvious non-uniform distribution with the floor in residential buildings. The NO emission from residential natural gas heating also shows an obvious impact on the microenvironment around buildings, which is primarily determined by the emission grade of the stoves. The findings highlight that HHS has become a non-negligible source of indoor NO pollution in China. It is urgently necessary to formulate NO emission limit standards for wall-mounted gas stoves in Chinese urban areas and upgrade traditional natural gas heaters with efficient emission reduction technologies.