Emissions of nitrogen oxide from household gaseous cooking.

Incomplete combustion of polluting solid fuels contributes severe air pollution, threatening public health, thus the transition toward clean modern energies is emphasized as one important Sustainable Development Goal. While gaseous fuels have been documented to significantly reduce particulate matter emissions, growing concerns highlight pollutants such as nitrogen oxides (mainly NO and NO2) generated during the gas cooking could pose non-negligible risks to indoor air quality and residents' health. This study systematically reviewed studies on nitrogen oxide emissions from household gas combustion, compiling fundamental datasets of emission factors (EFs) and emission rates (ERs), and evaluated factors contributing to emission variability. EFs are not normally distributed. The median values of NO and NO2 EF are 22.2 ng/J and 9.1 ng/J, respectively, and the median EF for NOx, expressed as NO2 equivalent, is 42.5 ng/J. The ERs for NO, NO2 and NOx are 30.8, 11.6, and 58.8 mg/h, respectively. Using the bootstrap resampling method, the ratio of NO to NO2 emissions is estimated as 2.65 (2.32-2.99 as the 95% confidence interval); however, this ratio varies depending on factors such as the air-to-fuel ratio, burner power, and loading height. Input power affects combustion temperature and thereby the formation of thermal NO, thus is an important factor determining NOx emissions. This study also compares methods of full-capture dilution tunnel facility, chamber, and carbon balance approach in emission studies, highlighting their technical advantages and limitations in different application scenarios. These results provide a scientific reference for formulating emission control strategies, optimizing appliance design standards, and establishing indoor air quality guidelines.