Underestimated Formaldehyde Emissions from Residential Coal Combustion in Rural China

Formaldehyde (HCHO) is an important precursor to atmospheric radicals, yet its complex sources remain poorly constrained. This study quantifies primary formaldehyde emissions from residential coal combustion and biomass burning using wintertime observations at a rural site in North China. A bimodal diurnal pattern of formaldehyde was observed, with morning peaks linked to coal combustion and afternoon peaks associated with biomass burning, supported by its strong correlations with combustion tracers (e.g., CO and SO₂) and intermittent household heating behaviors. Primary emission ratios of formaldehyde were determined to be 0.41 ppbv ppbv^-1 relative to SO₂ for coal combustion and 0.004 ppbv ppbv^-1 relative to CO for biomass burning based on the minimum R squared method. While the field-derived emission ratio for biomass burning agrees with current inventories, the ratio for coal combustion exceeds inventory estimates by up to an order of magnitude, indicating a significant underestimation of formaldehyde emissions from residential coal combustion. This underestimation is further corroborated by direct measurements from household coal combustion experiments and independent observations at another rural site. Chemical box model simulations show that observed formaldehyde significantly influences atmospheric chemistry, contributing 28, 16, and 21% to modeled HO₂, RO₂, and OH concentrations, respectively; 22% to ozone production rate; and 0.64 μg m^-3 to particulate hydroxymethanesulfonate concentrations. These findings highlight the critical role of formaldehyde in winter atmospheric chemistry and the urgent need to revise its emission estimates from residential coal combustion in the current inventory.