Biomass Burning is a Non‐Negligible Source for Ammonium During Winter Haze Episodes in Rural North China: Evidence from High Time Resolution 15n‐stable Isotope

The refined source apportionment of ammonium (NH4+), especially for biomass burning emissions, is unclear. Hourly resolution δ15N‐NH4+ values for three winter haze episodes (EP1–EP3) were determined in the North China Plain (NCP) in 2019 to identify and quantify the contribution of biomass burning. A reasonable source‐resolved structure of NH4+ was obtained after using the corrected source signatures of 15N and considering biomass burning emissions. The time‐series variation in biomass burning and vehicle sources identified by δ15N was more reasonable and matched better with their tracers (e.g., levoglucosan). The non‐negligible contribution of biomass burning in the NCP was found and contributed 13.0% ± 11.4% to NH4+ in haze episodes, but in special stages was as high as 29% ± 11% in local emission of EP1 and 23% ± 15% in southwest transportation of EP2. The redistribution of NH4+ sources further emphasize the contribution of biomass burning to haze episodes, as its contribution to PM2.5 increased up to 5% (without considering SO42− and NO3−). Considering NH4+ uniqueness in the formation of new particles, the role of biomass burning during haze episodes should be considered. This study indicates that further studies need to be conducted to reduce biomass burning emissions in the NCP.