A chemical ionization mass spectrometer (CIMS) was deployed to measure ambient ammonia (NH₃) with a time-resolution of ∼5 min. Unexpectedly high levels of NH₃, peaking at 15.5 ppbv and averaging at 2.1±1.9 ppbv, were observed near an agricultural field in Nanjing, China, during wintertime, a slack season for farming. Re-partitioning from aerosols can only explain a small portion of this NH₃ due to the considerably low air temperature, whereas the major source was emission from soil, as indicated by significant positive correlations of the NH₃ concentration with the ground surface temperature, solar radiation intensity, and particularly the surface-air temperature difference. The dissolved/adsorbed NH₃ and NH₄NO₃ in soil likely originated from conversion of nitrogen-containing organic matter by microorganisms through ammonification and nitrification. The upward transfer of NH₃ released from the soil was facilitated by convective mixing driven by solar radiation, which produced the required high soil-air temperature gradient. This winter release of NH₃ from the soil is not captured by commonly used long-term, passive-sampler based methods, giving rise to an incorrect assumption that during winter, a slack season for farming, soil emits no NH₃ and so this source has remained unresolved by typical emission inventory models. Our results indicate that contrary to the common assumption, agricultural background emissions are a non-negligible source of NH₃ even in wintertime, which may exert a significant impact on regional air pollution formations. Hence, a refined NH₃ emission inventory is critically needed to account for the haze events in wintertime northern China.