Radiatively driven NH3 release from agricultural field during wintertime slack season

Abstract

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.