Ammonia exchange flux over a tropical dry deciduous forest in the dry season in Thailand

Abstract. Ammonia (NH3) is a significant contributor to total nitrogen deposition in East Asia. However, process-based observations that specifically focus on the air–surface exchange of NH3 remain limited in this region, especially in Southeast Asia. To clarify the bi-directional exchange process of NH3 under tropical climatic conditions, we present the first observations of NH3 exchange flux over a tropical dry deciduous forest in Thailand during two periods with different canopy and meteorological conditions in the dry season using the aerodynamic gradient method. NH3 concentrations exhibited strong positive correlations with air temperature and negative correlations with wind speed during the leafy period. However, there was no clear correlation between concentrations and meteorological elements during the leafless period. Measured NH3 fluxes fell within the ranges presented in recent studies, with a weighted mean and standard deviation of 0.148 ± 0.240 µg m−2 s−1, and consistently larger during daytime. During the dry season, the tropical dry deciduous forest acted as an emission source of NH3. Across both observation periods, NH3 emissions were governed by air temperature, relative humidity, friction velocity, and solar radiation. While no clear difference in fluxes magnitude was observed between the leafy (0.140 ± 0.240 µg m−2 s−1) and the leafless (0.158 ± 0.239 µg m−2 s−1) periods, the main source of NH3 emission in the tropical dry deciduous forest probably shifted dynamically from stomata to leaf litter due to the changes in meteorological, canopy, and forest floor conditions.