Ammonia (NH.sub.3) is an important agent involved in atmospheric chemistry and nitrogen cycling. Current estimates of NH.sub.3 emissions from biomass burning (BB) differ by more than a factor of 2, impeding a reliable assessment of their environmental consequences. Combining high-resolution satellite observations of NH.sub.3 columns with network measurements of the concentration and stable nitrogen isotope composition ([delta].sup.15 N) of NH.sub.3, we present coherent estimates of the amount of NH.sub.3 derived from BB in the heartland of Southeast Asia, a tropical monsoon environment. Our results reveal a strong variability in atmospheric NH.sub.3 levels in time and space across different landscapes. All of the evidence on hand suggests that anthropogenic activities are the most important modulating control with respect to the observed patterns of NH.sub.3 distribution in the study area. N-isotope balance considerations revealed that during the intensive fire period, the atmospheric input from BB accounts for no more than 21Â±5 % (1Ï) of the ambient NH.sub.3, even at the rural sites and in the proximity of burning areas. Our N-isotope-based assessment of the variation in the relative contribution of BB-derived NH.sub.3 is further validated independently through the measurements of particulate K.sup.+, a chemical tracer of BB. Our findings underscore that BB-induced NH.sub.3 emissions in tropical monsoon environments can be much lower than previously anticipated, with important implications for future modeling studies to better constrain the climate and air quality effects of wildfires.