The air in urban Beijing becomes polluted with small particles when the winter season comes. One third of these small particles are ammonium aerosols that are formed via chemical reactions between ammonia and other acidic gases. However, the control of acidifying sulfur and nitrogen oxides in recent years has resulted in excess ammonia in many regions. Thus, reducing ammonia emissions has been suggested as an appropriate method to address the urgent need to reduce the frequent occurrence of haze events in urban regions.

Identifying the major sources of ammonia is not an easy task. Due to the high levels of ammonia volatilization following the application of fertilizer on farmlands, agricultural emissions have been suggested as the dominant source of ammonia in urban regions. However, this traditional belief is not necessarily a pervasive one, especially considering the reduction in fertilizer use in the winter season. In addition, stagnant weather on hazy days is unfavorable for the transport of ammonia from distant farmlands to urban regions; and although vehicular emissions are known to be an important source of ammonia in urban environments, quantifying their contribution remains an open question.

Measuring the nitrogen isotope composition of ammonia is a useful technique and can help trace the sources of ammonia in the urban atmosphere if we know the isotopic signatures of each ammonia source (e.g., fertilizer, pipelines, coal combustion). However, previous source signatures have almost always been characterized by passive samplers, which, compared to active samplers, underestimate isotopic composition of ammonia by 15.4 parts per thousand on average. This large bias results in misleading source apportionment results for ammonia, and hence makes life difficult for policymakers when designing control strategies to reduce ammonia.

In a recent study published by Advances in Atmospheric Sciences, Dr. PAN Yuepeng and colleagues from the Institute of Atmospheric Physics, Chinese Academy of Sciences, corrected the isotopic signatures of ammonia sources with updated active sampling techniques and then revisited the sources of ammonia in urban Beijing as an example. With the new source signatures, Pan and his team confirmed that the major source of ammonia in urban Beijing is not from agricultural emissions, which is different from traditional opinion. In contrast, it was found that non-agricultural emissions contributed 66% of ammonia in urban Beijing.

Agriculture was traditionally regarded as the major source of ammonia emissions, but now – based on robust isotopic evidence – vehicular emissions have been identified as the key player.