A Century Ammonium Record Retrieved From the Central Tibetan Plateau

The limited observations pose challenges to understanding long-term variations of atmospheric ammonium (NH4+) deposition. Glaciers on the margins of the Tibetan Plateau (TP) adjacent to heavily populated regions can capture and preserve unique records of natural and anthropogenic ammonium aerosol. However, relatively little has been known about spatiotemporal variations and anthropogenic influences on ammonium deposition on the central TP. Here, we present a high-resolution ammonium record spanning the period 1900-2011, reconstructed from the Qiangtang (QT) ice core on the central TP. Empirical orthogonal function and interspecies correlation analysis reveal that NH4+ has a source and/or transport route that differs from the dust species, especially after the 1950s. The QT ammonium record is also likely influenced by temperature-dependent biogenic emissions from northwestern South Asia. Comparison with other ice core records shows that intensified anthropogenic emissions have caused a widespread increase in ammonium records in the inner TP after the 1950s. Furthermore, the significant positive relationship with the December- April Arctic Oscillation (AO) suggests that enhanced westerlies during the positive AO phase transport NH4+ aerosol more efficiently to the QT. As a result, the decrease in ammonium concentrations after 1990, when anthropogenic emissions were still increasing, could be attributed to the weakening of westerlies associated with the transitioning of the AO phase from positive to negative. Our results may enhance the understanding of the influence of the westerlies on the transport of atmospheric pollutants to the central TP.Plain Language Summary Glaciers on the Tibetan Plateau (TP) are ideal natural archives that capture and preserve air pollutants released by human activities. Ammonium is one of the cations widely measured in ice cores from the TP glaciers. In the last decades of the 20th century, the enhancement of ammonia from anthropogenic emissions (such as fertilizer application, animal waste, biomass burning, and industrial processes) contribute to the increase in ammonium concentrations in ice cores retrieved from the south and west margins of the TP. However, the ammonium record in the inner TP should be affected by a combination of variations in climatic and environmental conditions. Here, we analyzed ammonium record extracted from the Qiangtang (QT) glacier, located in the central TP. Results show that ammonium has discernible noncrustal inputs in the late 20th century. QT ammonium concentration also displays an increasing trend after the 1950s, which could be attributed to the enhanced agricultural emissions in south and central Asia. However, after 1990, when anthropogenic emissions were still increasing, the decrease in ammonium concentrations could be attributed to the weakening of the westerlies.

Automated summary

The analysis of the Qiangtang ice core in the central Tibetan Plateau reveals that ammonium concentrations were mainly influenced by agricultural emissions from South and Central Asia in the late 20th century, resulting in an increasing trend after the 1950s. However, after 1990, despite continuing anthropogenic emissions, the decrease in ammonium concentrations can be attributed to the weakening of the westerlies.