Most of the planet’s glaciers exist in the Himalayas and the Tibetan plateau. This area, with thousands of lakes, is also known as the Third Pole (TP). Essentially a huge reservoir of water, the TP provides water and food security to billions of people in Asia. Numerous studies have linked air pollutants from South Asia to significant environmental and climatic impacts in the region and beyond.
Chemically, the isotopic composition of nitrate deposited in the atmosphere in masses of ice, such as glaciers, plays a central role in identifying environmental and climatic changes, past and present. Recent research by Mark Thiemens of UC San Diego, Mang Lin and Naohiro Yoshida of the Tokyo Institute of Technology and scientists of the Chinese Academy of Sciences (CAS) provides insight into the complete isotopic compositions of nitrate in aerosol, snow, ice and water samples collected across sub-regions of the TP. The scientists’ findings, recently published in the Journal of Geophysical Research Atmospheres as a cover article, reduce the complex nitrogen cycles at work in the atmosphere, cryosphere, hydrosphere and, potentially, the biosphere.
Thiemens, a distinguished professor of chemistry and biochemistry with expertise in atmospheric chemistry, explained that the ice bodies in the TP serve as a record of past changes in atmospheric and climatic conditions in this important region.
âDue to the unique location of the TP (at mid-latitudes), ice cores from this region record valuable paleoclimatic information that cannot be obtained from polar regions and link records from high and low latitudes to establish a global paleoclimatic perspective, âhe said.
Despite the importance of PT, studies of isotopic ratios of atmospheric nitrates in the region are limited. Yet, the high sensitivity of nitrate values ââto atmospheric oxidant reactions makes the triple isotopic analysis of nitrate in the atmosphere and ice samples a unique way to follow atmospheric chemical processes over time. Additionally, since atmospheric nitrate is the only source of nitrate that has unique oxygen isotopic values, its composition offers a new tracer to quantify its contribution to various ecosystems, from deserts to forests. The technique for doing this was discovered and developed at UC San Diego in the doctoral work of Greg Michalski (’99, BA, environmental chemistry; ’00, MA, ’03, PhD, chemistry), now a professor at the ‘Purdue University.
“As a major component of anthropogenic pollution and nitrogenous nutrients, nitrate in snow, ice and runoff on TP plays an important role in hydrological ecosystems not only of TP but also of Asia.” said Lin (’18, PhD, chemistry), first author of the research paper, professor at the Guangzhou Institute of Geochemistry at CAS, and alumnus of UC San Diego who won the Chancellor’s Thesis Medal in as a doctoral student researching the same subject at Thiemens Lab. “TP is the tallest and largest plateau in the world, and it strongly influences Asian monsoon and global atmospheric circulation patterns through dynamic and thermal effects.”
To conduct their study, the scientists used samples of snow and ice collected across Tibet. They melted the samples at room temperature, avoiding exposure to sunlight. All water samples were passed through quartz microfiber filters to remove insoluble material. They also dissolved filter papers with aerosol samples in pure water and removed insolubles. The solution was used for nitrate / nitrite concentration and isotopic analysis with an ion chromatograph and isotope ratio mass spectrometer, tools used for water chemistry and isotope analysis, respectively.
According to the scientists, their research presents a new comprehensive isotopic analysis of nitrate in aerosol, snow, ice and stormwater samples collected from sub-regions of the PT, enriching knowledge about the cycle of the nitrogen in the area.
âThe new measurements cover a large geographic area and reveal a spatially and temporally heterogeneous pattern of isotopic compositions of nitrates,â Thiemens explained. “Our study sheds new light on the complicated atmospheric and hydrologic nitrogen cycle on PT now and potentially in the past.”
The researchers propose further investigation strategies, noting the challenges of the remoteness and difficulty of sampling in the TP region.
This research was funded by the National Natural Science Foundation of China (grants nos. 41630754 and 41721091); the second scientific expedition and research program on the Tibetan plateau (grant n Â° 2019QZKK0605); Japan Society for the Promotion of Science (JSPS) KAKENHI (grant nos. JP18F18113, JP18K19850, JP19H01143, JP20H04305, JP16H05884 and JP17H06105 of the Ministry of Education, Culture, Sports, Science and Technology) and a JSPS Postdoctoral Fellowship for Research in Japan.