Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 111; no. 16; pp. 5808 - 5812
• Main Authors: Geng, Lei, Alexander, Becky, Cole-Dai, Jihong, Steig, Eric J., Savarino, Joël, Sofen, Eric D., Schauer, Andrew J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 22.04.2014; National Acad Sciences
• Subjects: Acidity; Acids - chemistry; Aerosols; Air pollution; Air pollution effects; anthropogenic activities; Anthropogenic factors; Atmosphere - chemistry; atmospheric deposition; Atmospherics; Climate change; Correlations; ecosystems; emissions; Environmental impact; Environmental Sciences; Fractionation; Greenland; Human Activities; Humans; Ice; Ice - analysis; Ice cores; ions; Isotope fractionation; Isotopes; Lake sediments; lakes; Nitrates; Nitrates - analysis; Nitric Acid; nitric oxide; Nitrogen; Nitrogen cycle; Nitrogen Isotopes; North America; Physical Sciences; Pollutant emissions; Pollution control; Protons; Sciences of the Universe; Sediments; Snow; Stable isotopes; Sulfates; Sulfates - analysis; Sulfur dioxide; North America; Greenland; proxy; fossil fuel emissions; industrial; clean air act; acid deposition
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1319441111

Nitrogen stable isotope ratio (δ ¹⁵N) in Greenland snow nitrate and in North American remote lake sediments has decreased gradually beginning as early as ∼1850 Christian Era. This decrease was attributed to increasing atmospheric deposition of anthropogenic nitrate, reflecting an anthropogenic impact on the global nitrogen cycle, and the impact was thought to be amplified ∼1970. However, our subannually resolved ice core records of δ ¹⁵N and major ions (e.g., [Formula], [Formula]) over the last ∼200 y show that the decrease in δ ¹⁵N is not always associated with increasing [Formula] concentrations, and the decreasing trend actually leveled off ∼1970. Correlation of δ ¹⁵N with H ⁺, [Formula], and HNO ₃ concentrations, combined with nitrogen isotope fractionation models, suggests that the δ ¹⁵N decrease from ∼1850–1970 was mainly caused by an anthropogenic-driven increase in atmospheric acidity through alteration of the gas−particle partitioning of atmospheric nitrate. The concentrations of [Formula] and [Formula] also leveled off ∼1970, reflecting the effect of air pollution mitigation strategies in North America on anthropogenic NO ₓ and SO ₂ emissions. The consequent atmospheric acidity change, as reflected in the ice core record of H ⁺ concentrations, is likely responsible for the leveling off of δ ¹⁵N ∼1970, which, together with the leveling off of [Formula] concentrations, suggests a regional mitigation of anthropogenic impact on the nitrogen cycle. Our results highlight the importance of atmospheric processes in controlling δ ¹⁵N of nitrate and should be considered when using δ ¹⁵N as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.