Nitrate record of the 2022 Mt. Logan ice core suggests highest elevation regions sensitive to atmospheric acidity

Anthropogenic nitrogen oxide (NO_x = NO + NO₂) emissions have increased since the Industrial Revolution as a result of fossil fuel burning, contributing to increasing atmospheric acidity and changes to the oxidative capacity of the atmosphere. Oxidation of NO_x leads to the formation of atmospheric nitrate both in the gas phase (HNO_3(g)) and aerosol phase (p-NO₃^–), which may then be removed from the atmosphere via wet and dry deposition. Ice core records of nitrate may thus be used to infer past changes in atmospheric NO_x concentrations and atmospheric acidity given high enough accumulation rates to prevent substantial post-depositional photolytic loss from the snowpack. Increasing trends innitrate concentrations over the 20^th century have been observed in ice core records throughout the Northern Hemisphere including Greenland and the North Pacific. However, two ice cores (1980 NW Col and 2002 PR Col ice cores) retrieved from the summit plateau (5,334 m a.s.l.) of Mt. Logan, the second tallest mountain in North America located in the glaciated region of the St. Elias Mountains in southwest Yukon, revealed no long-term trend in acid chemistry. This lack of sensitivity to increasing atmospheric acidity was largely attributed to the high elevation of the site within the free troposphere and the efficient scrubbing of atmospheric pollutants during transit across the Pacific. Here, we present a nitrate record from the new 2022 Mt. Logan ice core since 1912 CE (~256 m depth). Reconstructed accumulation at the site is extremely high with an average rate of 2.97 m weq a^-1 from 1912 to 2020, implying excellent preservation of volatile species coupled with low average temperatures (-26.9°C). The nitrate record suggests a statistically significant (p < 0.01) increasing trend since 1912 CE, in contrast to both the NW Col and PR Col records. The record agrees with other Northern Hemisphere ice core nitrate records including Summit (Greenland; r = 0.49, p < 0.01, 1912–2006), Begguya (Alaska; r = 0.44, p < 0.01, 1912–2012), and Eclipse (Yukon; r = 0.30, p < 0.01, 1912–2001). These results indicate that the highest elevation regions of the North Pacific, such as Mt. Logan, are indeed sensitive to anthropogenic NO_x emissions, with ice cores providing rare insight into mid-tropospheric acid chemistry where preservation is adequate.