Increased nitrate and decreased δ¹⁵N–NO₃⁻ in the Greenland Arctic after 1940 attributed to North American oil burning

Abstract. Nitrogen oxides (NO_x = NO + NO₂) have an important impact on the atmosphere and biosphere through controls on oxidant concentrations and the formation of nitric acid (e.g. acid rain). Anthropogenic emissions from fossil fuel combustion, land use change, and agriculture have altered the global nitrogen cycle since the beginning of the Industrial Revolution but contributions from specific sources are difficult to quantify, hindering understanding of changes to the natural environment and design of effective mitigation strategies. Ice core records reveal changes in the nitrogen cycle over time in connection to climate, atmospheric chemistry, and the biosphere. Here we use a seasonally resolved ice core record of the nitrogen isotopic composition of nitrate (δ¹⁵N–NO₃⁻), together with a broad suite of highly resolved (> 22 samples y⁻¹) elemental and chemical tracers, to investigate sources of nitrate deposited in central Greenland from 1760 CE to present. A marked negative trend in δ¹⁵N–NO₃⁻ since ~ 1940 CE paralleled a nearly three-fold increase in nitrate concentration. Based on correlated increases in tracer concentrations, independent emission estimates of oil burning, and an isotope mixing model, North American oil combustion likely was the primary driver of recent changes in the nitrogen cycle recorded in Greenland.