Light carboxylic acids in Greenland ice: A record of past forest fires and vegetation emissions from the boreal zone

Abstract

Measurements of light carboxylic acids (formate, HCOO−; acetate, CH3COO−; glycolate, C2H3O3−; and oxalate, C2O4−−) were performed along Summit (central Greenland) ice cores covering the last two centuries and a complete glacial‐interglacial cycle. High‐latitude biomass burning contributes between 20% and 30% to the formate, oxalate, glycolate, and ammonium deposited on central Greenland over the last 200 years. This biomass burning contribution is very weak for nitrate and acetate. A decreasing trend in formate, parallel to the increasing strong acidity (H2SO4 and HNO3) is observed over the last decades. This lack of record of the growing anthropogenic emissions of formic acid and of their precursors (nonmethane hydrocarbons and CH4) likely results from a less efficient scavenging of this weak acid from the gas phase into precipitation due to the recent acidification of the atmosphere. In contrast, the acetate profile shows a well‐marked increase over the last three decades. This difference in the formate and acetate trends remains difficult to explain in the light of our present knowledge of anthropogenic emissions. The role of peroxyacetyl nitrate (PAN) acting as a reservoir for acetic acid in polar regions is a possibility which needs to be investigated. Aside from high‐latitude biomass burning events which frequently occurred in the past except during the ice age maximum (15,000 to 40,000 years B.P.), the background levels of HCOO− and CH3COO− are lower during the ice age with respect to values observed during warm stages. Changes in vegetation emissions from North America are suggested to be responsible for the variations of formate background level observed in Greenland ice over the last 100,000 years. Greenland ice therefore contains information on the history of the Laurentide ice sheet, suggesting a well‐established Laurentide ice sheet between 15,000 and 78,000 years B.P. and large fluctuations of its extent before 70,000 years B.P. Although the interpretation of the acetate profile remains more speculative, our data suggest that marine emissions of hydrocarbons may be an additional significant source of acetate.