Ice core records of dicarboxylic acids, w-oxocarboxylic acids, pyruvic acid and a-dicarbonyls from south Alaska: Implications for climate change in the Northern Hemisphere since 1734–2008

180 m long (ca. 274 years) ice core was drilled in the saddle of the Aurora Peak of Alaska, which is located southeast of Fairbanks (63.52°N; 146.54°W, elevation: 2,825 m). Samples were directly transported to the Institute of Low Temperature Science (ILTs), Hokkaido University, Japan. Ice core has been analyzed for a homologous series of normal chain dicarboxylic acids (C2 - C11) branched chain saturated dicarboxylic acids (iC4 - iC6), unsaturated dicarboxylic acids (maleic, fumaric, methylmaleic and phthalic), multifunctional dicarboxylic acids (malic, oxomalonic and 4-oxopimelic), w-oxocarboxylic acids (ωC2 - ωC9), pyruvic acid, glyoxal and methylglyoxal to better understand historical changes in water soluble organic aerosols in the northern high latitudes over the past 274 years by using gas chromatograph (GC; HP 6890) and mass spectroscopy system (GC/MS; Agilent).

We found a predominance of oxalic acid (av. 7.17±4.18 ng/g-ice), followed by adipic acid (av. 4.99±10.5 ng/g-ice) and succinic acid (av. 4.31±3.20 ng/g-ice). Oxalic acid (C2) is an end product of oxidative chain reactions of low molecular weight dicarboxylic acids, ketoacids and a-dicarbonyls. C2 showed sporadic peaks in the years of 1746, 1795, 1809, 1813, 1846-1851, 1910, 1966, 1989, 1999 and 2005. Among the longer chain diacids (C5 to C11), adipic acid (C6) is the most abundant, followed by azelaic (C9) and suberic acid (C8). The historical trend of adipic (C6) in 1740s -1840s is similar to those of glutaric (C5), pimelic (C7), suberic (C8) and azelaic acid (C9). The profiles of sebacic (C10) and undecanedioic acid (C11) are different than those of other species, which could be formed from the bacterial activities in the marine atmosphere.

Molecular distributions of ω-oxocarboxylic acids are characterized by the predominance of 9-oxononanoic acid (1.13±1.22 ng/g-ice), followed by 4-oxobutanoic acid (0.99±1.12 ng/g-ice), and glyoxylic acid (0.93±1.05 ng/g-ice). 9-Oxononanoic acid is produced by photochemcial oxidation of biogenic unsaturated fatty acids emitted from ocean surface. Total oxoacids showed sporadic peaks in the years of 1746, 1751, 1760, 1768, 1795, 1840-1856, 1983, 1989 -1999 (except for 1993 and 1999). In contrast, pyruvic acid (Pyr) shows different multi-decadal historical trends than α-dicarbonyls and others species, which could be influenced from in-cloud isoprene oxidation. Historical increases in the ratios of malonic (C3) to succinic (C4) acid (range: 0.04 – 3.4, ave. 0.25) and C2 to C4 acid (range: 0.73 - 8.5, ave. 2.4) show that photochemical oxidation process has been enhanced significantly from 1734 to the present (2008). Historical increases of Pyr and C3/C4 and C2/C4 ratios suggest that atmospheric oxidizing capability for these organic compounds are associated with an increase in solar radiation in the northern North Pacific region.

The molecular distribution of diacids and related compounds is completely different than Greenland Site-J ice core, in which C4 is generally more abundant than C2. Historical concentrations of diacids, oxoacids and α-dicarbonyls along with correlation and principal component analyses for 1734-2008 and backward trajectories suggest that they are mainly formed by the atmospheric oxidation of unsaturated fatty acids emitted from the ocean surfaces and are derived from heavy biomass burning in the source regions (e.g., significant concentrations of NH₄⁺, SO₄^2-, NO₂^- and levoglucosan were found in the same ice core). Their historical trends for the last three decades are similar to each other and somewhat consistent with lower tropospheric temperature anomalies (NOAA'S TIROS-N weather satellites data from surface to ~ 6 km), which is thought to be climate driven, following the past atmospheric oxidizing capability and climate variability in the Northern Hemisphere. (568)